Nucleic acids and ATP to assess microbial biomass and activity in a marine biosedimentary system

 Sedimentary DNA, RNA and ATP concentrations were measured at six stations on a tidal flat in the German Wadden Sea from April to December 1995 to determine factors controlling microbial abundance and activity. Two stations (1 and 2) were located in a mussel bed (Mytilus edulis) in the middle of the flat, and the other four were arranged in a line following ebb tide direction. DNA and ATP concentrations were converted into carbon equivalents using literature coefficients and considered to represent total microbial biomass and viable microbial biomass, respectively. The difference between C_DNA and C_ATP was considered as detrital biomass. C_DNA and detrital C_DNA biomass both increased from spring to summer and decreased in autumn to reach lowest values in December. The most prominent seasonal increase was observed at the mussel bed stations and two adjacent stations. C_ATP biomass peaked in early spring, especially at the mussel bed stations, and showed a secondary peak in summer. Regression analysis yielded a highly significant relation between the silt content of the sediment and C_DNA and C_ATP. However, C_DNA and C_ATP biomass are not correlated if silt is taken into account. The relation between silt and microbial biomass in combination with the decrease of the silt content with distance from the mussel bed indicate the importance of mussel beds for the microbial activity on the tidal flat, most likely through deposition of feces and pseudofeces. The low silt content and microbial biomass in winter are attributed to resuspension events. The zero intercepts of the regressions between DNA and ATP or RNA suggest that extracellular DNA is absent in our samples. Hence, detrital C_DNA biomass in our samples probably consists of intact but dormant cells. Compared to other marine habitats the percentage of detrital C_DNA biomass on the tidal flat is quite high, i.e. up to >95% of the total C_DNA. However some doubt is expressed about the validity of factors used to convert ATP and DNA to carbon. The proportion of active biomass, expressed by the ratio C_ATP/C_DNA being the complement of the detrital C_DNA percentage shows a steady decrease with time and with diminishing distance towards the mussel beds. The role of a mussel bed as a biosedimentary system influencing spatial and temporal trends in specific activity is discussed. Received: 26 January 2000 / Accepted: 3 July 2000