Relations between active bacteria and heterotrophic potential in the sea

Measurements of heterotrophic substrate uptake and turnover rates of water-soluble substances suggest that these parameters are closely related to the degree of eutrophication of a water body. Respiration ratios (i.e. the respired fraction of the gross uptake) of organic substances by heterotrophic bacteria seem to be very similar under different conditions. The efficiency of incorporation is considerably higher than in other organisms (e.g. 44% for aspartic acid, 72% for glucose). Organic substrates excreted by phytoplanktonic algae are a food source for the heterotrophs. Autoradiographic experiments demonstrate that mainly heterotrophic bacteria are responsible for the substrate uptake from the water, though common and dominant algal species in brackish water also show slight substrate uptake even at nearly natural substrate concentrations (30 μg C l-1). Free living bacteria, not forming colonies on routine agar media but actively metabolizing organic substrates, are predominant inhabitants of offshore marine regions. Average annual numbers of active bacteria in the western Baltic vary from 945000 ml-1 in polluted inshore to 450 000 ml-1 in unpolluted offshore waters. In summer up to 56% of the total bacterial flora is metabolically active, whereas in winter sometimes only about 10% shows detectable substrate uptake in autoradiographic experiments. Colony forming bacteria range from 0.01% to 12.5% of the actively metabolizing bacterial population. The predominant active bacterial flora is well-adapted to the seasonal changes of water temperature and the uptake velocity of tritiated substrates is correlated with the number of active cells. (Auth.)