Microbial processes of the carbon and sulfur cycles in an ice-covered, iron-rich meromictic lake Svetloe (Arkhangelsk region, Russia).

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-1  day-1 ) 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-1  day-1 ) 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-1  day-1 ) below the chemocline zone. The rates of methane oxidation were as high as 1.8 µmol CH4  L-1  day-1 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.

Lichen, moss and peat control of C, nutrient and trace metal regime in lakes of permafrost peatlands.

Permafrost thaw in continental lowlands produces large number of thermokarst (thaw) lakes, which act as a major regulator of carbon (C) storage in sediments and C emission in the atmosphere. Here we studied thaw lakes of the NE European permafrost peatlands - shallow water bodies located within frozen peat bogs and receiving the majority of their water input from lateral (surface) runoff. We also conducted mesocosm experiments via interacting lake waters with frozen peat and dominant ground vegetation - lichen and moss. There was a systematic decrease in concentrations of dissolved C, CO2, nutrients and metals with an increase in lake size, corresponding to temporal evolution of the water body and thermokarst development. We hypothesized that ground vegetation and frozen peat provide the majority of C, nutrients and inorganic solutes in the water column of these lakes, and that microbial processing of terrestrial organic matter controls the pattern of CO2 and nutrient concentrations in thermokarst lakes. Substrate mass-normalized C, nutrient (N, P, K), major and trace metal release was maximal in moss mesocosms. After first 16 h of reaction, the pCO2 increased ten-fold in mesocosms with moss and lichen; this increase was much less pronounced in experiments with permafrost peat. Overall, moss and lichen were the dominant factors controlling the enrichment of the lake water in organic C, nutrients, and trace metals and rising the CO2 concentration. The global significance of obtained results is that the changes in ground vegetation, rather than mere frozen peat thawing, may exert the primary control on C, major and trace element balance in aquatic ecosystems of tundra peatlands under climate warming scenario.