Biological and Ecological Features, Trophic Structure and Energy Flow in Meromictic Lakes

Vertical structure of the bacterial diversity in meromictic Fayetteville Green Lake

The permanently stratified water columns in euxinic meromictic lakes produce niche environments for phototrophic sulfur oxidizers and diverse sulfur metabolisms. While Green Lake (Fayetteville, New York, NY) is known to host a diverse community of ecologically important sulfur bacteria, analyses of its microbial communities, to date, have been largely based on pigment analysis and smaller datasets from Sanger sequencing techniques. Here, we present the results of next-generation sequencing of the eubacterial community in the context of the water column geochemistry. We observed abundant purple and green sulfur bacteria, as well as anoxygenic photosynthesis-capable cyanobacteria within the upper monimolimnion. Amidst the phototrophs, we found other sulfur-cycling bacteria including sulfur disproportionators and chemotrophic sulfur oxidizers, further detailing our understanding of the sulfur cycle and microbial ecology of euxinic, meromictic lakes.

Anoxygenic photo- and chemo-synthesis of phototrophic sulfur bacteria from an alpine meromictic lake

Meromictic lakes are interesting ecosystems to study anaerobic microorganisms due their permanent stratification allowing the formation of a stable anoxic environment. The crenogenic meromictic Lake Cadagno harbors an important community of anoxygenic phototrophic sulfur bacteria responsible for almost half of its total productivity. Besides their ability to fix CO2 through photosynthesis, these microorganisms also showed high rates of dark carbon fixation via chemosyntesis. Here, we grew in pure cultures three populations of anoxygenic phototrophic sulfur bacteria previously isolated from the lake, accounting for 72.8% of the total microbial community and exibiting different phenotypes: (1) the motile, large-celled purple sulfur bacterium (PSB) Chromatium okenii, (2) the small-celled PSB Thiodictyon syntrophicum and (3) the green sulfur bacterium (GSB) Chlorobium phaeobacteroides. We measured their ability to fix CO2 through photo- and chemo-synthesis, both in situ in the lake and in laboratory under different incubation conditions. We also evaluated the efficiency and velocity of H2S photo-oxidation, an important reaction in the anoxygenic photosynthesis process. Our results confirm that phototrophic sulfur bacteria strongly fix CO2 in the presence of light and that oxygen increases chemosynthesis at night, in laboratory conditions. Moreover, substancial differences were displayed between the three selected populations in terms of activity and abundance.

Microbial Processes and Microbial Communities in the Water Column of the Polar Meromictic Lake Bol'shie Khruslomeny at the White Sea Coast

Microbiological, molecular ecological, biogeochemical, and isotope geochemical research was carried out at the polar Lake Bol'shie Khruslomeny at the coast of the Kandalaksha Bay, White Sea in March and September 2017. The uppermost mixolimnion was oxic, with low salinity (3-5%). The lower chemocline layer was brown-green colored, with very high content of particulate organic matter (up to 11.8 mg C L-1). The lowermost monimolimnion had marine salinity (22-24%) and very high concentrations of sulfide (up to 18 mmol L-1) and CH4 (up to 1.8 mmol L-1). In the chemocline, total microbial abundance and the rate of anoxygenic photosynthesis were 8.8 × 106 cells mL-1 and 34.4 μmol C L-1 day-1, respectively. Both in March and September, sulfate reduction rate increased with depth, peaking (up to 0.6-1.1 μmol S L-1 day-1) in the lower chemocline. Methane oxidation rates in the chemocline were up to 85 and 180 nmol CH4 L-1 day-1 in March and September, respectively; stimulation of this process by light was observed in September. The percentages of cyanobacteria and methanotrophs in the layer where light-induced methane oxidation occurred were similar, ∼2.5% of the microbial community. Light did not stimulate methane oxidation in deeper layers. The carbon isotope composition of particulate organic matter (δ13C-Corg), dissolved carbonates (δ13C-DIC), and methane (δ13C- CH4) indicated high microbial activity in the chemocline. Analysis of the 16S rRNA gene sequences revealed predominance of Cyanobium cyanobacteria (order Synechococcales) in the mixolimnion. Green sulfur bacteria Chlorobium phaeovibrioides capable of anoxygenic photosynthesis constituted ∼20% of the chemocline community both in March and in September. Methyloprofundus gammaptoteobacteria (family Methylomonaceae) were present in the upper chemocline, where active methane oxidation occurred. During winter, cyanobacteria were less abundant in the chemocline, while methanotrophs occurred in higher horizons, including the under-ice layer. Chemolithotrophic gammaproteobacteria of the genus Thiomicrorhabdus, oxidizing reduced sulfur compounds at low oxygen concentrations, were revealed in the chemocline in March. Both in March and September archaea constituted up to 50% of all microorganisms in the hypolimnion. The percentage of putative methanogens in the archaeal community was low, and they occurred mainly in near-bottom horizons.

Spatio-temporal insights into microbiology of the freshwater-to-hypersaline, oxic-hypoxic-euxinic waters of Ursu Lake

Ursu Lake is located in the Middle Miocene salt deposit of Central Romania. It is stratified, and the water column has three distinct water masses: an upper freshwater-to-moderately saline stratum (0-3 m), an intermediate stratum exhibiting a steep halocline (3-3.5 m), and a lower hypersaline stratum (4 m and below) that is euxinic (i.e. anoxic and sulphidic). Recent studies have characterized the lake's microbial taxonomy and given rise to intriguing ecological questions. Here, we explore whether the communities are dynamic or stable in relation to taxonomic composition, geochemistry, biophysics, and ecophysiological functions during the annual cycle. We found: (i) seasonally fluctuating, light-dependent communities in the upper layer (≥0.987-0.990 water-activity), a stable but phylogenetically diverse population of heterotrophs in the hypersaline stratum (water activities down to 0.762) and a persistent plate of green sulphur bacteria that connects these two (0.958-0.956 water activity) at 3-3.5 to 4 m; (ii) communities that might be involved in carbon- and sulphur-cycling between and within the lake's three main water masses; (iii) uncultured lineages including Acetothermia (OP1), Cloacimonetes (WWE1), Marinimicrobia (SAR406), Omnitrophicaeota (OP3), Parcubacteria (OD1) and other Candidate Phyla Radiation bacteria, and SR1 in the hypersaline stratum (likely involved in the anaerobic steps of carbon- and sulphur-cycling); and (iv) that species richness and habitat stability are associated with high redox-potentials. Ursu Lake has a unique and complex ecology, at the same time exhibiting dynamic fluctuations and stability, and can be used as a modern analogue for ancient euxinic water bodies and comparator system for other stratified hypersaline systems.

Geomicrobiology of the carbon, nitrogen and sulphur cycles in Powell Lake: a permanently stratified water column containing ancient seawater

We present the first geomicrobiological characterization of the meromictic water column of Powell Lake (British Columbia, Canada), a former fjord, which has been stably stratified since the last glacial period. Its deepest layers (300-350 m) retain isolated, relict seawater from that period. Fine-scale vertical profiling of the water chemistry and microbial communities allowed subdivision of the water column into distinct geomicrobiological zones. These zones were further characterized by phylogenetic and functional marker genes from amplicon and shotgun metagenome sequencing. Binning of metagenomic reads allowed the linkage of function to specific taxonomic groups. Statistical analyses (analysis of similarities, Bray-Curtis similarity) confirmed that the microbial community structure followed closely the geochemical zonation. Yet, our characterization of the genetic potential relevant to carbon, nitrogen and sulphur cycling of each zone revealed unexpected features, including potential for facultative anaerobic methylotrophy, nitrogen fixation despite high ammonium concentrations and potential micro-aerobic nitrifiers within the chemocline. At the oxic-suboxic interface, facultative anaerobic potential was found in the widespread freshwater lineage acI (Actinobacteria), suggesting intriguing ecophysiological similarities to the marine SAR11. Evolutionary divergent lineages among diverse phyla were identified in the ancient seawater zone and may indicate novel adaptations to this unusual environment.