Global emergent responses of stream microbial energetics to glacier shrinkage

Abstract Mountain and polar glaciers are melting worldwide. However, the downstream impacts of this unprecedented environmental change on elemental fluxes and microbial energetics in the glacier-fed streams (GFS) remains poorly understood at a global scale. This contrasts the relevance of GFSs in initiating the flow of some of the world’s largest river networks and providing clean water to large human populations. Here, by studying resource stoichiometry in 154 GFSs from Earth’s major mountain ranges, we show that these ecosystems are potentially carbon (C) and phosphorus (P) limited, where P limitation may become exacerbated and C limitation alleviated as glaciers shrink. Modeling threshold elemental ratios from extracellular enzymatic activities, we show that the microbial metabolism in GFSs is indeed C and P limited. This is consistent with low microbial carbon use efficiencies (CUE; median: 0.15) indicative of maintenance metabolism. Using space-for-time substitution approaches, we found increasing biomass of benthic primary producers potentially relieving the microbial metabolism from resource limitation as glaciers shrink. Furthermore, increasing streamwater temperature will stimulate microbial growth (temperature sensitivity: 0.62 electronvolts) in GFSs. Our study unveils responses of the microbial energetics to a greening and warming GFS environment, shifting ecosystem metabolism to autotrophy with consequences for related ecosystem C cycling.