Historical observations and contradictions
The unique landscape, changing colors, fragrance, and salt crust beaches of GSL are certainly striking to the observer, and it is not surprising that the solitude of the place would conjure notions of stillness and lifelessness. Stansbury wrote, “Save the dashing of the waves against the shore absolutely nothing is heard. Not the jumping of a fish, the chirp of an insect, nor any of the least thing betokening life, unless it be that very rarely a solitary gull is disturbed in his midnight rumination and flys screaming away. All is stillness and solitude profound” (Fig. 4) (Stansbury 1855). The Captain was so convinced that the water was sterile and antiseptic (without microorganisms), he had his men experiment with using the GSL brine for salting meat for storage on the return trip home. The salt did indeed prevent spoilage, but gave a bad taste according to the men, and they referred to it as “salt junk.” The meat likely grew harmless microorganisms from the salty brine, as is the case with salted fish from which our first laboratory investigations of halophilic microbiology emerged (Browne 1922; Larsen 1986). This, along with the salt, would have augmented the flavor, hence the revulsion from the sailors.
Underscoring the idea of lifeless waters, Mormon pioneers continued to make parallels between GSL and the Dead Sea, an irresistible confirmation of Utah as a spiritual place (The Church of Jesus Christ of Latter-Day Saints 1997). Parley Pratt, exploring the valley for farming possibilities, just 2 days before Brigham Young’s wagon arrived, wrote in his journal that as they came closer to GSL, the soil had “a more sterile appearance.” Alfred Lambourne, an artist and writer who lived for a while on Gunnison Island in the middle of GSL, wrote: “At twilight a wild and thrilling spectacle….Dim and pale, the moon, the ghost of a dead world, lifted above the distant Wasatch peaks and stared at the acrid waters of a dead sea” (Lambourne 1909).
Though the water of GSL may at times appear dead, it is teaming with microbial life (Baxter et al. 2005) and actually, microorganisms enrich the waters of the Dead Sea as well (Oren 1999). Early human observers noticed smells and colors, but only later did they attribute this to the microbial communities. Frémont and his team described a “disagreeable smell in stirring up the mud” as they portaged a boat into the salty waters (Frémont 1845). Later, in 1877, John Muir had a more enjoyable take on the smell while swimming in GSL “…the cool, fragrant brine searches every fibre of your body…” (Muir 1877).
There are historical references to the color palette of GSL as well as the aroma. Frémont commented on the color, referencing the algae without realizing it, “The water continued to deepen as we advanced; the lake becoming almost transparently clear, an extremely beautiful bright green color…” (Frémont 1845). In fact, over time, many wrote about the seasonal changing colors of GSL, including Dale Morgan, “Visitors have called its waters bright emerald, grayish green and leaden gray; they have called them sapphire and turquoise and cobalt -- and they have all been right. Its color varies with the time of day, the state of the weather, the season of the year, the vantage point from which it is seen” (Morgan 1947). And we know now that the colors and smells do change over time, as the microbial communities shift in response to oscillations in temperature, lake level, and salinity gradients (Almeida-Dalmet et al. 2015; Baxter et al. 2005; Lindsay et al. 2017; Meuser et al. 2013; Rushforth and Felix 1982).
Historical microbiology studies of Great Salt Lake
The nineteenth century brought pioneers, fortune seekers, and religious people to the Salt Lake valley, but it also brought scientists. Microbiologists were curious about what could survive in the brine of GSL, and there were attempts to isolate and describe life here through the mid twentieth century, when much of this investigation ceases to appear in the literature. Some of the early work was archived in bulletins, collections, and theses, and much of it was lost to later microbiologists relying on published literature.
The first microbial isolations from GSL trace back to Alpheus Spring Packard Jr. (Fig. 5a), who was a member of the Ferdinand Vandeveer Hayden expedition, a geological and biological study of the western US territories in the 1870s (Hayden 1873). Packard, who later became a professor of zoology and geology at Brown University (Rhode Island), was an expert in moths who had more than 100 species attributed to him and was mainly a part of the team due to his entomological expertise (Cockerell 1920). While in Utah traveling by stagecoach, he collected algae samples and in 1871, sent them to botanist, William Gilson Farlow (Fig. 5b), to characterize (Spencer 1904). Later described in a journal, “Professor W.G. Farlow, of Harvard University, soaked out and examined the dried material, which he found to consist largely of grains of sand and remains of small animals, mixed with which were three species of Algae” (Crisp et al. 1880). Farlow isolated and identified these three species of algae in his laboratory: the cyanobacteria Polycystis packardii, Rhizoclonium sp., and Ulva marginata (Crisp et al. 1880; Harvard University 2018; Packard Jr. 1879). The latter two species likely occur only in freshwater seeps and marshes (Rushforth and Felix 1982). These algae samples were pressed and are currently stored in the Farlow Herbarium (Fig. 5c) (Harvard University 2018).
A little later, a geologist, Rothpletz, connected cyanobacteria (Gleothece and Gleocystis) to the carbonates in the lake (Rothpletz 1892), pointing to the role of the microbiota in carbonate formation. At the time, his work was suspect: “We have been unable to find [these species] in the part of the lake studied, and it might be said, too, that the connection between these and the ooliths has not been generally accepted, even by geologists” (Dainels 1917). We now have a deeper understanding of the role of cyanobacteria in carbonate formation [e.g., (Lindsay et al. 2017)], which may validate Rothpletz’s hypothesis.
Perhaps the very first on-site systematic study of microorganisms in GSL was in the 1890s by a remarkable woman, Josephine Tilden, the first female professor at University of Minnesota and an algae specialist (Horsfield 2016). She embarked on a science exploration of the west, sampling and isolating algae from extreme environments, including Yellowstone and Great Salt Lake (Fig. 6a) (Tilden 1898; Tilden 1910). Tilden reported five species of algae from the lake: Aphanothece utahensis Tilden, Polycystis packardii Farlow, Dichothrix utahensis Tilden, Enteromorpha tuhulosa (Kiitzing) Reinbold, and Chara contraria Braun. In addition, she stored samples in herbariums which are still available today (Fig. 6b) (Macroalgal Herbarium Portal 2018). In her description of the sampling site (Garfield Beach) for Polycystis packardii Farlow, Tilden included details that provide significant context, “Forming irregularly-shaped balls of masses of a firm gelatinous structure, showing various tints of pink, brown and green. In thick masses around edge of lake for a distance of forty feet out from shore and one to two feet in depth. Often washed ashore and left in beds on sand” (Tilden 1898). Several scientists who followed subsequently isolated Tilden’s Aphanothece species from GSL brine [e.g., Flowers 1934; Kirkpatrick 1934].
In the same decade as Tilden’s work, looking to farm oysters in GSL, biologist Henry Moore authored a study that also commented on the presence of diatoms at freshwater interfaces, which would work to feed the shellfish (Moore 1899). A Mormon religious leader and geologist, James Talmage, wrote about diatom colonies along the shore and in the water column (Talmage 1889; Talmage 1900). Lyman Daines, a graduate student and then professor at the University of Utah was able to culture GSL diatoms in the laboratory in 1910 (Daines 1910; Daniels 1917). All of these studies pointed at the question of whether the discovered diatoms washed in from freshwater or if they were truly hypersaline. The most complete early study of diatoms was done by Ruth Patrick, who surveyed these algae around the lake and in cored sediment samples, noting many species that were vestige paleo-remains, some that were freshwater in areas with springs or river input, and some that were hypersaline (Patrick 1936). As a young woman in science, Patrick at the time of this study was working as an unpaid “volunteer” for the Academy of Natural Sciences (ANS) (Academy of Natural Sciences 2018). Over the next decade, she became a paid employee and one of the world’s diatom experts, and the ANS limnology research center was named in her honor in the 1980s. Recent studies confirm the presence of Patrick’s hypersaline diatoms (Lindsay et al. 2017).
Green algae such as Dunaliella viridis, which are important for the survival of Artemia (Belovsky et al. 2011; Rushforth and Felix 1982) were observed in 1910 by Daines but called Chlamydomonas sp. at that time (Daines 1910; Daniels 1917), “The presence of plants [sic algae] is not so evident to the casual observer, although, at certain times of the year, clumps of greenish material, which must at least suggest a vegetable growth, are very plentiful.” Others isolated and described these chlorophytes in detail later on [e.g., Kirkpatrick 1934], and we now know their significance and enormous abundance in the overall GSL ecosystem (Belovsky et al. 2011). The brine shrimp thrive when the glycerol-packed Dunaliella is available (Fig. 3). Later we learned that at salt saturation, a carotenoid-containing orange species, Dunaliella salina, inhabits the hypersaline north arm (May 1978).
The first reference to prokaryotic life (outside of cyanobacteria) was from Daines in his thesis (Daines 1910): “The fact that putrefaction and decay are taking place in the water, especially near to shore, where organic material is abundant, shows conclusively that bacteria are present.” He performed colony counts from plating brine, from a south shore site, on salt agar and determined that the concentration was between 200 and 625 culturable cells per milliliter. Daines also isolated five species of bacteria/archaea, three of which were producing “abundant pigment” colored yellow, orange and violet. These carotenoid pigments are by now well-associated with photoprotection in halophilic archaea (Fig. 7) (Jones and Baxter 2017), but this may be the first notation of such. In 1924, Elfriede Frederick published a thesis describing bacteria from GSL (Frederick 1924). From brine samples, she cultivated and identified 12 strains, including the pink-colored “Serratia salinaria,” a halophilic archaea species found world-wide in salty places, and later renamed Halobacterium salinarum. The coloration of regions of GSL by halophilic archaea as seen in Fig. 1 (a and b) was rarely remarked upon by the scientists working on the lake before the causeway was installed. Tilden described specific areas of the lakeshore colored red (Tilden 1898), and others commented on the pigmented isolates from GSL (Daines 1910; Frederick 1924; Kirkpatrick 1934; Smith 1936), but pink water is not discussed. The periodic high salinity of the lake was likely not a stable enough condition in which the halophilic archaea could thrive, until the separation of the north arm which allowed the water to be at salt-saturation year-round.
Daines became a professor of bacteriology at the University of Utah, and was listed as a committee member for Winslow Smith, whose 1936 master’s thesis was an exploration of the idea of sterility in GSL (Smith 1936). He surveyed the halophilic microbiology literature of the time and hypothesized that GSL would have prokaryotic life in all areas, despite localized high salinity conditions. He began a collaboration with Claude Zobell, a marine microbiologist at the Scripps Institution of Oceanography in La Jolla, California. Zobell used a technique of submerging slides in water onsite for a period then staining and counting colonies that grew on the slides (Fig. 8a) (Zobell et al. 1937) This technique, for the first time, gave visual information about communities rather than isolated species. Smith argues for studying communities: “…pure culture studies and the usual culture methods present only fragmentary information about what certain bacteria do when freed of competition for life and placed in an ideal but unnatural environment” (Smith 1936). Smith manufactured a similar device and was astounded by the salt crystals that permeated the slide holder when he reeled in the apparatus (Fig. 8b). He reported that not a single sampling location resulted in sterile samples. It seemed as if GSL was teaming with microorganisms (Smith 1936; Zobell et al. 1937). However, Smith missed one important observation when he threw away plates that “were overgrown with mold” without realizing he may have disposed of the first evidence of fungi in GSL. We now know that fungi are part of the lake’s microbial community (Baxter and Zalar 2018).
Smith (Smith 1936) also observed the same colorful archaea described by Daines (Daines 1910) and Kirkpatrick (Kirkpatrick 1934), “The colonies that developed on these different media were highly bizarre. Violet, red, orange, tan, brown and yellow colonies were to be observed as well as white and colorless ones. Many of them were of picturesque morphology that defies description.” Smith, working with Zobell, focused on recording some environmental parameters for the first time, such as: dissolved oxygen, osmotic pressure, ion concentrations, wave action, and freshwater inputs. He went to great lengths to do careful sterility controls on all sampling and culturing materials such that he could confirm the presence of microorganisms in his work was indeed from GSL. Smith also was concerned with the sewage that was being dumped into GSL, and he designed experiments to test for the viability of pathogens in lake water, concerned with the fact that humans were recreating and soaking in the lake’s many resorts. Working with Zobell, Smith determined that enteric bacteria, like Escherichia coli, were killed by the hypersaline water (Zobell et al. 1937). Later this issue was contested with new membrane filter technique, and once again concerns were raised about raw sewage entering the lake as E.coli was shown to persist with this method (Fraser and Argall 1954).
The first reference to protozoa, in 1917, was by a zoologist at the University of Utah, Charles T. Vorhies, who was interested in the diets of brine shrimp and brine flies (Vorhies 1917). He described several protozoan species including an amoeba (likely Amoeba flowersi Jones), a ciliate (likely a Uroleptus species), and a species of Euglena (likely Euglena chamberlini Jones). Shortly after, Dean A. Pack, a Botany instructor, described Uroleptus packii and Prorodon utahensis (Pack 1919). Using dilution methods, Pack noted changes in the protozoa as he lowered salinity. The protozoa were re-discovered and better described by D.T. Jones in 1944 (Jones 1944), who named the Euglena for Ralph Chamberlin, a prolific biologist at the University of Utah and one of his mentors. Chamberlin was a taxonomist known not only for naming thousands of invertebrates, but also for building the University’s zoology department and medical school (Wintrobe 1982). Jones named the Amoeba he was studying after Seville Flowers, a colleague and botanist who had first found the salty species (Behle 1984; Jones 1944). Frederick Evans and collaborators later worked on the protozoa and found several new yet unidentified species and described eight south arm protozoan taxa (Evans 1958; Evans 1960; Evans and Thompson 1964; Flowers and Evans 1966), and Reddy described a GSL ciliate, Euplotes, in a later thesis (Reddy 1971). The only reference to north arm hypersaline protozoa was from Post in 1977 (Post 1977), who observed them in aquaria microcosms, but these were not his primary area of study and were not identified. Sadly, the field of protozoa of GSL has not progressed since this time.
In addition to his work on invertebrates, Ralph Chamberlin also mentored some GSL microbiology graduate student projects [e.g., Kirkpatrick 1934; Smith 1936]. In the 1930s, Chamberlin served as a mentor for Ruth Kirkpatrick (Fig. 9), who earned a Masters in the biology department at the University of Utah (Kirkpatrick 1934). Previously, as an undergraduate engineering major, in 1931 she was crowned “Queen of the Engineers” on the same night that her male colleagues were elected officers (The Salt Lake Tribune 1931). Her subsequent master’s thesis reviewed the GSL microbiology literature well and added some important in-depth observations (Kirkpatrick 1934). To the notion regarding freshwater microorganisms that may have been incidental in GSL sampling due to stream flows into the lake, “…the writer believes the only possible way to determine the species normally present in Great Salt Lake is by growing the algae in cultures over a period of time, long enough to eliminate any forms that may have come from extraneous sources.” Thus, this was her approach. Kirkpatrick gave credit to Daines (Daines 1910; Daniels 1917) for using good cultivation practices, and she used these techniques to observe cultures over prolonged periods under conditions varying salinity (Kirkpatrick 1934). She also was the first to collect microorganisms both spatially and temporally. She noted changing salinity of GSL over seasons and at different sites affecting what researchers could isolate at different times in various studies. In general, Kirkpatrick’s work was done at a time when the lake level was low, and the salinity measured as much as 26%, which was very high since this was before the construction of the causeway that created the isolated north arm. She collected from sediment and water at Black Rock beach on the southern shore of GSL, at the east shore near Syracuse, and off the west shore of Promontory Point. She also sampled exposed “algae covered rocks” (likely microbialites) at Antelope Island. She worked out culturing conditions for diatoms (Navicula), green algae (Dunaliella) (Fig. 9b), and various protozoa. Kirkpatrick described five “colonial forms” of cyanobacteria (Fig. 9c) linking these back to prior studies and suggesting that Aphanothece utahensis and Polycystis (Microcystis) packardii co-colonize in some areas, making their distinction difficult. She noted that most of the isolated strains did not thrive in the laboratory, an important observation from microbiologists at that time, which led future scientists studying microbial diversity to adopt non-cultivation methods of study decades later [e.g., Almeida-Dalmet et al. 2015; Lindsay et al. 2017; Meuser et al. 2013].
In the middle of the twentieth century, the center of studies of microbiology of GSL, the University of Utah, ceased to sponsor GSL projects with such frequency. Likely, key faculty retired. Also, the department of bacteriology collapsed into the biology department at some point, and in the early 1970s, a clinical microbiology department, focused on pathology, appeared at the university’s medical school (University of Utah 2018). No longer were aquatic biologists exploring GSL in Utah in a time when microbiological methodology was improving. In 1959–1961, the Union Pacific railroad causeway was built to bisect GSL (Cannon and Cannon 2002; Madison 1970). As discussed above, this separation changed the lake’s salinity gradient and thus, its microbiology. Studies after this time must contain the caveat of this change in the ecosystem.
Post-causeway researchers often separated their studies spatially with respect to the north arm, which immediately began its approach towards perpetual salt-saturation, and the south arm, for which salinity was decreased, resulting in an observed increase in microbial diversity (Evans and Thompson 1964; Rushforth and Felix 1982). There also was a concern that organic matter was accumulating in the hypersaline north arm due to a lack of bacterial degraders (Post 1977). The state of Utah welcomed the brine shrimp industry to the south arm as it ramped up in the 1970s (Wotipka 2014) and brought a new economic interest in the lake’s health. Doyle Stephens worked as a research hydrologist for the US Geological Survey, and he was a GSL researcher who approached the lake from several different angles. He had an undergraduate degree in biology, a master’s in entomology and his PhD in limnology (FRIENDS of Great Salt Lake 2018). He spent his career focusing on the sustainable management of the GSL ecosystem, including the economically important brine shrimp industry and the algae that feed these tiny crustaceans. Stephens, as an expert on the unique ecology of terminal lakes, was the right scientist to monitor the lake’s changing salinity with the construction of the railroad causeway (Stephens 1974; Stephens 1990; Stephens 1998; Stephens and Gillespie 1972; Stephens and Gillespie 1976). His work with the brine shrimp industry (Great Salt Lake Artemia Association 2018) for the Utah Division of Wildlife’s Great Salt Lake Ecosystem Program (State of Utah 2018c) set up a collaborative science-based management strategy that still exists today (Belovsky et al. 2011). This approach, focused on measuring the abundance of brine shrimp and their cysts as well as the distribution and types of phytoplankton, resulted in an ongoing, predictive model for the state which regulates the industry.
During this time of a changing lake and a growing industry, there were two important contributors to the microbiological literature on GSL. Sam Rushforth, a biology professor at Brigham Young University and then Utah Valley University, led many studies describing, in particular, the algae of the lake in the changing context of salinity (Felix and Rushforth 1977; Felix and Rushforth 1979; Felix and Rushforth 1980; Rushforth and Felix 1982; Rushforth and Merkley 1988). He and colleagues carefully traced the historical contributions (Rushforth and Felix 1982) and cataloged the algae discovered at that time (Rushforth and Merkley 1988). Further north at Utah State University in Logan, Fred Post was a microbiologist who entered GSL studies because there was so little work being done in the 1970s. He characterized several isolates of halophilic archaea and bacteria and published a few articles including important reviews (Cronin and Post 1977; Post and Stube 1988; Post 1975; Post 1977; Post 1980a; Post 1980b; Post 1981; Post et al. 1983). He investigated ecological phenomena such as gas domes in the salt crust (Post 1980b) and nitrogen utilization (Post and Stube 1988). Post remained focused on prokaryotes, but he recorded and published other observations that noted viruses (Post 1981), protozoa (Post et al. 1983), and a fungal species in brine-soaked wood (Cronin and Post 1977). With his location in the northern part of the state, he focused on the north arm more than any other researcher had up until that point in time (Post 1980a, 1981), in part because the hypersalinity there since the construction of the causeway was still a relatively new condition. Post retired before molecular techniques like gene and genome sequencing were common, and before the Domain of archaea was understood and readily applied to taxonomy (Woese and Fox 1977), but he laid the groundwork for studying halophilic archaea from GSL.
Recent microbiology studies of Great Salt Lake
The historical efforts to build an understanding of GSL microbial diversity seemed to lose momentum during the molecular revolution in the 1980s that would color modern studies. This author began working in Utah in 1998, and recent microbiology studies on GSL were scant or non-existent. I met with Fred Post, then retired, with my notion of collecting GSL halophilic archaea to use as DNA repair models (Baxter et al. 2007; Jones and Baxter 2016, 2017). Post sent me to the lake to find my model microorganisms as his were destroyed, after he retired, in a freezer-cleaning incident, which he reported with much sadness. With Post’s maps in hand, I began working on GSL microbiology with molecular training but little to no skill in microbial ecology. The dearth of work brought other Utah scientists to the table from Brigham Young University (Shen et al. 2012; Tazi et al. 2014), Weber State University (Shen et al. 2012), and Utah State University (Parnell et al. 2009, 2010, 2011; Weimer et al. 2009). We were building momentum, but it was clear that there was much work to do to understand the microbial foundation of this iconic ecosystem. Intentionally, I reached out to other researchers around the world who studied halophilic microorganisms elsewhere (Baxter et al. 2005; Baxter and Tate-Wright 2018). Ten years ago, we started Great Salt Lake Institute at Westminster College (Westminster College 2018), which carried the mission of facilitating large research projects on the lake, including helping scientists with access to sampling sites and logistical aid (Fig. 10). As a result, we have engaged researchers in our state with scientists bringing new ideas, insights and techniques to GSL. Whether facilitated by our institute or driven independently by current scientific questions, GSL microbiology is now an active area of research. A current review of the known GSL genera of bacteria, archaea, and eukaryotic microbial life is in press (Baxter and Zalar 2018).
Cultivation studies of GSL microorganisms are still relevant, as one can isolate lab strains to explore physiology and biochemistry (Almeida-Dalmet et al. 2018; Baxter et al. 2007; Baxter and Zalar 2018; D'Adamo et al. 2014; Pugin et al. 2012). Researchers have cultivated a number of GSL strains of bacteria and archaea, isolating in laboratory conditions in salty media, to study their physiology and genetics. There are currently 14 GSL strains, one archaeal 13 bacterial, stored and maintained in the following culture banks: the American Type Culture Collection (ATCC, USA); the Biological Resource Center (NBRC, Japan), the Leibniz Institute DSMZ (DSM, Germany), and the All-Russian Collection of Microorganisms (VKM, Russia) [Reviewed in Baxter and Zalar 2018].
Current researchers have elucidated much about the microbial communities of the lake from molecular work. Studies that utilize techniques that assess the DNA of an environment, though the SSU rRNA genes or from metagenomes, give a more complete depiction of the community members since these methods detect the species that are not culturable. From such studies, we know that the microbial communities in GSL are composed predominantly of halophilic archaea and bacteria (Almeida-Dalmet et al. 2015; Baxter et al. 2005; Meuser et al. 2013; Parnell et al. 2011; Tazi et al. 2014; Weimer et al. 2009). We now understand that assemblages of microorganisms must be dynamic, responding to the changes in salinity and temperature that accompany the seasons GSL experiences (Almeida-Dalmet et al. 2018). Salinity gradients in the less saline south arm of the lake have been shown to influence the composition of planktonic species (Boyd et al. 2014; Lindsay et al. 2017; Meuser et al. 2013). However, the hypersaline north arm microbial communities are more stable over time and not as impacted by changes in temperature and salinity (Almeida-Dalmet et al. 2015). These stable groups of microorganisms also have a lower phylogenetic diversity relative to communities in the south arm (Parnell et al. 2009, 2010, 2011). Though we still do not know much about the specific roles of microorganisms in GSL, we are concerned about their role in the bioaccumulation and modification of heavy metal pollutants (Boyd et al. 2014; Wurtsbaugh et al. 2011), a particularly difficult problem to solve in a terminal lake.
Modern studies have broadened our knowledge of life forms in GSL and how they work together. We now understand that viruses are plentiful (Baxter et al. 2011; Motlagh et al. 2017; Shen et al. 2012), especially in the north arm where they are the only predators of the bacterial and archaeal residents (Baxter et al. 2011). GSL Fungi were ignored in the past, but recently 32 strains of fungi were isolated from both north and south arms (Baxter and Zalar 2018) though we do not know what role they play in the communities. We have learned that GSL halophilic archaea are resistant to desiccation and UV radiation as well as high salinity (Baxter et al. 2007; Jones and Baxter 2016; Jones and Baxter 2017), and GSL bacteria may tolerate high temperatures or high pH (Pugin et al. 2012), making GSL halophiles polyextremophiles. We have begun thinking about GSL microorganisms as members of richer assemblages or microorganisms that work together in a community (Fig. 10c) [e.g., Almeida-Dalmet et al. 2015; Lindsay et al. 2017; Meuser et al. 2013].
And we have collaborated across disciplines to understand how geology and biology work together in GSL (Fig. 10a). One example of this is the calcium carbonate microbialite structures that line the lake bottom, which are likely precipitated from the action of photosynthesis, and resulting localized chemistry changes, by their associated cyanobacteria (Chidsey Jr et al. 2015; Lindsay et al. 2017). These mounds grow and power the lake, converting solar energy and forming a critical component of GSL primary production. In addition, the formation of halite (salt) and gypsum crystals, in the hypersaline sediment (Fig. 10b), may preserve GSL life forms over time, and this provides a link to potential mineral biosignatures on Mars (Perl et al. 2016). Indeed, GSL as a Martian analogue brings these historical studies into the future.