The occurrence of bottom-water hypoxia is increasing in bodies of water around the world. Hypoxia is of concern due to the way it negatively impacts lakes and estuaries at the whole ecosystem level. During 2015, we examined the influence of hypoxia on the Muskegon Lake ecosystem by collecting surface- and bottom-water nutrient samples, bacterial abundance counts, benthic fish community information, and performing profiles of chlorophyll and phycocyanin as proxies for phytoplankton and cyanobacterial growth, respectively. Several significant changes occurred in the bottom waters of the Muskegon Lake ecosystem as a result of hypoxia. Lake-wide concentrations of soluble reactive phosphorus (SRP) and total phosphorus increased with decreasing dissolved oxygen (DO). Bacterial abundance was significantly lower when DO was less than 2.2 mg L⁻¹. Whereas there were no drastic changes in surface chlorophyll a concentration through the season, phycocyanin increased threefold during and following a series of major wind-mixing events. Phycocyanin remained elevated for over 1.5 months despite several strong wind events, suggesting that high SRP concentrations in the bottom waters may have mixed into the surface waters, sustaining the bloom. The fish assemblage in the hypolimnion also changed in association with hypoxia. Overall fish abundance, number of species, and maximum length all decreased in catch as a function of bottom DO concentrations. The link between hypoxia and wind events appears to serve as a positive feedback loop by continuing internal loading and cyanobacterial blooms in the lake, while simultaneously eroding habitat quality for benthic fish.

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.

Who’s cooking, who’s cleaning, and who’s got the remote control within the waters blanketing Earth? Anatomically tiny, numerically dominant microbes are the crucial “homemakers” of the watery household. Phytoplankton’s culinary abilities enable them to create food by absorbing sunlight to fix carbon and release oxygen, making microbial autotrophs top-chefs in the aquatic kitchen. However, they are not the only bioengineers that balance this complex household. Ubiquitous heterotrophic microbes including prokaryotic bacteria and archaea (both “bacteria” henceforth), eukaryotic protists, and viruses, recycle organic matter and make inorganic nutrients available to primary producers. Grazing protists compete with viruses for bacterial biomass, whereas mixotrophic protists produce new organic matter as well as consume microbial biomass. When viruses press remote-control buttons, by modifying host genomes or lysing them, the outcome can reverberate throughout the microbial community and beyond. Despite recognition of the vital role of microbes in biosphere housekeeping, impacts of anthropogenic stressors and climate change on their biodiversity, evolution, and ecological function remain poorly understood. How trillions of the smallest organisms in Earth’s largest ecosystem respond will be hugely consequential. By making the study of ecology personal, the “housekeeping” perspective can provide better insights into changing ecosystem structure and function at all scales.

Hypolimnetic hypoxia is expanding globally due to anthropogenic eutrophication and climate warming. Muskegon Lake, a Great Lakes estuary, experiences annually recurring hypoxia, impairing ecological, social, and economic benefits. Using high-frequency, time-series Muskegon Lake Observatory (MLO) data, we quantified the dynamics of hypoxia and developed a hypoxia severity index to estimate the spatiotemporal extent of hypoxia during 2011–2021. We also analyzed United States Geological Survey’s temperature and discharge data on the Muskegon River to explain the annual variability in the hypoxia severity index. Severe hypoxia occurred in warmer years with greater stratification, fewer wind mixing events, warmer winter river temperatures, and less winter and spring precipitation, as in 2012 and 2021. Conversely, milder hypoxia was prevalent in colder years with a later stratification onset, more mixing events, colder river temperatures, and more winter and spring precipitation, as in 2015 and 2019. Thus, knowledge of environmental conditions prior to the onset of stratification may be useful for predicting the potential severity of hypoxia for any year. While consistent multi-year trends in hypoxia were not discernible, our findings suggest that temperature and precipitation are major drivers of hypoxia and that as surface waters warm, it will lead to the further deoxygenation of Earth’s inland waters.

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.

Animal migrations mark the largest daily movement of biomass on Earth today, but who performed the first diurnal migration? Extant benthic microbial mats inhabiting Lake Huro’s low-oxygen, high-sulfur submerged sinkholes that resemble life on early Earth, may offer some answers. Herein, mats are dominated by motile filaments of purple-pigmented cyanobacteria capable of oxygenic and anoxygenic photosynthesis, and pigment-free chemosynthetic sulfur-oxidizing bacteria. We captured time-lapse images of diurnal vertical migration between phototactic cyanobacteria and chemotactic sulfur-oxidizing bacteria – dramatically turning the mat surface purple at dawn and white at dusk. Alternating waves of vertically migrating photosynthetic and chemosynthetic filaments rapidly tracked diurnally fluctuating light; observations corroborated with intact mats under simulated day-night conditions. Both types of filaments increased in surface coverage non-linearly, albeit at different rates. During their respective surface takeovers, maximum nightly rate of movement for white chemosynthetic filaments occurred an hour before that of purple cyanobacteria during the day. However, though slow to start at dawn, the cyanobacteria’s maximum rate of movement was double that of the chemosynthetic bacteria, leading to greater total coverage over the span of the day. Such synchronized diurnal “tango” might have been the largest daily mass movement of life during the long Archean and Proterozoic eras, when the biosphere was mostly benthic, and played a critical role in optimizing photosynthesis, chemosynthesis, carbon burial, and oxygenation. Further studies of extant microbial “mat worlds” will add to the expanding knowledge of Earth’s biodiversity and physiologies, and may aid our ongoing search for life in extraterrestrial waters.