As we expand the search for life beyond Earth, a water-dominated planet, we turn our eyes to other aquatic worlds. Microbial life found in Earth's many extreme habitats are considered useful analogs to life forms we are likely to find in extraterrestrial bodies of water. Modern-day benthic microbial mats inhabiting the low-oxygen, high-sulfur submerged sinkholes of temperate Lake Huron (Michigan, USA) and microbialites inhabiting the shallow, high-carbonate waters of subtropical Laguna Bacalar (Yucatan Peninsula, Mexico) serve as potential working models for exploration of extraterrestrial life. In Lake Huron, delicate mats comprising motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis and pigment-free chemosynthetic sulfur-oxidizing bacteria lie atop soft, organic-rich sediments. In Laguna Bacalar, lithification by cyanobacteria forms massive carbonate reef structures along the shoreline. Herein, we document studies of these two distinct earthly microbial mat ecosystems and ponder how similar or modified methods of study (e.g., robotics) would be applicable to prospective mat worlds in other planets and their moons (e.g., subsurface Mars and under-ice oceans of Europa). Further studies of modern-day microbial mat and microbialite ecosystems can add to the knowledge of Earth's biodiversity and guide the search for life in extraterrestrial hydrospheres.

We chronicled the seasonally recurring hypolimnetic hypoxia in Muskegon Lake - a Great Lakes estuary over 3 years, and examined its causes and consequences. Muskegon Lake is a mesotrophic drowned river mouth that drains Michigan's 2nd largest watershed into Lake Michigan. A buoy observatory tracked ecosystem changes in the Muskegon Lake Area of Concern (AOC), gathering vital time-series data on the lake's water quality from early summer through late fall from 2011 to 2013 (www.gvsu.edu/buoy). Observatory-based measurements of dissolved oxygen (DO) tracked the gradual development, intensification and breakdown of hypoxia (mild hypoxia <4 mg DO/L, and severe hypoxia <2 mg DO/L) below the similar to 6 m thermocline in the lake, occurring in synchrony with changes in temperature and phytoplankton biomass in the water column during July-October. Time-series data suggest that proximal causes of the observed seasonal hypolimnetic DO dynamics are stratified summer water-column, reduced wind-driven mixing, longer summer residence time, episodic intrusions of cold DO-rich nearshore lake Michigan water, nutrient run off from watershed, and phytoplankton blooms. Additional basin-wide water-column profiling (2011-2012) and ship-based seasonal surveys (2003-2013) confirmed that bottom water hypoxia is an annually recurring lake-wide condition. Volumetric hypolimnetic oxygen demand was high (0.07-0.15 m, DO/Liter/day) and comparable to other temperate eutrophic lakes. Over 3 years of intense monitoring, similar to 9-24% of Muskegon Lake's volume experienced hypoxia for similar to 29-85 days/year - with the potential for hypolimnetic habitat degradation and sediment phosphorus release leading to further eutrophication. Thus, time-series observatories can provide penetrating insights into the inner workings of ecosystems and their external drivers. (C) 2018 The Authors. Published by Elsevier B.V. on behalf of International Association for Great Lakes Research.

Many lacustrine systems, despite management efforts to control eutrophication, are hypoxic during stratified periods. Hypoxia is a major concern, not only for its impact on aquatic life but also for its potential to stimulate production of the greenhouse gases, methane (CH4) and nitrous oxide (N2O). We investigated the drivers of hypoxia in Muskegon Lake, a temperate dimictic freshwater estuary that experiences frequent hypolimnetic mixing due to atmospheric forces, riverine inputs, and intrusion of oxic water from coastal upwelling in Lake Michigan. Primary production and respiration (R) rates obtained from a δ18O mass balance model were similar to other mesotrophic environments (0.56–26.31 and 0.57–13.15 mmol O2 m−3 day−1, respectively), although high P/R (≥2 in mid-summer) indicated there is sufficient autochthonous production to support hypoxia development and persistence. The isotopic enrichment factor for respiration (εobs) varied markedly and was least negative in August of both sampling years, consistent with high R rates. Hypoxic conditions were associated with accumulation of N2O but not CH4, and emissions of N2O are among the highest reported from lakes. The average N2O site preference value of 25.4‰ indicates that the majority of N2O was produced by nitrification via hydroxylamine oxidation, despite the presence of resilient hypoxia. While it has been hypothesized that denitrification acts as a sink for N2O in hypoxic lakes, it is clear that Muskegon Lake functions as a strong source of N2O via nitrification. Further considerations of lakes as global sources of N2O thus warrant a closer evaluation of nitrification-fueled N2O production.

The free-water dissolved-oxygen (FWDO) method employs high-frequency time series of dissolved oxygen (DO) and several other variables to estimate gross primary production (GPP), total respiration (R), and net production (NP) of a lake. It requires a process-based model of DO dynamics to represent the roles of oxygenic photosynthesis, aerobic respiration, and atmospheric exchange in determining observed DO concentrations, as well as a procedure for estimating parameter values. We review approaches that have been used in conjunction with the FWDO method for modeling DO dynamics in lakes and estimating model parameters. Most studies have used mixed-layer models of DO dynamics that assume stratified lakes have a well-mixed upper layer (mixed layer) whose DO concentration is not affected by deeper water, which therefore is excluded from the models. We have found that mixed-layer models often cannot adequately account for observed diel dynamics of DO in our study system (Muskegon Lake, USA), indicating that model improvement is necessary. We outline various alternative forms of mixed-layer DO models that are compatible with the FWDO method (including several new ones), review methods for estimating GPP, R, and NP with them, and illustrate their performance in Muskegon Lake. None of the alternative forms resolve the problem of frequent poor model fits. It therefore appears that an important process is missing from mixed-layer DO models. We suggest the main problem is that the mixed-layer assumption is too simple to adequately represent the key physical transport and mixing processes in Muskegon Lake on most days, and we argue that this problem is likely to arise in other lakes, as well. We conclude by discussing several alternatives to mixed-layer models that address the entire water column, are straightforward to implement, and retain compatibility with the FWDO method.

During the summers of 2002–2013, we measured rates of carbon metabolism in surface waters of six sites across a land-to-lake gradient from the upstream end of drowned river-mouth Muskegon Lake (ML) (freshwater estuary) to 19 km offshore in Lake Michigan (LM) (a Great Lake). Despite considerable inter-year variability, the average rates of gross production (GP), respiration (R) and net production (NP) across ML (604 ± 58, 222 ± 22 and 381 ± 52 µg C L−1 day−1, respectively) decreased steeply in the furthest offshore LM site (22 ± 3, 55 ± 17 and −33 ± 15 µg C L−1day−1, respectively). Along this land-to-lake gradient, GP decreased by 96 ± 1%, whereas R only decreased by 75 ± 9%, variably influencing the carbon balance along this coastal zone. All ML sites were consistently net autotrophic (mean GP:R = 2.7), while the furthest offshore LM site was net heterotrophic (mean GP:R = 0.4). Our study suggests that pelagic waters of this Great Lakes coastal estuary are net carbon sinks that transition into net carbon sources offshore. Reactive and dynamic estuarine coastal zones everywhere may contribute similarly to regional and global carbon cycles.

Exchange of carbon between the biosphere and atmosphere is dominated by rates of photosynthetic CO2 uptake and respiratory CO2 release by aquatic and terrestrial ecosystems worldwide. Obtaining accurate estimates of these rates is therefore important. In lakes, the most common estimation method is based on a model of dissolved-oxygen (DO) dynamics and a corresponding time series of DO concentrations measured in freely moving lake water. O-2 production and consumption are inferred from changes in DO concentration, then converted to estimates of carbon uptake and release using photosynthetic and respiratory quotients. The traditional method of this type uses a simple accounting procedure to estimate daily gross primary production (GPP), total respiration (R), and net production (NP). Assuming that measured DO concentrations contain no error, it attempts to back-calculate GPP, R, and NP from an observed time series without using statistical techniques. This method produces valid estimates of GPP and the nighttime component of R, but it is unable to estimate the daytime component of R and hence cannot estimate R or NP for a complete dark-light cycle. To obtain estimates of these quantities, one must subjectively assume a value for daytime respiration. We present three new methods for estimating GPP, R, and NP that resolve this problem and also facilitate assessment of model adequacy. We illustrate use of the methods and compare their parameter estimates by applying them to monitoring data from Muskegon Lake, Michigan (USA). (C) 2013 Elsevier B.V. All rights reserved.

Lake Huron’s submerged sinkhole habitats are impacted by high-conductivity groundwater that allows photosynthetic cyanobacterial mats to form over thick, carbon-rich sediments. To better understand nutrient cycling in these habitats, we measured the stable isotopic content of carbon and nitrogen in organic and inorganic carbon pools in Middle Island sinkhole, a ~23 m deep feature influenced by both groundwater and overlying lake water. Two distinct sources of dissolved CO2 (DIC) were available to primary producers. Lake water DIC (δ 13C = −0.1 ‰) differed by +5.9 ‰ from groundwater DIC (δ 13C = −6.0 ‰). Organic carbon fixed by primary producers reflected the two DIC sources. Phytoplankton utilizing lake water DIC were more enriched in 13C (δ 13C = −22.2 to −23.2 ‰) than mat cyanobacteria utilizing groundwater DIC (δ 13C = −26.3 to −30.0 ‰). Sinkhole sediments displayed an isotopic signature (δ 13C = −23.1 ‰) more similar to sedimenting phytoplankton than the cyanobacterial mat. Corroborated by sediment C/N ratios, these data suggest that the carbon deposited in sinkhole sediments originates primarily from planktonic rather than benthic sources. 210Pb/137Cs radiodating suggests rapid sediment accumulation and sub-bottom imaging indicated a massive deposit of organic carbon beneath the sediment surface. We conclude that submerged sinkholes may therefore act as nutrient sinks within the larger lake ecosystem.

Cyanobacteria are renowned as the mediators of Earth's oxygenation. However, little is known about the cyanobacterial communities that flourished under the low-O(2) conditions that characterized most of their evolutionary history. Microbial mats in the submerged Middle Island Sinkhole of Lake Huron provide opportunities to investigate cyanobacteria under such persistent low-O(2) conditions. Here, venting groundwater rich in sulfate and low in O(2) supports a unique benthic ecosystem of purple-colored cyanobacterial mats. Beneath the mat is a layer of carbonate that is enriched in calcite and to a lesser extent dolomite. In situ benthic metabolism chambers revealed that the mats are net sinks for O(2), suggesting primary production mechanisms other than oxygenic photosynthesis. Indeed, (14)C-bicarbonate uptake studies of autotrophic production show variable contributions from oxygenic and anoxygenic photosynthesis and chemosynthesis, presumably because of supply of sulfide. These results suggest the presence of either facultatively anoxygenic cyanobacteria or a mix of oxygenic/anoxygenic types of cyanobacteria. Shotgun metagenomic sequencing revealed a remarkably low-diversity mat community dominated by just one genotype most closely related to the cyanobacterium Phormidium autumnale, for which an essentially complete genome was reconstructed. Also recovered were partial genomes from a second genotype of Phormidium and several Oscillatoria. Despite the taxonomic simplicity, diverse cyanobacterial genes putatively involved in sulfur oxidation were identified, suggesting a diversity of sulfide physiologies. The dominant Phormidium genome reflects versatile metabolism and physiology that is specialized for a communal lifestyle under fluctuating redox conditions and light availability. Overall, this study provides genomic and physiologic insights into low-O(2) cyanobacterial mat ecosystems that played crucial geobiological roles over long stretches of Earth history.

We investigated functional and structural responses of periphyton communities to simulated nonpoint source (NPS) pollution over a 2-year period. Periphyton communities were examined in three unique but hydrologically connected habitats in the lower Muskegon River: river, wetland, and lake. Our study addressed several major questions: (1) what are the effects of nonpoint source pollutants on periphyton structure and function, (2) to what degree do environmental conditions, including the pollutant delivery system and hydrologic conditions, affect periphyton response, and (3) do periphyton communities show synchronous responses to nonpoint source pollution? Nutrients were added over a 21-day period in each of the four seasons over 2 years. The effect of road salt on metabolism was evaluated in the winter and spring seasons in the river habitat only. In general, the periphyton community showed complex responses to the NPS amendments, but there was little evidence of nutrient limitation in this system based on community structure, biomass accrual, metabolism, or alkaline phosphatase activity. This result was unexpected given the relatively low ambient concentrations of phosphorus. We discuss the possibilities that nutrient delivery was ineffective or that herbivory and/or irradiance might have constrained periphyton growth, but ultimately conclude that our results suggest that periphyton communities in the lower Muskegon River watershed were (1) nutrient-replete, (2) tolerant of short-term salt additions, and (3) influenced more by local conditions (habitat) than by regional (watershed) factors. In systems where nonpoint source pollutant delivery is diffuse and hydrologic residence time is short, the influence of nonpoint source pollutants on periphyton may be either modest or too difficult to detect using traditional endpoints.

ABSTRACT Physicochemical characterization, automated ribosomal intergenic spacer analysis (ARISA) community profiling, and 16S rRNA gene sequencing approaches were used to study bacterial communities inhabiting submerged Lake Huron sinkholes inundated with hypoxic, sulfate-rich groundwater. Photosynthetic cyanobacterial mats on the sediment surface were dominated by Phormidium autumnale, while deeper, organically rich sediments contained diverse and active bacterial communities.

Recent underwater explorations have revealed unique hot spots of biogeochemical activity at several submerged groundwater vents in Lake Huron, the third largest of the Laurentian Great Lakes. Fueled by venting groundwater containing high sulfate and low dissolved oxygen, these underwater ecosystems are characterized by sharp physical and chemical gradients and spectacularly colorful benthic mats that overlie carbon‐rich sediments. Here, typical lake inhabitants such as fish and phytoplankton are replaced by communities dominated by microorganisms: bacteria and archaea that perform unique ecosystem functions. Shallow, sunlit sinkholes are dominated by photosynthetic microorganisms and processes, while food webs in deep aphotic sinkholes are supported primarily by chemosynthesis.

A variety of short-term responses were observed in inorganic nutrient enrichment bioassays on the plankton of Mona Lake - an urbanized and eutrophic drowned river-mouth lake that drains directly to Lake Michigan (USA). Chlorophyll a and phytoplankton production data indicated phosphorus (P) limitation in spring, nitrogen (N) and P co-limitation in summer, and no N or P limitation in autumn. Heterotrophic bacterial abundance, dissolved organic carbon levels, and plankton respiration were not stimulated by nutrient enrichment. Autotrophic biomass and production, but not heterotrophic biomass or respiration, were seasonally stimulated by the availability of P and/or N. Such imbalance between autotrophic and heterotrophic processes may contribute to the accumulation of phytoplankton biomass during the growing season. Cyanobacterial abundance in the lake increased from 22% of the plankton biomass in the spring when the TN:TP ratio was high to 68% in the autumn when the TN:TP ratio was low, suggesting that P availability helps drive a seasonal succession towards cyanobacterial dominance.

In the northern Great Lakes region, limestone sediments deposited some 400 million ybp during the Devonian era have experienced erosion, creating karst features such as caves and sinkholes. The groundwater chemical constituents of the shallow seas that produced these rock formations now contribute to the formation of a unique physical (sharp density gradients), chemical (dissolved oxygen-depleted, sulfate-rich) and biological (microbe-dominated) environment in a submerged sinkhole near Middle Island in freshwater Lake Huron. A variety of methods including aerial photography, physico-chemical mapping, time series measurements, remotely operated vehicle (ROV) survey, diver observations and bathymetric mapping were employed to obtain a preliminary understanding of sinkhole features and to observe physical interactions of the system's groundwater with Lake Huron. High conductivity ground water of relatively constant temperature hugs the sinkhole floor creating a distinct sub-ecosystem within this Great Lakes ecosystem. Extensive photosynthetic purple cyanobacterial benthic mats that characterize the benthos of this shallow sinkhole were strictly limited to the zone of ground water influence.

Dissolution of the Silurian-Devonian aquifer in the Lake Huron Basin has produced several karst formations in the bedrock (sinkholes), through which groundwater emerges onto the lake floor. During September 2003, we explored a recently discovered submerged sinkhole ecosystem (55 m × 40 m × ∼1 m) located at a depth of 93 m with a remotely operated vehicle (ROV) equipped with a conductivity-temperature-depth (CTD) system, an acoustic navigational system, a video camera, and a water sampling system. In addition to two morphotypes of benthic mats, a 1–2 m thick visibly cloudy near-bottom nepheloid-like layer (sinkhole plume) with a strong hydrogen sulfide odor prevailed just above the seepage area of clear water. Relative to lake water, water samples collected within the sinkhole plume were characterized by slightly higher (by 4°C) temperatures, very high levels of chloride (up to 175 mg l−1) and conductivity (1,700 μS cm−1), as well as extremely high concentrations of sulfate (1,400 mg l−1), phosphorus (3 mg l−1) and particulate organic matter (400 mg C l−1). Compared to background lake water, sinkhole plume water was characterized by approximately twofold lower C:N ratios and tenfold higher levels of dissolved organic carbon, bacterial biomass as well as heterotrophic bacterial production. Significant uptake of 14C-bicarbonate in dark incubations provided preliminary evidence for occurrence of chemosynthesis, possibly mediated by specialized Bacteria and Archea present in this submerged sinkhole ecosystem in the Laurentian Great Lakes.