CYCLING OF MARINE DISSOLVED ORGANIC-MATTER .1. AN EXPERIMENTAL SYSTEM

The degradation of dissolved organic carbon (DOC), together with bacte rial growth and mortality, was studied in a multi-stage flow-through s ystem. Particle-free sterile seawater was used as growth substrate and fed into vessels containing bacteria and viruses with or without flag ellates. The residence time spanned from 30 to 1800 h. The rate of net organic carbon consumption decreased 3 orders of magnitude with incre asing residence time (from 0.45 to 0.001 mu M C h(-1)). Interestingly, we found a significantly higher DOC consumption in the presence of fl agellates (1.3 to 10 times higher). Viruses were abundant in all cultu re vessels (1.7 to 6.5 x 10(10) l(-1)), indicating active virus produc tion within the system. The number of nucleoid-containing bacteria dec reased with increasing residence time from 68% of the total count at t he shortest residence time to 30% at the longest residence time. Also, with bacteria only, the total number of bacteria decreased at the lon gest residence time. We argue that with a long residence time the inte rnal cycling of organic matter increased and that the fraction of viab le bacteria decreased substantially resulting in a large fraction of ' ghosts'.