Abstract
Abstract. 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 pCO2 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 β-glucosidase and leucine-aminopeptidase increased along the gradient of mesocosm pCO2. A decrease in seawater pH of 0.5 units almost doubled rates of both enzymes. Heterotrophic bacterial activity was closely coupled to phytoplankton productivity in this experiment. The bacterioplankton community responded to rising chlorophyll a 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 pCO2 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 pCO2-related effects on bacterial activity can be mitigated by the top-down control of bacterial abundances in natural microbial communities.
Highlights
The amendment of glucose alone and in combination with ammonium led to a significant increase in Bacterial protein production (BPP), bacterial abundance and β-glucosidase activity in comparison to the non-amended control within 4 days of incubation
Sole ammonium enrichment did not result in significant changes. These results show that growth of the bacterioplankton community in the fjord was limited by the deficiency of labile carbon or energy when the mesocosm study started
The present study investigates how elevated seawater pCO2 can impact the metabolic activity of a natural bacterioplankton community in an Arctic fjord system
Summary
After early investigations suggesting that heterotrophic bacterial remineralisation is low in perennially cold environments (Pomeroy and Deibel, 1986), a multitude of field observations and experimental studies over the last two decades revealed a fully active microbial loop in the polar oceans (Rivkin et al, 1996; Rich et al, 1997; Yager et al, 2001).The bottom-up regulation of bacterial activity in Arctic microbial food webs is mainly achieved by interactions with temperature and dissolved organic matter (DOM) (Pomeroy and Wiebe, 2001; Kirchman et al, 2005, 2009a). After early investigations suggesting that heterotrophic bacterial remineralisation is low in perennially cold environments (Pomeroy and Deibel, 1986), a multitude of field observations and experimental studies over the last two decades revealed a fully active microbial loop in the polar oceans (Rivkin et al, 1996; Rich et al, 1997; Yager et al, 2001). Psychrophilic and psychrotolerant heterotrophic bacteria in polar marine environments are physiologically adapted to low temperatures. They often act far below their temperature optimum, so that cold ambient temperature has a high potential to reduce their metabolic activity and growth, thereby decreasing the rate of bacterial DOM degradation (e.g., Simon et al, 1999; Huston et al, 2004). Arctic marine systems are characterised by low temperature, and by a deficiency of carbon and energy resources as a second regulating factor of heterotrophic microbial growth (Kirchman et al, 2009a)
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