Interactive effect of temperature and resources on carbon cycling by freshwater bacterioplankton communities

Planktonic heterotrophic prokaryotes play an essential role in all aquatic ecosystems due to their short generation times and access to dissolved organic carbon and nutrients. In order to understand how rising temperatures and nutrient loading affect the biogeochemistry of whole lake ecosystems, it is essential to understand the interactive effects that temperature and resources have on the metabolism of natural bacterioplankton communities. To address this question we sampled bacterial communities from 1 mesotrophic and 1 oligotrophic lake in Clearwater County, Minnesota, USA, in winter and summer. Each community was exposed to a combination of carbon, nitrogen and phosphorus additions at 4 different levels (ambient, 2x, 5x, and 10x ambient) and 4 temperatures (4, 14, 24 and 34°C). Community metabolic response to temperature depended on the resource treatment, and the season when the community was sampled. Bacterial respiration increased more with temperature than bacterial growth, resulting in decreased bacterial growth efficiencies at higher temperatures. This result was most pronounced in the high resource treatments, while at lower resource levels the results were more ambiguous. In addition, differences between seasons and lakes suggested that the bacterial communities had adapted or acclimated to in situ temperature such that communities sampled from cold temperature environments had higher respiration at high temperatures than those sampled from warmer environments. These results suggest that the effect of temperature on carbon cycling mediated by the bacterial community depends on both the in situ resource pool and the extent to which the bacterial community is acclimated to a specific temperature regime.