Abstract
Nutrient enrichment of high-elevation freshwater ecosystems by atmospheric deposition is increasing worldwide, and bacteria are a key conduit for the metabolism of organic matter in these oligotrophic environments. We conducted two distinct in situ microcosm experiments in a high-elevation lake (Emerald Lake, Sierra Nevada, California, USA) to evaluate responses in bacterioplankton growth, carbon utilization, and community structure to short-term enrichment by nitrate and phosphate. The first experiment, conducted just following ice-off, employed dark dilution culture to directly assess the impact of nutrients on bacterioplankton growth and consumption of terrigenous dissolved organic matter during snowmelt. The second experiment, conducted in transparent microcosms during autumn overturn, examined how bacterioplankton in unmanipulated microbial communities responded to nutrients concomitant with increasing phytoplankton-derived organic matter. In both experiments, phosphate enrichment (but not nitrate) caused significant increases in bacterioplankton growth, changed particulate organic stoichiometry, and induced shifts in bacterial community composition, including consistent declines in the relative abundance of Actinobacteria. The dark dilution culture showed a significant increase in dissolved organic carbon removal in response to phosphate enrichment. In transparent microcosms nutrient enrichment had no effect on concentrations of chlorophyll, carbon, or the fluorescence characteristics of dissolved organic matter, suggesting that bacterioplankton responses were independent of phytoplankton responses. These results demonstrate that bacterioplankton communities in unproductive high-elevation habitats can rapidly alter their taxonomic composition and metabolism in response to short-term phosphate enrichment. Our results reinforce the key role that phosphorus plays in oligotrophic lake ecosystems, clarify the nature of bacterioplankton nutrient limitation, and emphasize that evaluation of eutrophication in these habitats should incorporate heterotrophic microbial communities and processes.
Highlights
Despite their relative isolation from human populations, highelevation ecosystems are increasingly affected by global change
The results presented here demonstrate that bacterioplankton growth, dissolved organic matter (DOM) removal and community composition respond rapidly to pulse inputs of inorganic nutrients, suggesting that the concept of nutrient limitation by N or P alone may apply to heterotrophic as well as autotrophic microbes
It is clear from these results that the response of a bacterial community to inorganic nutrients need not be mediated through the response of phytoplankton and subsequent alterations to the pool of dissolved organic matter: here phosphate stimulated bacterioplankton growth on extant terrestrially-derived DOM
Summary
Despite their relative isolation from human populations, highelevation ecosystems are increasingly affected by global change. Warming trends are disproportionately altering these habitats [1,2], and there is continuing evidence for large-scale anthropogenic depositional impacts to remote montane environments [3,4,5]. Atmospheric deposition of anthropogenic nitrogen has generated broad biogeochemical and ecological impacts in highelevation regions of the western United States [6]. In the Sierra Nevada of California (USA), atmospheric deposition of nitrogen (N) and phosphorus (P) represents a large fraction of the nutrient inputs to lakes at elevations greater than 2500 m above sea level (a.s.l) [9]. Two decades of monitoring suggest that lakes in the Sierra Nevada are undergoing eutrophication as well as a regional shift in nutrient limitation status [10,11]
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