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

Sedimentary DNA, RNA and ATP concentrations were measured at six stations o n a tidal flat in the German Wadden Sea from April to December 1995 to dete rmine 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 directi on. DNA and ATP concentrations were converted into carbon equivalents using literature coefficients and considered to represent total microbial biomas s and viable microbial biomass, respectively. The difference between C-DNA and C-ATP was considered as detrital biomass. C-DNA and detrital C-DNA biom ass both increased from spring to summer and decreased in autumn to reach l owest 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 secon dary peak in summer. Regression analysis yielded a highly significant relat ion 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. T he relation between silt and microbial biomass in combination with the decr ease of the silt content with distance from the mussel bed indicate the imp ortance of mussel beds for the microbial activity on the tidal flat, most l ikely through deposition of feces and pseudofeces. The low silt content and microbial biomass in winter are attributed to resuspension events. The zer o intercepts of the regressions between DNA and ATP or RNA suggest that ext racellular DNA is absent in our samples. Hence, detrital C-DNA biomass in o ur 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 ex pressed 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 bein g the complement of the detrital C-DNA, percentage shows a steady decrease with time and with diminishing distance towards the mussel beds. The role o f a mussel bed as a biosedimentary system influencing spatial and temporal trends in specific activity is discussed.