Arnosti C, Grossart HP, Mühling M, Joint I, Passow U
Enzyme, carbon cycle, pCO2 changes, Microbial loop, mesocosm, PEECE, 20m3, Bergen, Norway, Espegrend
As part of the PeECE II mesocosm project, we investigated the effects of pCO2 levels onthe initial step of heterotrophic carbon cycling in the surface ocean. The activities of microbial extracellularenzymes hydrolyzing 4 polysaccharides were measured during the development of a naturalphytoplankton bloom under pCO2 conditions representing glacial (190 ?atm) and future (750 ?atm)atmospheric pCO2. We observed that (1) chondroitin hydrolysis was variable throughout the pre-,early- and late-bloom phases, (2) fucoidanase activity was measurable only in the glacial mesocosmas the bloom developed, (3) laminarinase activity was low and constant, and (4) xylanase activitydeclined as the bloom progressed. Concurrent measurements of microbial community composition,using denaturing-gradient gel electrophoresis (DGGE), showed that the 2 mesocosms diverged temporally,and from one another, especially in the late-bloom phase. Enzyme activities correlated withbloom phase and pCO2, suggesting functional as well as compositional changes in microbial communitiesin the different pCO2 environments. These changes, however, may be a response to temporalchanges in the development of phytoplankton communities that differed with the pCO2 environment.We hypothesize that the phytoplankton communities produced dissolved organic carbon(DOC) differing in composition, a hypothesis supported by changing amino acid composition of theDOC, and that enzyme activities responded to changes in substrates. Enzyme activities observedunder different pCO2 conditions likely reflect both genetic and