Within the permanently ice-covered lakes of the McMurdo Dry Valleys (MDV), Antarctica, microbes dominate simple food webs supported largely by ecosystem-derived nutrients. Characterizing extracellular system enzyme activities (EEA) and the microbial carbon use efficiency (mCUE) expands current knowledge of how the MDV lakes’ microbial communities cycle nutrients. Given the relatively stable abiotic conditions and limited nutrient availability in the MDV, we hypothesize EEA and CUE will depend on depth and seasonality.
During early and mid-summer in 2015-2016, we collected water samples from MDV lakes at select water column depths and analyzed potential enzyme activities using fluorescent substrates as indirect indicators of microbial nutrient acquisition. Because proteins are a major source of carbon for bacterial communities, we also correlated enzymes to their respective nutrients. We calculated mCUE to determine energy and material flow from primary producers to higher trophic levels.
Results/Conclusions
EEA was 15-53% greater in the lakes during the early season than in mid-summer indicating microorganisms are acquiring complex nutrient compounds through limited bioavailable nutrients. In general, EEA decreased with depth as nutrient concentrations and flux increased. mCUE ranged from 20-60% in the lakes, the highest mCUE reported for any system. High mCUE suggests the microbial communities are tightly cycling the limited, ancient carbon. We conclude that biologically-mediated tight recycling of carbon should be considered alongside external inputs when building energy budgets.