Long-term effects of lowered pH on marine periphyton communities

The phenomenon of Ocean Acidification has been identified as a potential threat to several marine organisms and might lead to adverse disturbances of marine ecosystems. Although the knowledge about effects of rising acidity in the oceans is increasing for some species, the knowledge of these effects on the community level of biological complexity is very scarce. Still, community-level effect indicators are needed in order to predict direct and indirect effects of Ocean Acidification on marine ecosystems. In a community the organisms live in their realized niche with important ecological interactions (e.g. competition, grazing and predation) present. This gives community approaches in ecotoxicological tests high ecological relevance. Since lowered pH might eliminate species or strains that are sensitive to such stress, and select for the ones that are more competitive under this condition, a community approach have the potential to detect any pH-induced change in community structure or function. We have used marine periphyton communities in a long-term study of effects of increased partial pressure of CO2 in the water and the accompanied lowering of pH. Periphyton was allowed to colonize and grow on glass substrata for 3.5 weeks in flow-through microcosms. The pH in the microcosms was either that of the incoming natural surface water or was manipulated by bubbling of CO2 down to approximately 7.7. We used Pulse Amplitude Modulation (PAM) to detect effects on photosynthetic electron transport and estimate induced community tolerance to low pH. In addition, we used confocal microscopy to trace effects on biofilm architecture and biofilm thickness. We have also estimated microbial diversity and composition of trace elements in periphyton using PCR-TGGE and X-ray fluorescence respectively. First results indicate that long-term effects of lowered pH change the capacity of communities to tolerate further changes in pH, making them more sensitive to pH-stress. However, no long-term effects on photosynthetic electron transport or trace element composition was detected.