Nutrient parsimony shapes diversity and functionality in hyper-oligotrophic Antarctic soils

The balance of nutrients in soil is critical for microbial growth and function, and stoichiometric values below the Redfield ratio for C:N:P can negatively affect microbial ecosystem services. However, few studies have assessed the relationships between nutrient balance and biological productivity in extremely nutrient-poor habitats. The Mackay Glacier region of Eastern Antarctica is a hyper-oligotrophic ice-free desert and is an appropriate landscape to evaluate the effects of nutrient deficiency and imbalance on microbial community ecology. In a survey of multiple, widely dispersed soil samples from this region, we detected only low rates of microbial respiration, and observed that C:N:P ratios were well below those required for optimal activity. In silico metagenomic and soil isotopic ratio (δ15N) analyses indicated that the capacity for nitrogen fixation was low, but that soil microbial communities were enriched for soil nitrate assimilation processes, mostly associated with heterotrophic taxa. δ13C isotope ratio data suggested that carbon dioxide was fixed principally via the Calvin cycle. Genes involved in this pathway were common to all metagenomes and were primarily attributed to members of the dominant soil bacterial phyla: Bacteroidetes and Acidobacteria. The identification of multiple genes encoding non-photoautotrophic RUBISCO and carbon dioxide dehydrogenase enzymes in both the metagenomic sequences and assembled MAGs is suggestive of a trace-gas scavenging physiology in members of these soil communities.