| Summary: | Ocean acidification is expected to affect enzymatic hydrolysis, resulting in changes in microbial exopolymer decomposition.The effects of increasing CO2 concentrations on bacterial degradation of organic matter was studied during a combined chemostat and batch experiment in the frame work of the BIOACID project (Biological Impact of Ocean Acidification). Here, we report on the effect of pCO2 on the activity rates of extracellular enzymes. These process organic matter degradation as well as nutrient regeneration and hence play an important role in the turnover of dissolved organic matter (DOM). The diatom Thalassiosira weissflogii was pre-adapted in chemostat chambers under nitrogen limitation and different CO2 partial pressures (180, 380 and 780 ppm representing past, present-day, and future atmospheric pCO2, respectively). After that exudation was enhanced by growing the algae in pCO2 controlled batch cultures for three days. A natural bacterial community then degraded the produced exudates during a four-day incubation in the dark. Enzyme activities as well as concentration and composition of organic material were determined. In accordance with nutrient availability in the medium, we measured high aminopeptidase activity in all treatment but no phosphatase activity. Beta-glucosidase and aminopeptidase activity increased with pCO2 during the batch phase and at the beginning of the degradation phase. We discuss whether this effect derives from a change in biomass production in the high pCO2 treatments or whether it was a direct pH effect on enzyme activities. After 48h of degradation, no enzyme activities were detectable anymore and bacterial cell counts decreased significantly. These findings suggest that the active bacterial community degraded all labile organic matter within 48h and perished subsequently.
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