Seasonal changes in sedimentary microbial communities of two eutrophic bays as estimated by biomarkers

Abstract

Seasonal changes in sedimentary microbial communities were investigated using biomarker analysis from samples collected in spring and autumn 1995 from Hiroshima Bay and Suo Nada of the Seto Inland Sea of Japan. Surface sediment samples were analysed for several sediment parameters and phospholipid ester-linked fatty acids (PLFA). Total PLFA concentration, which is indicative of microbial biomass, was high in autumn and low in spring in Hiroshima Bay but no such significant seasonal difference of PLFA concentration was observed in Suo Nada. PLFA composition of the sediments were dominated by saturated PLFA, branched PLFA and monounsaturated PLFA. Regional variation in the PLFA composition was less pronounced than the seasonal variation. The presence of the characteristic polyunsaturated fatty acids (PUFA) revealed that microeukaryotes were always present in small proportions. The aerobic prokaryotes and eukaryotes group were predominant in spring, whereas in autumn, the Gram-positive bacteria group was predominant. Monounsaturated PLFA/branched PLFA ratio was high in spring and low in autumn, indicating the relative dominance of these biomarker fatty acids. Significant seasonal differences of sediment parameters, PLFA groups and microbial groups were observed in Hiroshima Bay. In Suo Nada, these parameters showed similar patterns to Hiroshima Bay but the differences of most of the parameters were insignificant. Furthermore, principal components analysis (PCA) revealed a significant shift in microbial community structure between the two seasons, indicating a clear response of microbial communities to changes in sediment parameters.