Introduction

Global climatic patterns and anthropogenic activities promote eutrophication in freshwater bodies, leading to rapid multiplication of photosynthetic cyanobacteria termed cyanobacterial harmful algal blooms (cHABs). Current trends in atmospheric and water temperatures are expected to increase the incidence and expand the biogeography of toxic cyanobacteria worldwide [1, 2]. In addition, cHAB formation in freshwater environments is stimulated by various abiotic factors such as light intensity, nutrient levels (nitrogen and phosphorus), pH, temperature, pollutants, and short-wavelength radiations [3,4,5]. Globally, cHABs are increasing in frequency, duration, and severity, posing significant health hazards to wildlife, recreation, and public health [1, 6]. Several major freshwater lakes have been impacted by cHABs including Lake Erie, USA; Lake Winnipeg, Canada; Lake Victoria Kenya; and Lake Taihu, China [7]. The dominant and toxin-producing cyanobacteria in lakes are species belonging to the genus Microcystis [8].

Microcystins (MCs) are considered the most abundant and toxic cyanobacterial toxins (cyanotoxins). MC concentrations in surface waters are variable, though elevated levels can impair water quality used for recreation and consumption. MC-LR, the most potent form of MC, is regarded as one potential carcinogen to humans [9]. MC-LR toxicity depends upon active transport into hepatic tissue via organic anion transporting polypeptides (OATPs). The World Health Organization (WHO) has adopted provisional guidelines for MC-LR in drinking water (1.0 µg/L) and recreational water (12 µg/L) [10, 11]. Nodularins (NODs) constitute a similar group of cyanotoxins, where NOD-R is the most frequently detected variant [12, 13]. NODs predominately exist in brackish waters; however, they can appear in conjunction with MCs in freshwater. The presence of the extremely toxic ADDA (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid) moiety in their cyclic structures makes them an excellent target for quantitation [12]. Since toxicological data for NODs remain sparse, the level of exposure and toxicity are based on MCs (0.04 µg/kg body weight/d) [14].

Other notable cyanotoxins produced by cyanobacteria include hepatotoxic cylindrospermopsins, neurotoxic anatoxins and saxitoxins, and the non-proteinogenic amino acid, β-N-methylamino-L-alanine (BMAA) [15]. A comprehensive database called “CyanoMetDB” offers detailed information on their biosynthesis, identification, occurrence, and toxicological risks [16].

Direct or indirect exposure to cyanotoxins inflicts harm on aquatic organisms, wild and domestic animals, plants, and humans. With respect to animal and human intoxications, direct exposure typically occurs from consuming toxin-producing cyanobacterial cells or ingesting contaminated drinking water harboring cyanotoxins [17].

Epidemiological studies in China and Serbia established a potential association between MC-contaminated drinking water and primary liver cancer [18, 19]. Two other studies linked elevated levels of alanine and aspartate transaminase in sera of fishermen and children to liver damage, perhaps from chronic exposure to contaminated drinking water and aquatic foods [20, 21]. More recently, a case–control study in China discovered an elevated risk of chronic kidney disease among cases jointly exposed to high levels of MC and cadmium, suggesting the likelihood of synergistic effect of environmental pollutants in drinking water [22].

Based on a literature review on cyanotoxin poisonings, recreational activities contribute to nearly 50% of all human intoxications worldwide [23]. Recreational exposures to cyanobacteria and their associated cyanotoxins (i.e., MCs) include contact with bloom-infested water, inhalation of aerosolized sprays, and accidental ingestion of contaminated water. Past surveys of participants engaging in water recreation reported a wide range of health effects including allergic reactions, headache, fever, gastroenteritis, hay fever-like symptoms, mouth sores, and pyritic skin rashes [24,25,26]. On the other hand, indirect exposure can occur when organisms of higher trophic levels consume contaminated animal or plant tissue. This is particularly concerning as concentrated toxin in tissue can bioaccumulate in the food chain and subsequently affect human health [23].

This mini-review seeks to enhance our current understanding of the toxicological and health risks associated MC and NOD toxins derived from freshwater cHABs.

Cyanotoxins

Cyanotoxins are potent secondary metabolites produced by cyanobacteria, with over 2000 identified to date [16]. Structurally, cyanotoxins are categorized as cyclic peptides (microcystins and nodularins), alkaloids (anatoxins, cylindrospermopsins, and lyngbyatoxins), and the non-proteinogenic amino acid, β-N-methylamino-L-alanine (BMAA) [27]. From a toxicological perspective, cyanotoxins are grouped by the organs they affect in living organisms, which include hepatotoxins, neurotoxins, dermatotoxins, irritant toxins, and cytotoxins [28]. These cyanotoxins vary in structure, mechanism, and toxicity (Table 1). For the purposes of this review, special attention is paid to the hepatotoxic microcystins and nodularins.

Table 1 Structural and functional characteristics of freshwater cyanotoxins and their toxic effects
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Microcystin toxicity

Microcystins (MCs) are monocyclic heptapeptides (cyclo-(D-Ala-L-X-D-MeAsp-L-Z-Adda-D-Glu-Mdha)) produced by cyanobacteria in freshwater, estuarine, and marine environments [43]. Various genera synthesize MCs in eutrophic waters including Aphanizomenon, Dolichospermum (formerly planktic Anabaena), Microcystis, Nostoc, Oscillatoria, and Planktothrix [44]. The Adda moiety (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid) is critical to hepatotoxicity induced by MCs. More than 300 variants have been identified, with many others yet to be explored. These variants mainly differ in two variable amino acid positions (2 and 4), although some exhibit modifications or substitutions at other positions [45]. Most of these occurrences involve replacement of a methoxy group with an acetyloxy or hydroxy group at C-9, resulting in 9-O-acetylDMAdda (ADMadda) and 9-O-desmethylAdda (DMAdda), respectively. Microcystin-LR (MC-LR), microcystin-RR (MC-RR), and microcystin-YR (MC-YR) are highly potent and well-studied toxins in freshwater bodies [39]. Of these three variants, MC-LR is arguably the most ubiquitous, analyzed, and toxic in surface waters. The WHO’s provisional guidance value of MC-LR in drinking water is 1 µg/mL, equivalent to a tolerable daily intake (TDI) of 0.04 µg/kg body mass [10]. MC-LR is displayed in Fig. 1, where leucine (L) and arginine (R) occupy C-2 and C-4 in the heterocyclic ring, respectively.

Fig. 1
figure 1

Chemical structure of the cyclic heptapeptide microcystin leucine-arginine (MC-LR)

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The primary route of MC exposure is oral consumption, and organic anion transporting peptides (OATPs) mediate its uptake into hepatocytes [46]. OATPs are differentially expressed in several essential organs including the brain, kidney, small and large intestines, and stomach [46, 47]. These membrane-bound transporters facilitate the uptake of endogenous compounds such as bile salts, hormones, toxins, drugs, and xenobiotics [48]. Differential expression of hepatic OATP 1B1 and 1B3 isoforms facilitates cellular uptake of MCs, which may explain varying degrees in uptake and toxicity of MC variants [49].

MCs inhibit protein phosphatases (PP) 1 and 2A in tissue via covalent binding to serine and threonine amino acids. Hepatotoxic MCs exert a stronger attraction towards PP1 and PP2A compared to PP2B [50]. A two-step mechanism has been proposed for interaction between PP1 and PP2B and MC-LR: (1) rapid binding and inactivation of PP-1c and PP-2Ac catalytic subunits and (2) formation of adducts through prolonged covalent interactions [51]. Since PP1 and PP2 regulate protein dephosphorylation, their inhibition can induce hyperphosphorylation of intracellular proteins, culminating in hepatocyte disintegration, internal hemorrhage, and hepatic necrosis or apoptosis [49]. PP1 and PP2 inactivation can also lead to cytoskeleton disruption, DNA damage, oxidative stress, and mitogen-activated protein kinase (MAPK) deregulation [52]. MC toxicity extends beyond the liver and has been implicated in organs such as the kidneys, lungs, and heart [40]. Experimental studies on MC toxicity in these organs are detailed in Table 2.

Table 2 Toxicological studies on MC-LR-induced toxicity in renal, respiratory, and cardiovascular systems
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Microcystin epidemiology

Sporadic epidemiological investigations on MC exposure have been conducted across various continents including Asia, North America, and South America (Table 3).

Table 3 Epidemiological investigations of freshwater microcystin and human health effects
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Hemodialysis

Arguably the most renown outbreak of MC poisoning occurred in a hemodialysis center in Caruaru, Brazil, where 101 case patients received MC-contaminated dialysate, 50 whom had acute liver failure and succumbed after exposure. Affected patients who died were older than survived patients (median age, 47 vs. 35 years, p < 0.001). Toxicological analysis of MCs in liver tissue of 17 case patients revealed MC concentrations ranging from 0.03 to 0.60 g/kg [60].

Drinking water

Additionally, multiple studies in China previously associated MC exposure with primary liver cancer [18], colorectal cancer [61], and liver damage [20, 21]. A three-trial survey in Haimen city correlated MCs in drinking water sources to primary liver cancer incidence. A similar survey in Fusui, Guangxi Province, reported a high occurrence of MC contamination in water samples collected from ponds/ditches and rivers [18]. In the case of colorectal cancer, the relative risk (RR) was significantly higher among those who consumed pond (RR = 7.70) and river water (RR = 7.94). MC concentrations in these drinking water sources also positively correlated with colorectal cancer incidence (ρ = 0.881, p < 0.01). Overall, consistent findings were observed between studies, with higher exposures of MCs in drinking water sourced from ponds and rivers.

Another study examined chronic exposure to MCs in a population of fishermen who lived on fishing ships and consumed drinking water from Lake Chaohu. The presence of MCs in fishermen sera (average 0.39 ng/mL), accompanied by elevated serum enzymes (alanine aminotransferase (ALT), alkaline phosphatase (ALP), and aspartate aminotransferase (AST), indicated the possibility of liver damage [20]. Similarly, a cross-sectional study in the Three Gorges Region investigated chronic exposure to MCs through drinking water among high and low exposed children in relation to liver damage. High exposed children had elevated levels of ALP and AST compared to low exposed children when analyses excluded subjects who used hepatotoxic medications or were positive for hepatitis B infection [21].

A more recent case–control study attempted to understand the combined effect of MC and cadmium (Cd) in drinking water with chronic kidney disease (CKD). For combined high exposures of MC and Cd, the odds of developing CKD 2.58 times greater than the reference group (low MC and low Cd) [22]. Findings of the study demonstrated that the combined effect between environmental pollutants in drinking water can significantly increase the risk of chronic disease.

Recreation

Recreational exposure to MCs during water activities has also received attention in epidemiology. In a small lake enduring an algal bloom, exposed participants reported more respiratory symptoms (i.e., cough and sore) 7 days before partaking in recreational activities than 7–10 days after partaking in recreational activities. Unexposed participants complained of dermatologic complaints immediately prior to engaging in recreational activities compared to after engaging in recreational activities. However, MC levels in aerosol (< 0.1 ng/m3), water (2–5 µg/L), and blood samples of participants (< 0.147 µg/L) were relatively low. The results indicated that low-level exposure to MC aerosols might occur during recreational activities [25]. In a different field study, exposed participants reported more upper respiratory symptoms 7 days before partaking in water recreational activities [26]. MC concentrations in two ‘Bloom Lakes’ varied from as low as < 10 µg/L to as high as > 500 µg/L, while nasal swabs (< 0.1–5 ng/m3) and blood samples (< 1.0 µg/L) contained low levels of toxin. These findings coincided with the earlier study, supporting inhalation as one potential exposure route for MCs during  recreational activities.

Nodularin toxicity and epidemiology

Nodularins (NODs) are cyclic pentapeptides produced by planktonic, filamentous Nodularia spumigena and benthic Nodularia sphaerocarpa in brackish waters, and less commonly, in freshwater [42]. NOD-producing blooms have occurred in many parts of the world including the Baltic Sea, Northern Europe, Australia, and the USA [40]. Like those in MCs, NODs contain the amino acid residues D-erythro-β-methylaspartic acid, L-arginine, Adda, D-glutamic acid, and N-methyldehydrobutyrine [42]. Because of their shared chemical properties, NODs and MCs are usually co-studied by the same analytical method. Currently, ten variants of NODs are recognized, with NOD-R (arginine) being the most common (Fig. 2) [63]. The presence of arginine (R) as opposed to valine (V) at C-2 differentiates NOD-R from mutoporin, a hepatotoxin derived from the marine sponge Theonella swinhoei [64].

Fig. 2
figure 2

Chemical structure of the cyclic pentapeptide nodularin-R

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Few studies on NOD toxicokinetics are currently available. However, cellular uptake and biological activity of NOD are similar to MC. Uptake transporters (OATPs), Oatp1d1 and Oatp2b1, are mainly expressed in the liver of zebrafish. A study demonstrated that Oatp1d1 (drOatp1d1) mediates uptake of NOD in the permanent zebrafish cell line ZFL. As for PP inhibition, NOD non-covalently binds to PP2A, which can ultimately enhance the production of tumor necrosis factor α (TNF-α), as evidenced by a study in primary rat hepatocytes [65, 66]. Additionally, exposure to 2.5 nM NOD for 24 h was shown to induce TNF-α in human primary liver. Consequently, molecular induction of TNF-α stimulates the expression of interleukin-8 (IL-8) and activation of MAPK, thereby contributing to the toxicity and tumor-promoting activities of NOD in hepatocytes [67]. Because of the lack of exposure data, the International Agency for Research on Cancer (IARC) classifies NOD as non-carcinogenic to humans [13].

NODs are regarded as important hepatotoxins to human health. However, no epidemiological studies have explicitly investigated the relationship between NOD exposure and health outcomes at the population level. Health effects of NODs are generally inferred from limited epidemiological studies on MCs (Table 3). NOD exposure may therefore cause a variety of signs and symptoms including allergic reactions, skin rashes, gastrointestinal illness, nausea, liver damage, and bleeding [68]. Future epidemiological studies could start by simultaneously assessing the co-exposure of MC and NOD in freshwater  environments and examining their relationship with liver disease.

Conclusion

The findings of this review emphasize the health impacts of microcystin (MC) and nodularin (NOD) toxins derived from freshwater cyanobacterial harmful algal blooms (cHABs). It is imperative to gain a deeper understanding of the pathways through which these cyanotoxins are encountered in the aquatic environment, as this knowledge is essential for mitigating toxic exposures. Moreover, it can pave the way for the implementation of regulatory guidelines to ensure appropriate levels of exposures and toxicities to humans. Given recent epidemiological findings, the synergistic effect of MC and NOD with other environmental pollutants on chronic health conditions merits further exploration. In short, there is a pressing need to conduct toxicological experiments, exposure assessments, and epidemiological investigations to appreciate the human health impacts of chronic MC and NOD exposures.