Desiccation of ecosystem-critical microbialites in the shrinking 3 Great Salt Lake , Utah ( USA )

Great Salt Lake hosts an ecosystem that is critical to migratory birds and international aquaculture, yet it is currently threatened by falling lake elevation and high lakewater salinity resulting from water diversions in the upstream watershed and the enduring megadrought in the western United States. Microbialite reefs underpin the ecosystem, hosting a surface microbial community that is estimated to contribute 30% of the lake’s primary productivity. We monitored exposure, desiccation, and bleaching over time in an area of microbialite reef. During this period, lake elevation fell by 1.8 m, and salinity increased from 11.0% to 19.5% in open-water portions of the outer reef, reaching halite saturation in hydrologically closed regions. When exposed, microbialite bleaching was rapid. Bleached microbialites are not necessarily dead, however, with communities and chlorophyll persisting beneath microbialite surfaces for several months of exposure and desiccation. However, superficial losses in the mat community resulted in enhanced microbialite weathering. In microbialite recovery experiments with bleached microbialite pieces, partial community recovery was rapid at salinities ≤ 17%. 16S and 18S rRNA gene sequencing indicated that recovery was driven by initial seeding from lakewater. At higher salinity levels, eventual accumulation of chlorophyll may reflect accumulation and preservation of lake material in halite crusts vs. true recovery. Our results indicate that increased water input should be prioritized in order to return the lake to an elevation that submerges microbialite reefs and lowers salinity levels. Without quick action to reverse diversions in the watershed, loss of pelagic microbial community members due to sustained high salinity could prevent the recovery of the ecosystem-critical microbialite surface communities in Great Salt Lake.

135 depth, visibility, salinity (using a handheld 0-28% refractometer with automatic temperature 136 compensation; measurements are reported as a % by mass), density (using a brewing 137 hydrometer), and temperature (using a digital aquarium thermometer) measurements were 138 collected monthly to seasonally, along with microbialite surface observations. Lake elevation 139 data were obtained from two U.S. Geological Survey monitoring sites in the lake's South Arm 140 (Fig 1A), one near Saltair (ES1: Station 10010000), and the other on the railroad causeway (ES2: 141 Station 10010024) [24]. Multiple manual field measurements of site water depths at each site 142 were then used to determine depth offsets vs. lake level (i.e., site elevation).
143 Weather data 144 Weather data for 2019-November 2020 was obtained from a station on Antelope Island, 145 located 4 km from the field site and operated by Antelope Island State Park (WS1: 146 KUTSYRAC22, Ambient Weather WS-2090; Fig 1A); the station was non-operational 147 beginning in November, 2020. Data for nearby stations available on WeatherUnderground 148 (wunderground.com) were analyzed to find a new station with values consistent with those 149 measured at KUTSYRAC22; S11 File); the station with the best coverage and closest similarity 150 to KUTSYRAC22 was determined to be a private station located 14 km from the field site (WS2: 151 KUTSYRAC27, Ambient Weather WS-2902; Fig 1A), with publicly available data retrieved and 152 used in this study with permission from the station owner. For analytical purposes, measured 153 weather values were averaged when data from both sites were available.
This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting.  This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv 343 Over short timescales, bleaching was observed to be superficial, temporary, and influenced by 344 water-soluble salts (especially halite); monitored microbialites re-greened following rain events 345 (Figs 6 and S12 Fig), and samples collected were usually green just beneath the surface (< 1 cm).
346 Indeed, microscopy revealed no obvious changes in surface samples, with Euhalothece clumps 347 persisting in surface samples even in microbialites that appeared superficially bleached (Fig 8A).

383
This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv 384 Lab desiccation experiments 385 Lab desiccation experiments accelerated, but otherwise mimicked field observations: 386 surficial bleaching occurred rapidly, due at least in part to the accumulation of surface salts, and 387 was immediately reversed upon rinsing with distilled water (Fig 10). Bleaching did, however, 388 have an impact on microbialite weathering: during each rinse, surface material (which itself re-389 greened upon exposure to distilled water) sloughed off, and the microbialite lost dry mass.
390 Material that was previously resistant to weathering was easily removed following each 391 additional period of bleaching. Surface color measurements and masses are available in S8 File.

401
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Results in 2021
404 In 2021, we observed exponential increases in extractable chlorophyll and DNA over the 405 40-day period the experiment ran (Fig 11A-B; exponential best fit line r 2 values > 0.96 and 0.83, 406 respectively), despite salinity levels exceeding 17% and falling water temperatures (Fig 4). In 407 both cases, the growth rate was greater at the channel site (RA) vs. the reef interior site (RB).
408 Extrapolating the exponential best fit lines to levels of chlorophyll a and DNA comparable to 409 that of surrounding microbialites with healthy periphyton suggested a possible time to full 410 recovery of ~60 and 90-130 days for RA and RB, respectively (Fig 11C-D). 424 observed during the initial three weeks of the experiment (Fig 11A-B). However, a substantial 425 increase in extractable chlorophyll was observed by incubation day 72, even though halite 426 saturation had been reached (Fig 5B), and continued to increase through day 100 of the 427 experiment ( Fig 11A). Fitting an exponential best fit line indicated a slower rate of recovery than 428 for the 2021 experiments ( Fig 11C). Recovery experiment measurements are available in S9 File.

429
Community analysis from DNA sequencing 430 DNA sequencing results from the 2021 experiment indicated a consistent shift in 431 community composition over time driven largely by a relative increase in Proteobacteria and 432 Bacterioidota in bacterial 16S sequences (Fig 12A), and an increase in diatoms in eukaryotic 18S 433 sequences (Fig 12B).

434
This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv 457 community, and more similar to water than healthy periphyton (Fig 12C). Amplicon sequence 458 variant (ASV) tables of sequence abundance in each sample are reported in S10 File. This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. Although decreases in surface coloration were observed over a several week period (Fig   497 6), with a white zone of bleaching extending gradually downward on the microbialites (Fig 7), 498 pigment extracts and microscopy from core samples revealed that bleaching was initially 499 superficial, with substantial pigmented cellular material, including large Euhalothece clumps, 500 persisting even in the upper ~1 cm samples (Figs 8A and 9). This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv

542
When we repeated the experiment in the higher-salinity water of summer-autumn, 2022 543 (18-27%), recovery was markedly slower but did eventually appear to occur, with extracted 544 chlorophyll a values reaching those seen in 2021 roughly seven weeks later, and increasing 545 further in the following month (Fig 11A). Euhalothece clumps were visible in microscopic 546 analyses in samples collected after 2-3 months incubation (Fig 8C), supplying additional 547 evidence of recovery. This is remarkable, considering the salinity was at or near halite saturation 548 when this apparent recovery was occurring. It is possible that the apparent recovery was simply a 549 gradual accumulation of cells from the surrounding water (supported by the faster recovery rates 550 of microbialites exposed to more lakewater flow; Fig 11A- (Figs 12C-D). The recovery sample 571 communities that are most similar to lakewater were unsurprisingly those incubated for the 572 shortest periods, and evolved away from lakewater over time, suggesting an initial seeding by 573 lakewater. It is also notable that the 2021 recovery experiment site located in a channel away 574 from the greater reef and exposed to more water flow (Site RA) experienced significantly faster 575 recovery rates than was observed at the reef-interior site (RB).

576
The recovery sample community also evolved over time to become less similar to that 577 seen in microbialites with healthy periphyton, despite the persistence of Euhalothece. Most 578 notable is the relative dearth of Desulfobacterota in recovery experiment samples and high 579 abundance of Flavobacteriales, Rhodobacterales, and Gammaproteobacteria, especially 580 Marinobacteraceae and Oceanospirillales (Figs 12A-B). The low abundance of Desulfobacteria 581 can be explained by the high salinity levels present during the recovery experiments; even 582 halotolerant strains are not known to tolerate salinity levels above 13% [42,43]. It could, 583 however, also be a consequence of an immature microbial mat lacking the anoxic zones required 584 to support sulfur-reducing metabolisms. What is unclear is whether-given enough time and 585 development of a robust biofilm, then microbial mat-the community would begin to shift to 586 more closely resemble that seen in microbialites with a healthy periphyton.

587
Primary seeding from lakewater indicates that microbialite recolonization is dependent on 588 the health of the lakewater microbial ecosystem and the presence of viable Euhalothece and 589 other organisms in lakewater. While microbialite communities may be able to survive, or at least 590 be preserved, through periods of subaerial exposure and at high salinity, this is not true of 591 lakewater communities. In the north arm, where salinity values routinely exceed 24%, 592 Euhalothece sequences are absent [36], and the community composition is markedly different 593 than that in the lake's south arm. Thus, a high-salinity lake would not be able to re-seed healthy 594 microbialite periphyton communities. It appears that the lake's south arm is the ultimate 595 reservoir for the organisms and metabolisms that support the broader Great Salt Lake ecosystem, 596 including the microbialite microbial communities.

598
Great Salt Lake's microbialites and their surface microbial communities are in peril from 599 declining lake water levels and a concurrent increase in the salinity of the lake, which is 600 amplified in hydrologically closed areas of microbialite reef. In recent years, dramatic bleaching 601 of newly-exposed microbialites has been observed, causing concern about the future of the 602 microbialites and impacts on the broader ecosystem. Here, we have shown that microbialite 603 bleaching is initially superficial, with an endolithic survival mode allowing the microbialite 604 communities to be resilient for months after surface bleaching is observed. We also showed that 605 portions of the microbialite surface communities, including the ecosystem-critical cyanobacterial 606 component, can recover when bleached, desiccated microbialites are re-submerged and seeded This manuscript is a preprint and has not been peer reviewed. The copyright holder has made the manuscript available under a Creative Commons Attribution 4.0 International (CC BY) license and consented to have it forwarded to EarthArXiv for public posting. license EarthArXiv