Biogeochemical, isotope geochemical and microbiological investigation of Lake Svetloe (White Sea basin), a meromictic freshwater was carried out in April 2014, when ice thickness was ∼0.5 m, and the ice-covered water column contained oxygen to 23 m depth. Below, the anoxic water column contained ferrous iron (up to 240 μμM), manganese (60 μM), sulfide (up to 2 μM) and dissolved methane (960 μM). The highest abundance of microbial cells revealed by epifluorescence microscopy was found in the chemocline (redox zone) at 23-24.5 m. Oxygenic photosynthesis exhibited two peaks: the major one (0.43 μmol C L day ) below the ice and the minor one in the chemocline zone, where cyanobacteria related to Synechococcus rubescens were detected. The maximum of anoxygenic photosynthesis (0.69 μmol C L day ) at the oxic/anoxic interface, for which green sulfur bacteria Chlorobium phaeoclathratiforme were probably responsible, exceeded the value for oxygenic photosynthesis. Bacterial sulfate reduction peaked (1.5 μmol S L day ) below the chemocline zone. The rates of methane oxidation were as high as 1.8 μmol CH L day at the oxi/anoxic interface and much lower in the oxic zone. Small phycoerythrin-containing Synechococcus-related cyanobacteria were probably involved in accumulation of metal oxides in the redox zone.

The colloidal distribution and size fractionation of organic carbon and trace elements were studied in a seasonally stratified, organic-rich boreal lake, Lake Maselga, located in the European subarctic zone (NW Russia, Arkhangelsk region). This study took place over the course of 5 years in winter (glacial) and summer periods and during the spring and autumn overturn. A newly developed in situ dialysis technique (1, 10, and 50 kDa) and traditional frontal filtration and ultrafiltration (20, 10, 5, 0.22, and 0.025 lm) were used to assess element concentrations at different depths. No significant changes in element concentrations occurred during filtration through sub-colloidal pore-size membranes (20-0.22 lm), suggesting a negligible amount of particulate Fe, OC, and associated trace metals. Large colloids (0.025-0.22 lm) were found to be the main carriers of poorly soluble elements (Fe, Al, Ti, Zr, REEs, Th, and U) during the summer and winter stratification. There was also a clear change in the vertical pattern of the percentage of colloidal Al, Ti, V, Cr, Fe, and Ni during different seasons, and the greatest proportion of colloidal forms was observed during the spring and autumn overturn. This pattern is most likely linked to the dominance of soil (allochthonous) organic carbon, which complexes with trace metals during these periods. During the summer seasons, autochthonous production of small exometabolites or photodegradation increases the concentration of the lowmolecular weight fractions (\1 kDa) that dominate the speciation of divalent heavy metals Electronic supplementary material The online version of this article (in surface horizons. The colloidal status of As (30-60%), which was documented in different seasons along the full depth of the water column, is most likely linked to the presence of organic complexes. The overall results of this study suggest that changes in the colloidal speciation of trace elements with depth in different seasons depend on changes in the redox conditions, the input of soil OM, the biodegradation of plankton biomass releasing dissolved organic matter in the bottom horizons, and in upward diffusion from the sediments.

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Located on the shore of Kandalaksha Bay (the White Sea, Russia) and previously separated from it, Trekhtzvetnoe Lake (average depth 3.5 m) is one of the shallowest meromictic lakes known. Despite its shallowness, it features completely developed water column stratification with high-density microbial chemocline community (bacterial plate) and high rates of major biogeochemical processes. A sharp halocline stabilizes the stratification. Chlorobium phaeovibrioides dominated the bacterial plate, which reached a density of 2 × 10 cell ml and almost completely intercepts H S diffusion from the anoxic monimolimnion. The resulting anoxygenic photosynthesis rate reached 240 μmol C l day , exceeding the oxygenic photosynthesis rate in the mixolimnion. The rates of other processes are also high, reaching 4.5 μmol CH l day for methane oxidation and 35 μmol S l day for sulfate reduction. Metagenomic analysis demonstrated that the Chl. phaeovibrioides population in the bacterial plate layer had nearly clonal homogeneity, although some fraction of these cells harbour a plasmid. The Chlorobium population was associated with bacteriophages that share homology with CRISPR spacers in the host. These features make the ecosystem of the Trekhtzvetnoe Lake a valuable model for studying regulation and evolution processes in natural high-density microbial systems.

Acidophilic archaea of the ARMAN group from the uncultured candidate phylum ‘Micrarchaeota’ have small genomes and cell sizes and are known to be metabolically dependent and physically associated with their Thermoplasmatales hosts. However, phylogenetically diverse ‘Ca. Micrarchaeota’ are widely distributed in various non-acidic environments, and it remains uncertain because of the lack of complete genomes, whether they are also devoted to a partner-dependent lifestyle. Here, we obtained nine metagenome-assembled genomes of ‘Ca. Micrarchaeota’ from the sediments of a meromictic freshwater lake, including a complete, closed 1.2 Mbp genome of ‘Ca. Micrarchaeota’ Sv326, an archaeon phylogenetically distant from the ARMAN lineage. Genome analysis revealed that, contrary to ARMAN ‘Ca. Micrarchaeota,’ the Sv326 archaeon has complete glycolytic pathways and ATP generation mechanisms in substrate phosphorylation reactions, the capacities to utilize some sugars and amino acids as substrates, and pathways for de novo nucleotide biosynthesis, but lacked an aerobic respiratory chain. We suppose that Sv326 is a free-living scavenger rather than an obligate parasite/symbiont. Comparative analysis of ‘Ca. Micrarchaeota’ genomes representing different order-level divisions indicated that evolution of the ‘Ca. Micrarchaeota’ from free-living ‘Ca. Diapherotrites’-like ancestor involved losses of important metabolic pathways in different lineages and gains of specific functions in the course of adaptation to a partner-dependent lifestyle and specific environmental conditions. The ARMAN group represents the most pronounced case of genome reduction and gene loss, while the Sv326 lineage appeared to be rather close to the ancestral state of the ‘Ca. Micrarchaeota’ in terms of metabolic potential. Importance The recently described superphylum DPANN includes several phyla of uncultivated archaea with small cell sizes, reduced genomes, and limited metabolic capabilities. One of these phyla, ‘Ca. Micrarchaeota,’ comprises an enigmatic group of archaea found in acid mine drainage environments, the Archaeal Richmond Mine Acidophilic Nanoorganisms (ARMAN) group. Analysis of their reduced genomes revealed the absence of key metabolic pathways consistent with their partner-associated lifestyle, and physical associations of ARMAN cells with their hosts were documented. However, ‘Ca. Micrarchaeota’ includes several lineages besides the ARMAN group found in non-acidic environments, and none of them has been characterized. Here we report a complete genome of ‘Ca. Micrarchaeota’ from a non-ARMAN lineage. Analysis of this genome revealed the presence of metabolic capacities lost in ARMAN genomes that could enable a free-living lifestyle. These results expand our understanding of genetic diversity, lifestyle, and evolution of ‘Ca. Micrarchaeota.’

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