Introduction

At a formative level, one of the goals of high school is for teachers to help students acquire knowledge and skills that will enable them to make responsible decisions as citizens. In some schools, where sustainability is part of the curriculum, students may exercise their rights and obligations to implement sustainable development in the country successfully. However, teachers’ beliefs and knowledge significantly influence teaching–learning processes (García & Aguado 2011). Therefore, teachers’ practices and knowledge are strongly linked to the quality of education. Any approach to teaching sustainability, or any other subject, cannot be successfully implemented without the teachers’ active participation. Hence, it is essential to determine what teachers know and understand about sustainability, to identify how they teach this subject, and how important they think the teaching of sustainability is within the subject matter they teach because of the climate emergency declared by the Common Worlds Research Collective (2020), where it is recommended that all educational systems should consider the teaching of sustainability as an instrument of behaviour transformation (Gayford, 2009). Nevertheless, in Mexico, as in many other places, this has not happened because this subject is taught as part of scientific disciplines such as biology or chemistry, and it is not seen as an independent cross-curricular subject that takes elements from other topics (Reyes & Quispe 2018). In this sense, this work seeks to characterise and analyse the pedagogical content knowledge (PCK) related to sustainability in in-service teachers by exploring how high school science teachers teach this subject, what concepts are essential, and what considerations they make to make sustainability comprehensible for students.

Currently, moving towards a sustainable future has become one of the biggest challenges to guarantee the continuity of all animal and plant species in the face of the imminent planetary emergency (Bybee 1991) caused by the increase in environmental difficulties such as ecosystem degradation, depletion of natural resources, loss of biodiversity, pollution, climate change, the COVID-19 pandemic, and the increase in social inequalities, among others. In this context, education for sustainability (ES) is being promoted by researchers as a tool for social transformation (Aznar-Minguet and Ull-Solís 2019) to promote the change from traditional educational models to sustainability-based teaching–learning approaches (Murga-Menoyo 2015). Education for sustainability aims to improve the quality of education under a human rights approach that fosters contextualised learning, promotes social cohesion and interculturality, and is oriented towards action (Vilches & Gil 2013). Regarding scientific education, some universities also seek to incorporate essential sustainability skills in teacher training (Wiek et al. 2011), encouraging its nature in teaching–learning (UNESCO 2009; UNESCO 2012).

In the literature (Wakefield 2003), it is reported that sustainability is mainly taught in informal education, and many researchers have focused their projects on this kind of education (Carvajal-Oses et al. 2023). However, in formal education, sustainability is taught as part of subjects, such as chemistry and biology, more as fragments of phenomena rather than a whole. In this sense, teachers teach sustainability from the subject’s point of view and use specific strategies related to their perspective. For these reasons, this research is essential because it could provide a perspective on what teachers think, believe, and do when they must teach the concept of sustainability through the lens of the subject they usually teach. (Ware et al. 2019; Fagnani et al. 2020; Wissinger et al. 2021).

Some teaching practices reported in the literature around sustainability involve e-learning (using digital environments such as labs, simulations, and virtual field trips) and inquiry practices that allow students to reflect on fundamental ideas related to this subject, such as green chemistry, biodiversity, and the environment. Espejel and Flores (2017) reported one exciting nonformal education project, where two successful strategies that promote the four dimensions of environmental consciousness (cognitive, affective, attitudinal, and active) are reported. The primary model is environmental urban education (MEUE), with two programmes: environmental activities from school to community and ecological projects (ludic and creatives). Both try to promote environmental education inside the community through students taking the high school ecology course.

PCK framework

Pedagogical content knowledge (PCK) has become one of science education research’s most used theoretical constructs in the last 30 years. It is a benchmark of professional knowledge possessed by teachers, and it is a fundamental tool for inquiring into teachers’ thinking about specific content (such as sustainability) and pre-service or in-service teacher training (Abell 2008). As Shulman (1987) pointed out, teachers can transform understanding, performance skills, and desired values for attitudes into pedagogical actions and representations. This is how PCK makes it possible to enquire about teachers’ thinking on specific content and their interaction with pedagogy since all educational activity is supported by a series of implicit beliefs and theories that are part of the teacher’s thinking and which, in turn, guide their ideas about the knowledge, the construction of their teaching, and the student’s learning (Loughran et al. 2008; Garritz 2007). PCK is a particular knowledge that differs from expert content and general pedagogical knowledge.

In general terms, there are different models related to PCK, which have evolved from Shulman’s first proposal. Among these, the latest (Carlson & Daehler 2019) proposes three different ways of conceptualising PCK: collective PCK (c-PCK), which is represented by different approaches and strategies that a group of teachers decides to implement in their classrooms; personal-PCK, which describes how a teacher understands their practice, the discipline and how to implement and assess it, considering not only the domain itself but also the possible filters (perceptions, emotions, beliefs, etc.) that can make the discipline understandable to their students; and finally, enacted-PCK, which teachers use to make decisions about which actions to take and how to put them into practice. In this research, personal PCK (p-PCK) will be considered. We will try to integrate collective PCK (c-PCK), understood as knowledge shared by different teachers (who have yet to meet each other) about teaching some subject. So, c-PCK will be built based on the results of the different teachers.

Finally, the study of teachers’ thinking on sustainability is an opportunity to contribute to educational research in the field of education for sustainability (ES): a type of education based on sustainable social transformation, promoting a rethinking of teaching and learning, as well as current educational practices around sustainability.

Models of personal PCK

There are several models to understand p-PCK, whose knowledge or components have been defined by many researchers (Van Driel et al. 1998, 2002; Magnusson et al. 1999). In the present work, the analysis of teachers’ way of thinking to characterise PCK was done through Padilla and Van Driel’s (2011) proposal, where the different components and subcomponents of PCK were described in a simple and organised way. This proposal was considered a valuable tool for studying teachers’ thoughts and actions in their teaching practices because it allows researchers to identify many characteristics of PCK. Table 1 shows in detail the five components of PCK initially reported by Magnusson et al. (1999) and later adapted by Padilla and Van Driel (2011).

Table 1 Description of components and subcomponents used to categorise teachers’ PCK. These codes were used for the CoRe analysis
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Characterising PCK

To portray PCK is a complex task that generally requires a combination of strategies to improve the data analysis regarding teachers’ thinking. These data are collected and analysed using different instruments, such as questionnaires with open and closed questions (Gess-Newsome et al. 2017), class observations carried out by teachers in training or in-service, concept maps, graphic representations of their ideas, questionnaires related to their teaching practices, and semi-structured interviews (Parga-Lozano and Mora-Penagos 2008).

Therefore, we aim to analyse teachers’ thinking and pedagogies associated with sustainability at the high school level and characterise this thinking as PCK. This goal will be achieved using the PCK model proposed by Padilla and Van Driel (2011) and a modified version of content representation matrices (CoRe) (Loughran et al. 2004). The following research questions were raised: What is the teacher’s thinking related to sustainability and the way they usually teach it? What are the pedagogical strategies that teachers use to teach sustainability? How is c-PCK related to sustainability?

Methodology

Participants

The teachers who agreed to participate in this study work in Mexico’s three main types of high schools. The differences between these high schools lie in the teaching approach they favour and the class size. Two of these high schools are associated with the national university; one teaches using a science-technology-society teaching approach; the second one uses an inquiry-based approach, whilst the third high school is part of the city government, and its students are of low socioeconomic status; so, the school’s approach is more student-centred and has much smaller groups than the first two.

The sample consisted of ten teachers who taught either chemistry or biology, with five in each discipline. All of them were in-service teachers with 1 to 32 years of experience (see Table 2). Pseudonyms were used to identify them: Chem for Chemistry teachers and Bi for biology. Therefore, participants are referred to as Chem1, Chem2, Chem3, Chem4 and Chem5 and Bi1, Bi2, Bi3, Bi4 and Bi5. For data collection, once the teachers had agreed to participate in the research, we sent them the CoRe in an Excel® file and gave them 2 to 3 weeks to complete it.

Table 2 Teachers’ general characteristics
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Data collection

Two of the most widely used methodologies to recognise and characterise PCK that help to describe and document teachers’ thinking at primary and secondary levels, particularly on subjects in the field of natural sciences, are the instruments called the Matrix of content representation (CoRe) and the Repertories of Professional and Pedagogical Experience (Re-PyP) (Loughran et al. 2004, 2008). This research uses CoRe to characterise the elements necessary for teachers to make sustainability understandable. Besides the questions (see Fig. 1), it was necessary to identify central ideas related to sustainability that teachers most used. So, teachers answered a questionnaire about their ideas regarding sustainability that guide their teaching practice and what teaching strategies they use most when teaching sustainability, among others. From this questionnaire, we selected five central ideas for each subject matter, which are the following: ecological balance, natural resources, biodiversity, ecosystems, and the environment (Biology) and the environment, natural resources, energy, climate change, and pollution (Chemistry). Once we had the central ideas and the CoRe matrix, this last was sent to teachers in Excel®, asking them to return the file with their answers as soon as possible. We use their answers for data analysis.

Fig. 1
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Questions included in the CoRe matrix and how each relates to the PCK components. Red lines relate the CoRe to PCK’s components. The other coloured lines relate each component to questions in the CoRe

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Analysis of data

The analytic strategy employed deductive qualitative analysis via the Atlas.ti (9.0)® software Muhr (1993). The data analysis was performed using Padilla and Van Driel’s (2011, 2012) categorisation (see Table 1), so the first author carried out the coding process, and the first step was to identify the components and subcomponents of PCK using the teachers’ answers in the CoRe matrix. This process helped precisely describe each component and subcomponent according to the subject they taught (biology or chemistry) and draw a preliminary detailed description of a personal teaching profile. In this way, each teacher was analysed as an individual case study to enable a joint analysis of them in a second stage. The codes were based on the components and subcomponents of PCK shown in Table 1, which describe the knowledge, beliefs, purposes, and goals of science teaching taken from the Author’s proposal, based on Magnusson et al.’s (1999) model. The ATLAS.ti (9.0) programme was used to improve the data analysis, where the teachers’ answers were introduced and coded, and then data were transferred to tables and Sankey diagrams. One advantage of using Atlas.ti is that frequencies (occurrences) in which each component appears are provided, as well as the co-occurrences in which the codes are related. Therefore, Sankey diagrams are ideal to show this type of result. These diagrams show data flows among codes and the width of lines proportionally. The width of the line represents the magnitude of the relationships among codes (frequencies or occurrences). Therefore, the wider the line is, the greater the relationship among codes (Datasketch 2023). In this paper, the width of the line represents the frequency with which codes appear in teachers’ answers. Finally, co-occurrence in Sankey diagrams is represented by showing those connections among PCK components as two joined lines.

Findings

The results are presented according to the subject taught by teachers, showing one specific case of each teacher and then comparing all the teachers in each group.

Chemistry teachers

Teacher Chem3

We chose teacher Chem3 because he shows a robust way of understanding his teaching of sustainability. In this case, different components appear in his answers, some of which have more frequency than others (see Fig. 2). Therefore, regarding approaches to the teaching (ATSc) of sustainability and the central topics analysed, environment, natural resources, energy, climate change, and pollution, only two components are shown: a process approach (ATSc-P) and conceptual change approach (ATSc-Cc), as observed in the following answer: It is very important, it must be part of the scientific culture of all citizens. In this way, we hope to mitigate environmental problems and that they (students) realise that they (environmental issues) are tangible and measurable problems. I ask them to imagine they are part of a team that must provide solutions (ATSc-Cc).

Fig. 2
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Sankey diagram of the teacher profile of Professor Chem3. The thickness of the lines represents the frequency of each subcomponent, so in this case, specific strategies (KIS-2), goals in learning (KScC-1), and beliefs of what students know (KSU-4) are the most frequent components used by this teacher

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Regarding the knowledge of the curriculum (KScC), the subcomponent teachers’ ideas about students’ goals (KScC-1) predominates in the teacher’s discourse, as can be recognised in the following paragraphs: (1) If we are not sustainable, there will be severe environmental damage (KScC-2, goals and guidelines across topics) and (2) Sustainability helps to have more rational consumption patterns, so we demand less goods and energy mitigating climate change (KScC-1). Consequently, it reflects that in most of the topics taught by Chem3, the teacher’s ideas prevail over the content and teaching objectives. KScC-1 is the most frequent subcomponent related to knowledge and beliefs about the curriculum. KScC-2, KScC-3 (students’ previous conceptions), and KScC-4 (teacher’s knowledge of specific curricular programmes) also appear, but with much less frequency.

Another component that appears very frequently is the knowledge of the students’ scientific understanding (KSU). In this case, subcomponent KSU-4 (beliefs about what students do or do not know) is the most frequent of all, and it is related to beliefs about what knowledge students do or do not have or what the teacher thinks students should learn. The following phrase from teacher Chem3 illustrates this: If we are unaware that the environment allows us to live since it provides water, air, and soil for subsistence, we will deplete it, endanger our survival, and become endangered as a species (KSU-4).

Regarding assessment knowledge (KASc), the subcomponent (KASc-2, dimensions of science learning to assess) is mainly present, as seen in the following answer: I have used case studies (KASc-2), they make posters (KASc-2), I have applied multiple-choice questionnaires (KASc-2) and portfolios (KASc-2). These results show the type of assessment strategies mainly employed by Chem3. Assessment is less important than the curriculum, teaching strategies, or students’ knowledge of PCK. This result is in line with those reported in the literature.

Finally, knowledge of instructional strategies (KIS) is as frequent as KSU (knowledge of students’ understanding). However, the most frequent subcomponent for Chem3 concerning KIS is a topic-specific representation (KIS-2). This can be seen in the following answer to the question (What procedures do you use for students to commit to the concept?): Analogies (KIS-2), examples (KIS-2), readings (KIS-2), discussions (KIS-2), laboratory practices (KIS-3, topic-specific activities), and simulations (KIS-3). The strategies above imply that Chem3 mainly uses specific strategies and representations to teach the selected topics, even using subcomponents (KIS-3) when proposing experimental activities and simulations.

As we have said, the p-PCK of Chem3 is shown in Fig. 2. As stated before, there are three dominant components in this teacher’s teaching profile: their ideas about why students learn the topic of sustainability (KScC-1, teachers’ ideas of students’ goals in learning the subject), what they believe students should or should not know or learn (KSU-4), and the type of specific teaching strategies used to make the topic understandable (KIS-2, goals and guidelines across topics). In other words, many of their decisions are based on their beliefs about how and why students learn about sustainability.

Figure 3 shows the relations among the most frequently mentioned components in Chem3. Teachers’ ideas of students’ goals in learning the subject (KScC-1) are strongly related to KSU-4, but at the same time, it is related to KSU-4 through goals and guidelines across topics (KScC-2). These relations show that this teacher’s ideas of students’ goals about the topic are strongly associated with his beliefs about what students should or should not learn. At the same time, these ideas (KScC-1) relate to teachers’ goals and guidelines across the subject, which relate to KSU-4 and ATSc-Cc (approach to conceptual change). This figure makes it easier to visualise the interrelationships among subcomponents, which can be more complicated than just one relation; for example, in Fig. 3, the relation KScC-1/KScC-2/KSU-4/ATSc-Cc (goals in learning the subject-guidelines across topics-beliefs’ about what students should learn-conceptual change) is shown as one type. Still, another example could be KScC-1/KSC-2/KSU-2/KSU-4 or one as simple as KScC-1/KSU-4. Therefore, this analysis allows us to understand teacher-PCK relationships.

Fig. 3
figure 3

Sankey diagram where interrelations (co-occurrences) among subcomponents are shown. A strong relation between goals of learning (KScC-1) and beliefs of what students know (KSU-4) can be seen, and at the same time, there is a relation between goals of learning (KScC-1)/guidelines across the topic (KScC-2)/beliefs of what students know (KSU-4) and KScC-1/KScC-2/students’ approaches (KSU-2)/KSU-4

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Variations in PCK components used by all chemistry teachers: c-PCK

After we analyse all the answers given by chemistry teachers, it is possible to notice that they all have a very similar teaching profile but quite different frequencies of components and subcomponents. In Fig. 4, the relationships among teachers’ profiles are shown. In addition, the most frequent components and subcomponents are KSU-4, KScC-1, and KIS-2 (topic-specific representations). That is, as in the case of Chem3, personal beliefs permeate sustainability teaching. It is important to note how teachers’ profiles differ in width and frequency. For example, Chem3 shows a more robust profile than Chem5, and this is quite clear in the Sankey diagram, as it offers a greater variety of subcomponents in Chem3’s PCK. These also appear thicker in the diagram. In contrast, the PCK of Chem5 shows less thickness at the origin of its profile and a thinner branching in the subcomponents used.

Fig. 4
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Sankey diagrams for chemistry teachers, which we call collective PCK for these teachers. Note that teachers’ ideas of students’ goals in learning sustainability (KScC-1) and beliefs about what students do or do not know (KSU-4) are the thickest, which means that those pieces of knowledge are the most used by teachers

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In Fig. 4, all the chemistry teachers’ profiles are shown, enabling a bird’s eye view of what collective PCK (Carlson & Daehler 2019) could be, which generally resembles the teachers’ personal PCK. It is important to note that there are teachers whose p-PCK is broader and more robust than others, meaning they use more elements in their teaching practice.

Another component with high occurrence is knowledge of instructional strategies (KIS), which also draws our attention because the teaching approach does not define these strategies but rather the teacher’s knowledge and beliefs about the curriculum and how students learn. Finally, the last component, which appears in a lower proportion, is knowledge of assessment (KASc). It is one of the most important types of knowledge because assessment allows teachers to decide if students understand and can apply what they have learned. Despite KASc showing lower frequencies, it is still essential for teachers since they focus more on pointing out methods without mentioning implicit reflection in the evaluation process.

We will now discuss the subcomponents with the highest occurrence in collective PCK. The component related to orientation towards the teaching of science that appears most frequently is approaches to conceptual change (ATSc-Cc). The following is an example of how this concept was demonstrated: I hope to generate a fundamental change in their behaviour that will impact their home and daily lives (Chem1). This component is related to the approach used by teachers to make sustainability a part of students’ scientific culture, and it is considered beneficial to teach in a specific context to achieve a fundamental change in students’ habits by helping the students generate awareness, reflect on environmental issues in their daily life, and propose possible solutions.

The component related to knowledge of the scientific curriculum with the highest frequency of appearance is KScC-1, which encompasses the teachers’ ideas about students’ goals to learn sustainability. Consequently, in most cases, the teachers’ ideas prevail over content and teaching goals regarding sustainability and the central topics analysed. For example, to show the importance of sustainability, teachers pointed out some issues, such as climate change, how resources are used, and pollution. It refers specifically to the sustainable use of resources by each generation without risking their availability (or looking for recycling or replacement alternatives) for new generations (Chem5).

Regarding the knowledge of students’ scientific understanding (KSU), the most common components are KSU-4 and students’ difficulties (KSU-3). The former refers to the teacher’s beliefs related to the knowledge that students may or may not possess or the knowledge that the teacher thinks students should learn, such as caring for the environment, properly managing natural resources, or addressing climate change. For instance, Chem1 mentions that all actions we take can impact the climate, so students must be aware of small to complex activities. Regarding understanding the causes of pollution, Chem2 says: Students realise that pollution is directly related to the excessive use of some resources.

In the case of KSU-3, knowledge of students’ areas of difficulty, the teachers mention that those difficulties are associated with the lack of genuine interest in the subject matter since students talked about caring for the environment and sustainability, but there is no real commitment to do so; they consider it a fad. KSU-3 also stands out when the teachers express the limitations connected to learning central topics; for example, Chem1 said that in their immediate reality, everyone lives in varied conditions that make them see the problem in different degrees of complexity and damage.

Concerning assessment, the most frequent component is methods of assessment (KASc-2). The assessment instruments that teachers used most frequently were case studies, questionnaires, preparation of posters, portfolios, and student participation. For example, Chem2 said: I usually use some questionnaires to evaluate what they normally do and questionnaires that reflect the consumption of luxury items and some necessities.

Finally, the most frequent component related to knowledge of topic-specific representations is KIS-2. This is associated with the use of representations in science, specifically in the teaching of sustainability. For example, Chem5 mentioned that the procedures focus on elaborating models, experimental work with low environmental impact, and demonstrations (videos or face-to-face) when this resource is ideal for safety and cost (Chem5). Other strategies mentioned for these components (KIS-1 (subject-specific strategies) and KIS-2) were using real-life examples, analysis of articles, videos, case studies, analogies, exhibitions, elaboration of infographics, digital resources, and ecological footprint calculations.

Biology teachers

Bi3

In this case (see Fig. 5), we start with approaches towards teaching sustainability (ATSc) and their relation to ecological balance, natural resources, biodiversity, ecosystems, and the environment, where the most used approach by this teacher is the (ATSc-Cc) focus of conceptual change. One example is for each to change their lifestyle to help mitigate this problem (ATSc-Cc). Another approach is activity-driven (ATSc-Ad), which also stands out as observed in the following sentence: Because it must be instilled in them, first, the knowledge of how delicate the ecological balance is and second, that they identify the attitudes and actions to take to maintain the environmental balance of the environment within sustainability (ATSc-Ad). Regarding the knowledge of the curriculum (KScC), the subcomponent related to students’ goals in learning the subject (KScC-1) predominates in the teacher’s discourse, as can be recognised in the following paragraphs: Ecological balance is the relationship between the human, natural, and built subsystem, where humans sustainably develop their activities and projects with environmental resources; it is to reduce and minimise ecological impacts that modify environments and produce imbalances on the planet, such as natural phenomena that affect the quality of the domain (KScC-1). We can conclude that in most subjects taught by Bi3, the teacher’s ideas prevail over content and teaching goals.

Fig. 5
figure 5

Sankey diagram of the teaching profile of Bi3. Notice that teachers’ ideas of students’ goals in learning sustainability (KSc-1) are the knowledge most used by this teacher

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Regarding the knowledge of the students’ scientific understanding (KSU), Bi3 perceives the following: Because it must be instilled in them, first, the knowledge of how delicate the ecological balance is and second, that they identify the attitudes and actions to take to maintain the ecological balance of the environment within sustainability (KSU-4). In this case, KSU-4 prevails, which means that the teacher imposes their beliefs related to the knowledge that students may or may not possess or the knowledge the teacher thinks students should learn. Regarding the knowledge of assessment (KASc), knowledge of dimensions of learning assessment (KASc-1) is most frequently represented, as seen in the following answer: The evaluation consists of the development of an investigation within a protected natural area (KASc-2, methods of assessment), and different criteria are requested that cover several topics, including those of sustainability (KASc-1) and ecological balance, as well as ecosystem services (KASc-1). These results show the type of assessment strategy that Bi3 uses and how students can apply the knowledge of those important concepts.

Regarding the knowledge of instructional strategies (KIS), this teacher mainly displays the KIS-2 subcomponent (topic-specific representations), as illustrated by the following sentence: First, we review videos (KIS-2) and some articles (KIS-2), and then I ask them to look for the ecological niche of three different animals. By doing this, they realise how important a species is within the ecological balance of an ecosystem (KIS-2). [A] Research (KIS-1, subject-specific strategies) and presentation (KIS-2) related to a protected natural area of Mexico City are carried out, highlighting the type of ecosystem to which it belongs, what the ecosystem services provided by this (KASc-1, dimensions of learning assessment), how it has been affected by human activity and which are the possible proposals to recover the area sustainably. In the last examples by Bi3, we can see that he uses strategies and specific representations using contextualised research methodologies and exposition of solutions.

In Fig. 5, the dominant knowledge in this teacher is students’ goals in learning the subject (KScC-1) and teachers’ beliefs about what students know (KSU-4), that is, his knowledge and beliefs about curriculum define and permeate the rest of the teacher profile. This approach mainly impacts the type of strategies used, highlighting representations of the discipline. It is worth noting that although the teaching profiles of Chem3 and Bi3 are similar, the chemistry teacher’s profile is more robust regarding the types of knowledge that comprise it.

In Fig. 6, the relations among the most frequent components that are present in the p-PCK of Bi3 are shown. We noticed complex relations, for example, activity-driven approach/conceptual change approach/ teachers’ beliefs about what students know (ATSc-Ad/ATSc-Cc/KSU-4). Still, at the same time, ATSc-Ad is related to KSU-4 through assessment methods (KASc-2), topic-specific representations (KIS-2), and students’ goals in learning the subject (KScC-1). Therefore, two components can be related through one or more different subcomponents. We can see that Bi3 combined two approaches, hands-on and conceptual change, with what they believe students should learn. At the same time, relations between assessment and teaching strategies on sustainability appear. Likewise, the activity-driven (ATSc-Ad) is related to conceptual change (ATSc-CC), and hence, there is a bifurcation where the assessment strategies (KASc-2) and the interests of the students (KScC-1) are considered, and these two, simultaneously, are related to beliefs about what students know (KSU-4). We consider that these relations are much more complex than those present in Chem3, and this occurs because sustainability is a more frequent content in biology curricula and programmes, so biology teachers have developed more complex thinking regarding teaching sustainability, and the strategies they use are more diverse than those used by chemistry teachers.

Fig. 6
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Sankey diagrams of Bi3 where interrelations among subcomponents (co-occurrences) are shown. Notice that this is more complex than that of Chem3

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Variations in PCK components used by all biology teachers: c-PCK

The comparative analysis of answers given by biology teachers is shown in Fig. 7 as a Sankey diagram, where all biology teaching profiles show the use of similar components but with different frequencies. The components that allow us to build their teaching profile are the conceptual change approach (ATSc-Cc), students’ goals in learning the subject (KScC-1), beliefs about what students know (KSU-4), assessment methods (KASc-2), and topic-specific representations (KIS-2). It is also possible to observe that the most robust teaching profiles are Bi1 and Bi3, with Bi3 having more variety of knowledge than Bi1. Another relevant aspect is that approaches used by teachers have little influence over other knowledge, but KScC (science curriculum), KSU (students’ understandings), and KIS (instructional strategies) do. This indicates that the teacher’s knowledge about the curriculum, the students, and the instructional strategies, predominantly in the case of Bi1, define teachers’ profiles. According to the analysis, the teacher with the least robust profile is Bi2, and although there are at least eleven components used, the flow lines are thinner than for other teachers.

Fig. 7
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Sankey diagram of the variations in PCK components used by biology teachers, which we call collective PCK for biology teachers

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In approaches to teaching science, conceptual change (ATSc-Cc) and activity-driven (ATSc-Ad) are the most frequent subcomponents. In the first one, we find ideas such as encouraging students to be critical, show an active role, reflect, pose personal questions, give their points of view, take action to solve local environmental problems, act with creativity and motivation, develop projects in their immediate environment, and promote changes in sustainable habits. For example, students could reflect on the impact of their actions on daily life (Bi5).

Following analysis, activity-driven approaches (ATSc-Ad) show the same occurrence as conceptual change (ATSc-Cc). For example, teachers mentioned that students participate in practical activities used for verification or discovery. The following is an example of a paragraph given by one of the teachers: [students can] calculate their ecological footprint, see their impact, and learn about actions to reduce their ecological footprint (Bi1).

Regarding knowledge of the scientific curriculum, KScC-1 has the highest occurrence among the components and is related to teachers’ knowledge of the goals and objectives of the course. Some examples are that I want students to understand the importance of contributing to sustainable development for their current and future lives, that they feel part of this process, and that they are not alien to sustainable development (Bi1); I consider it to be one of the most relevant topics in my course, I dedicate more time to the planning of activities, and I do not skimp on class time for reflection (Bi5).

Knowledge of student understanding (KSU) is one of the most frequent components; however, beliefs about what students know and do not know or what they should learn (KSU-4) have many occurrences. Some examples of what teachers said are as follows: At least in my teaching practice, with each concept, I intend to configure a ‘new vision’ regarding individual commitment to the impact on the environment and how this is crucial for a less harsh future for future generations (Bi5). Other ideas mentioned in this component are (i) that students integrate into their lives the respect and commitment to care for nature; (ii) that they recognise the importance of sustainability and that biodiversity is essential to achieve sustainable development and improve the quality of life; (iii) that they are aware of the consequences of their actions; (iv) that Mexico’s natural ecosystems provide many services crucial to the country’s development, including rainwater harvesting, pollination, and climate regulation; and (v) that natural resources and ecosystems must be managed sustainably in part to meet the needs of populations.

Regarding assessment, the most mentioned component was knowledge of evaluation strategies and methods (KASc-2), where the teachers mentioned that students must be able to pose and solve case studies and carry out group discussions, brainstorming, project development, directed investigations, concept mapping, presentations, tests, and problem-solving. For example, teacher Bi1 said, Their ability to explain certain phenomena considering their articulation of concepts in their arguments, clarity, and to offer samples.

The last knowledge, KIS-2, of specific strategies for a topic helped to identify that teachers used many teaching strategies, such as representations, case studies, videos, and group discussions (Bi4). Other strategies mentioned are analogies, examples, classification games, project development, use of real-life examples, analysis of images and articles, investigations, analysis of successful cases of sustainable development, drawing of diagrams, calculation of the ecological footprint, and exhibitions.

Discussion

In this research, the personal teaching profiles of the ten chemistry and biology teachers about teaching the subject of sustainability were obtained. The collective teaching profiles, which, depending on the subject, reflect teachers’ thinking, were also obtained. In both subjects in our study, chemistry and biology, the most robust knowledge that permeates other types of knowledge is the teachers’ knowledge of and beliefs about the objectives and goals of the course, as well as their knowledge about the curriculum (KScC). One way or another, this knowledge defines the rest of the knowledge in all cases, regardless of the discipline. This is noteworthy since teachers are more influenced by their personal beliefs about sustainability because, in Spanish, sustainable development and sustainability are not synonymous; the first concept is more related to the economy, and the second is environmental preservation.

In this work, it was possible to characterise the PCK of high school teachers from different areas of knowledge (chemistry and biology) related to sustainability in Mexico City. The most frequent components and subcomponents of PCK identified are teachers’ ideas of students’ goals to learn that subject (KScC-1), knowledge of requirements for learning (KSU-4), knowledge of subject-specific strategies (KIS2), conceptual change approach (ATSc-Cc), and knowledge of methods for assessment (KASc-2) for both groups of teachers studied. In addition, this research made it possible to link the teaching of sustainability to the curricula of chemistry and biology through the central topics analysed, and such knowledge can help to integrate sustainability into high school curriculums in the future. However, even when teachers are clear about the importance of the subject, this importance needs to be noticed because the approach proposed by education for sustainability does not permeate their teaching practices.

For the chemistry teachers, it was possible to characterise the subcomponents of the PCK they used when they teach sustainability, which means a shift from the conceptual change approach to the teaching of science (ATSc-Cc), as teachers accept sustainability as part of their scientific culture. This allowed us to obtain a teaching context in which students develop changes in habits and attitudes, enabling chemistry teachers to give perspectives on environmental problems, reflect on possible solutions, and develop critical students for decision-making. The two most predominant components of PCK were the subcomponent of curricular knowledge, which is what teachers think are students’ goals in learning the subject (KScC-1), and the knowledge of students understanding, specifically teachers’ beliefs about what students know or should learn (KSU-4). This leads to the conclusion that in most of the topics taught by teachers, their ideas about what students should learn, according to their living context, prevail over the contents and teaching goals that are relevant to sustainability and sustainable development, caring for the environment, and natural and clean energy sources.

The most frequent component is teaching beliefs about what students should know and how to learn it. However, they also emphasise aspects that are important for what they consider essential: that students recognise the importance of sustainability, feel part of the world, the environment, and biodiversity, be aware of the consequences of their actions and attitudes, and that they incorporate them into their lives, as well as respect and commitment to the care of nature, etc. Regarding the evaluation of knowledge, there is a predominant subcomponent that refers to the type of strategies that are used for that purpose (KIS-2), some of which are coherent explanation, argumentation, ability to offer examples, management of cases with worksheets, group discussions, written research, presentation, and review of the project. Likewise, researchers usually represent a subcomponent that refers to specific strategies and representations used by teachers, such as case studies, project development, use of real-life examples, analysis of images, articles and videos, research, and exhibitions, in addition to the ability to offer examples, case studies, group discussions, written research, project presentations, and tests.

In answering the research questions, we consider that the teaching profiles show various approaches and teaching strategies and demonstrate how teachers understand sustainability. Another important aspect is that the profiles of chemistry teachers differ from those of biology teachers. There is greater complexity in biology teachers because sustainability is more frequently found in the biology curriculum. It is commonly associated with ecology, biodiversity, and natural resources, a core part of the subject. In contrast, in chemistry, sustainability is related to environmental pollution and climate change, considered applications of knowledge and not necessarily a core part of the discipline. We also found that the ways of teaching are closely related to what is reported in the literature. That is, teachers use various types of approaches and strategies that promote critical thinking. A change in habits and attitudes that can impact students’ daily lives and decision-making is sought.

We found a few studies when we looked for related references. However, we cannot use them for this research because one of these research studies (Singer-Brodowski 2016) is focused on how students develop their conceptions about sustainability, and the research remarks that this is a fundamental idea of PCK. However, we consider this not a document where PCK is studied; in fact, the authors say so. Another reference that could be relevant was Birdsall (2015), where the author analyses how two elementary teachers translate their knowledge about sustainability into their classroom practices. Besides, the PCK model differs from the one used in this paper.

Considering that there are very few studies about teachers’ PCK and sustainability and, to the best of our knowledge, there is nothing about high school teachers (Birdsall 2015; Nousheen et al. 2022), we think that this research provides a view of chemistry and biology teachers’ concepts of sustainability and of how they make this idea comprehensible for students. Another critical issue is that sustainability is taught and interpreted as a fragmented subject that will depend on what content is taught and from what discipline (chemistry or biology). Consequently, teachers of either subject will have different PCK.

Conclusions

This study explored teaching practices related to sustainability in specific subjects with different goals, contexts, institutional realities, pedagogical approaches, and the most used representations of sustainability and its central topics. This reflection will contribute to creating new pedagogical proposals for teaching sustainability.

Finally, it is necessary to formulate new and better teaching models that focus not only on what teachers believe is important to teach but also on fostering reflection on environmental and social problems and the impact of the changes inflicted upon the planet. The objective should be to raise concern among citizens for the earth and the consequences of a lack of sustainability awareness. To this end, we recommend that teacher training courses be designed with approaches that promote the teaching of sustainability in a way that is cross-curricular, simple, and practical.