Coupling of heterotrophic bacteria to phytoplankton bloom development at different pCO2 levels: a mesocosm study
1Leibniz-Institute of Freshwater Ecology and Inland Fisheries; Department Limnology of Stratified Lakes; Alte Fischerhuette 2; D-16775 Stechlin-Neuglobsow, Germany
2Leibniz- Institute for Marine Sciences, University of Kiel, Duesternbrooker Weg 20, D-24105 Kiel, Germany
3Laboratory for Global Marine and Atmospheric Chemistry, School of Environmental Sciences, University of East Anglia, Norwich NR 7TJ, UK
4Department of Biology, Jahnebakken 5, University of Bergen, Norway
*currant address: DOE Joint genome Inst., Microbial Ecology Program, 2800 Mitchell Drive, Walnut Creek, CA 94598, USA
Abstract. The predicted rise in anthropogenic CO2 emissions will increase CO2 concentrations and decrease seawater pH in the upper ocean. Recent studies have revealed effects of pCO2 induced changes in seawater chemistry on a variety of marine life forms, in particular calcifying organisms. To test whether the predicted increase in pCO2 will directly or indirectly (via changes in phytoplankton dynamics) affect abundance, activities, and community composition of heterotrophic bacteria during phytoplankton bloom development, we have aerated mesocosms with CO2 to obtain triplicates with three different partial pressures of CO2 (pCO2): 350 μatm (1×CO2), 700 μatm (2×CO2) and 1050 μatm (3×CO2). The development of a phytoplankton bloom was initiated by the addition of nitrate and phosphate. In accordance to an elevated carbon to nitrogen drawdown at increasing pCO2, bacterial production (BPP) of free-living and attached bacteria as well as cell-specific BPP (csBPP) of attached bacteria were related to the C:N ratio of suspended matter. These relationships significantly differed among treatments. However, bacterial abundance and activities were not statistically different among treatments. Solely community structure of free-living bacteria changed with pCO2 whereas that of attached bacteria seemed to be independent of pCO2 but tightly coupled to phytoplankton bloom development. Our findings imply that changes in pCO2, although reflected by changes in community structure of free-living bacteria, do not directly affect bacterial activity. Furthermore, bacterial activity and dynamics of heterotrophic bacteria, especially of attached bacteria, were tightly correlated to phytoplankton development and, hence, may also potentially depend on changes in pCO2.