Find in Library

The effect of elevated seawater carbon dioxide (CO₂) on the activity of a natural bacterioplankton community in an Arctic fjord system was investigated by a mesocosm perturbation study in the frame of the European Project on Ocean Acidification (EPOCA). A <i>p</i>CO₂ range of 175–1085 μatm was set up in nine mesocosms deployed in the Kongsfjorden (Svalbard). The activity of natural extracellular enzyme assemblages increased in response to acidification. Rates of &beta;-glucosidase and leucine-aminopeptidase increased along the gradient of mesocosm <i>p</i>CO₂. A decrease in seawater pH of 0.5 units almost doubled rates of both enzymes. <br><br> Heterotrophic bacterial activity was closely coupled to phytoplankton productivity in this experiment. The bacterioplankton community responded to rising chlorophyll <i>a</i> concentrations after a lag phase of only a few days with increasing protein production and extracellular enzyme activity. Time-integrated primary production and bacterial protein production were positively correlated, strongly suggesting that higher amounts of phytoplankton-derived organic matter were assimilated by heterotrophic bacteria at increased primary production. Primary production increased under high <i>p</i>CO₂ in this study, and it can be suggested that the efficient heterotrophic carbon utilisation had the potential to counteract the enhanced autotrophic CO₂ fixation. However, our results also show that beneficial <i>p</i>CO₂-related effects on bacterial activity can be mitigated by the top-down control of bacterial abundances in natural microbial communities.