Bacterioplankton not only take up dissolved organic carbon (DOC) and convert it into biomass and CO*, 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, -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 -15 amol C cell-l h-l, which corresponds 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 -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.

It has recently been shown that marine bacterioplankton release copious amounts of capsular material as 'semi-labile' to 'refractory' dissolved organic carbon (DOC) into the ambient water. The fate of this bacterioplankton-derived DOC remains largely unknown. Here we investigate the capability of this bacterial-derived capsular DOC to coagulate to exopolymer particles under contrasting turbulence reglmes. Under high turbulence, fewer but larger particles (>2 pm In diameter) were detected, while the total exopolymer particle-mass (>0.2 pm) was higher under stagnant conditions. Under stagnant conditions most of the bacterial-derived particles remained in the size-class between 0.2 and 2 pm. The production rate of exopolymer particles was estimated to amount to about 4 am01 C cell-' h-', representing about 25 % of the previously estimated bacterioplankton DOC release of about 15 am01 C cell-' h-' Considering that bacterioplankton represent the largest living surface in the ocean, the release and subsequent coagulation of bacterioplankton-derived capsular DOC might be an important, thus far largely neglected mechanism of exopolymer particle formation in the ocean.

Little is known about the structure of microbial communities in Sphagnum peatlands, and the potential effects of the increasing atmospheric CO2 concentration on these communities are not known. We analyzed the structure of microbial communities in five Sphagnum-dominated peatlands across Europe and their response to CO2 enrichment using miniFACE systems. After three growing seasons, Sphagnum samples were analyzed for heterotrophic bacteria, cyanobacteria, microalgae, heterotrophic flagellates, ciliates, testate amoebae, fungi, nematodes, and rotifers. Heterotrophic organisms dominated the microbial communities and together represented 78% to 97% of the total microbial biomass. Testate amoebae dominated the protozoan biomass. A canonical correspondence analysis revealed a significant correlation between the microbial community data and four environmental variables (Na +, DOG, water table depth, and DIN), reflecting continentality, hydrology, and nitrogen deposition gradients. Carbon dioxide enrichment modified the structure of microbial communities, but total microbial biomass was unaffected. The biomass of heterotrophic bacteria increased by 48%, and the biomass of testate amoebae decreased by 13%. These results contrast with the absence of overall effect on methane production or on the vegetation, but are in line with an increased below-ground vascular plant biomass at the same sites. We interpret the increase in bacterial biomass as a response to a CO2induced enhancement of Sphagnum exudation. The causes for the decrease of testate amoebae are unclear but could indicate a top-down rather than a bottom-up control on their density.

The dynamics of dissolved organic matter (DOM) and microbial activity were monitored in the coastal North Sea over an annual cycle. DOM accumulated during the spring bloom towards the summer, associated with high phytoplankton extracellular release. Accumulation of dissolved organic carbon (DOC) occurred from April to June. During this period, based on the bacterial carbon demand (BCD) of the free-living bacteria and the photosynthetic extracellular release (PER), 85 µM C were derived from sources other than direct DOC release by phytoplankton. Thereafter, from the end of August until December, the DOM concentrations decreased by about half. During this period, at least 269 µM C was removed from the system via the utilization by the bacterial community and/or sedimentation and export. Overall, our data indicate a pronounced seasonal shift in DOM sources supporting BCD. From spring to summer, BCD is almost fully supported by PER alone while in fall and winter, BCD is supported about equally by PER and by the DOM accumulated in the spring-summer period and originating presumably from a variety of DOM production mechanisms.

We simultaneously assessed the metabolic activity and viability of individual bacterioplankton cells in the coastal and open North Sea. Three different techniques were applied to determine cell features related to the physiological status of the cell. SYBR Green I was used to estimate the nucleic acid content of the cell. Propidium iodide (PI) stains cells with a compromised cell membrane, commonly interpreted as indicative of dead cells. Microautoradiography (MA) with radiolabeled glucose and leucine was applied to indicate metabolically active cells. The relative abundance of metabolically active cells determined by MA was usually < 20% of the total abundance of bacteria. In contrast, the percentage of PI-positive cells in the total bacterial community was generally high (~80%). However, the overwhelming majority (97%) of cells taking up glucose and leucine were also PI-positive. Apparently, the uptake of radiolabeled substrate is related to PI accumulation in cells, indicating that PI is not a reliable stain to indicate non-active or dead bacteria. We suggest that several methods should be combined to assess the physiological status of individual cells in natural bacterioplankton communities.