Production and release of bacterial capsular material and its subsequent utilization by marine bacterioplankton

Bacterioplankton not only take up dissolved organic carbon (DOC) and convert it into biomass and CO2, but also release DOC into the water column. Because bacterioplankton represent the largest living surface in the world's oceans, and most intact bacterioplankton have a capsular envelope, we tested the hypothesis that most of the DOC released by bacterioplankton into the water column is derived from capsular material. Therefore, the bacterial uptake of radiolabeled glucose was differentiated between incorporation into intracellular and capsular pools to obtain production estimates for capsular material. Release of the radiolabeled material into ambient water was followed and its potential as a carbon source for bacterioplankton determined. Of the radiolabed organic carbon detected in bacterioplankton, similar to 55% was incorporated into intracellular material and 45% into capsular material. No significant difference was found between seawater cultures grown under P-limited and balanced nutrient conditions. After transfer of the radiolabeled bacteria into aged seawater, bacterial-derived, radiolabeled DOC was released into the ambient water at a rate of similar to 15 amol C cell(-1) h(-1), which corresponds to similar to 25% of the respired C. Incorporation and respiration rates of this bacterial-derived DOC by bacterioplankton were at least three orders of magnitude lower than the corresponding rates for glucose uptake. Incorporation of bacterial-derived DOC was only detectable when additional inorganic nutrients were added. Thus, we have evidence that bacterioplankton are constantly renewing parts of the capsule by releasing this material into ambient water. The release rate of capsular material represents similar to 25% of the bacterial respiration rate and suggests that a considerable portion of the oceanic DOC pool should consist of "semi-labile" bacterial-derived DOC.