MICROBIAL COMMUNITY RESPONSES TO NITROGEN-DEFICIENT AND PHOSPHORUS-DEFICIENT NUTRIENT INPUTS - MICROPLANKTON DYNAMICS AND BIOCHEMICAL-CHARACTERIZATION

Natural plankton communities from Masnou, a locality 20 km north of Ba rcelona (NW Mediterranean coast), were enclosed in 301 microcosms to t est the effect of different availability of nitrogen (N) and phosphoru s (P) on the biomass of the main microplankton groups, and the biochem ical composition (DNA, protein and chlorophyll concentration) of micro bial communities. Immediately after enclosure in microcosms, three dif ferent nutrient enrichments were performed:N-deficient, P-deficient an d nutrient-balanced. N and P deficiencies affected the structure and t he biochemical composition of the microbial communities. Phytoplankton assemblages showed similar temporal patterns under the three nutrient treatments, although the relative contribution of the different group s was notably affected. The lowest DNA concentration was measured in t he P-deficient treatment, suggesting that P availability imposes the l imits on the DNA levels in the ecosystem. The availability of N in the P-deficient microcosms allowed relatively high synthesis of chlorophy ll and protein until the end of the experiment. Significantly high chl orophyll:DNA and protein:DNA ratios characterized the P-deficient trea tment (where N was available) compared to the N-deficient microcosms. From the results obtained, we suggest that the protein:DNA ratios may constitute a biochemical indicator of the P versus N availability in n atural ecosystems.