COS 45-1 - Microbial community genes for cycling carbon and nitrogen transcribed within ice-lidded Antarctic cryoconite holes

Background/Question/Methods

Cryoconite holes, or sediment melt-holes in the top meter of glacial ice, are among the most extreme environments on Earth, but particularly so in Antarctica’s McMurdo Dry Valleys. Due to their frozen ice lids and resulting isolation from the atmosphere, cryoconite holes develop a wide range of extreme conditions including temperatures less than a degree above zero, oxygen supersaturation, and pH values of 8-10. Despite these extreme conditions, cryoconite holes support active microbial communities as indicated by community change over time, consistent with growth and turnover. Cycling of carbon and nitrogen within these icy ecosystems have been inferred from Redfield-like stoichiometry and stable isotope ratios, but the specific processes being carried out, and the organisms responsible, have not been resolved. To better understand nutrient cycling processes occurring and the species driving them, we performed metagenomic and metatransciptomic analyses on a total of 6 samples collected from melted cryoconite holes in Canada and Taylor glaciers in the McMurdo Dry Valleys, Antarctica. We additionally measured carbon and nitrogen concentrations and stable isotope ratios for further biogeochemical context.

Results/Conclusions

We recovered 45 bacterial metagenome-assembled genomes (MAGs) dominated by Cyanobacteria and Actinobacteria, confirming results of previous studies using 16S metabarcoding that these phyla dominate cryoconite communities. Cyanobacteria have previously been established as dominant (up to 30%) members of cryoconite bacterial communities, and total RNA suggested Cyanobacteria comprised an even greater proportion (close to 50%) of the active organisms, highlighting their ecological importance. Transcripts for the use of readily available organic carbon (e.g., organic carbon oxidation pathway genes) mapped to all MAGs, whereas only 3 out of 45 MAGs transcribed genes involved in metabolic pathways indicative of severe carbon limitation (e.g., methanotrophy or hydrogen oxidation genes). The carbon isotope values, which ranged from -12.5‰ to -22.5‰, further suggest prevalent active fixation of carbon. Genes involved in nitrogen cycle were associated with 11 of 45 MAGs and limited to nitrate, nitrite, nitrous oxide, and nitric oxide reduction and nitrite ammonification, consistent with limited available nitrogen being cycled. Nitrogen isotope values (0‰ to -4.5‰) were near atmospheric values, suggesting the presence of nitrogen inputs via fixation, perhaps in response to otherwise limited nitrogen sources. These results begin to resolve the picture of ecological interactions and carbon and nitrogen cycling in extreme cryoconite ecosystems.