Nutrient limitation of microbial phototrophs on a debris-covered glacier

• Growth in the debris on top of the Toklat Glacier is principally P-limited.
• Nutrient amendment significantly determined bacterial and eukaryal community structure.
• Microbial eukaryotes that respond differentially to N and P addition are identified.

Photosynthetic microbial communities are important to the functioning of early successional ecosystems, but we know very little about the factors that limit the growth of these communities, especially in remote glacial and periglacial environments. The goal of the present study was to gain insight into the degree to which nutrients limit the growth of photosynthetic microbes in sediments from the surface of the Toklat Glacier in central Alaska. Previous studies and historical observations indicated that this environment is dominated by unique soil algae, and that succession from a microbial to a plant-dominated system is very slow. We used a soil microcosm approach to determine if nitrogen (N) and/or phosphorus (P) additions would affect the development and final biomass of microbial phototroph communities in this system. We found that fertilization with P significantly increased the exponential growth rate (r), but P alone did not affect the final percent soil cover (K) by microbial phototrophs. Nitrogen alone had no effect on either r or K, but the combination of P and N dramatically increased K, thus showing that algal growth rate in this system is likely P-limited, but total productivity may be co-limited by P and N. In addition, nutrient treatments differentially stimulated microbial groups resulting in significantly different microbial communities among treatments. Overall, these results give a preliminary indication of the factors that might limit the development and productivity of photosynthetic microbial communities in an extreme and remote glacial system.