Glacial retreat due to global warming is exposing large tracts of barren glacial sediments that are quickly colonized by CO2-fixing microbial communities that can constitute the climax community in many high-Arctic, alpine, and Antarctic environments. Despite the potential importance of these processes, little is known about microbial community successional dynamics and rates of carbon (C) sequestration in environments where higher plants are slow or unable to establish. We analyzed microbial community succession and C and N accumulation in newly exposed sediments along an Antarctic glacial chronosequence where moss and microbial autotrophs are the dominant primary producers. During the first 4 years of succession (0 to 40 m from the glacier) algae (including diatoms) were the most relatively abundant eukaryotes, but by the second phase studied (8 to 12 years) moss amplicon sequence variants (ASVs) dominated. The rise in moss coincided with a significant buildup of C and N in the sediments. The final two phases of the successional sequence (16 to 20 and 26 to 30 years) were marked by declines in microbial species richness and moss relative abundance, that coincided with significant decreases in both total C and N. These retrogressive declines coincided with a large increase in relative abundance of predatory Vampyrellidae suggesting a possible mechanism for retrogression in this and perhaps other terrestrial ecosystems at the edge of the cryosphere. These findings have implications for understanding CO2 sequestration and ecosystem succession in microbial-dominated regions of the cryobiosphere where large tracts of land are currently undergoing deglaciation.

Biological nitrogen fixation (BNF) is a high energy-demanding process that may be inhibited by penguin guano. We tested this hypothesis in Ardley Island by measuring BNF in biological soil crusts (BSC) directly within a Penguin Colony and in sites unaffected by penguins. We also explored the effect of adding guano to BSCs in sites free of the influence of penguins. Water availability is also one of the most limiting elements for life in the Antarctica, and we expected that a wetter growing season would stimulate BNF. To evaluate the effect of moisture on BNF, we added water to BSCs under laboratory conditions and estimated BNF by means of the acetylene reduction assay during three growing seasons (2012, 2013 and 2014), with contrasting temperature and precipitation conditions. The results reveal an almost complete inhibition of N fixation in the BSCs of the Penguin Colony. In sites free of ammonium and phosphate in rainwater, BNF rates reached up to 3 kg N ha(-1) year(-1) during warmer and wetter years. The addition of guano to BSCs significantly inhibited the rates of BNF. In laboratory incubations, the addition of water significantly stimulated rates of BNF during the warmer growing season with more sunshine hours. The likely increases in soil moisture levels due to climate change and glacier melting in the Antarctic Peninsula may enhance the rates of BNF. However, this may be constrained by accompanying changes in the distribution of Penguin Colonies.