We now know that the abundance of free viruses in most marine environments is high. There is still, however, a lack of understanding of their occurrence and distribution and of in situ relationships between viral and host communities in natural environments. This may be partly due to methodological limitations. Our main aim was therefore to perform a case study in which a variety of methods were applied in order to give an improved, high-resolution description of the microbial communities in a natural environment. In order to do this we combined light microscopy (LM), transmission electron microscopy (TEM), flow cytometry (FCM), PCR denaturing gradient gel electrophoresis (PCR-DGGE) and pulsed-field gel electrophoresis (PFGE) and studied the diversity and succession of algae, bacteria and viruses in a nutrient enriched seawater enclosure. In the enclosure we experienced a situation where the development of the dominating algal population, which consisted of several flagellate species, was followed by proliferation of several different size-classes of viruses. The total bacterial number decreased markedly during the flagellate bloom but the community composition was maintained and the diversity remained high. Our results indicate a close linkage between various algal, bacterial and viral populations and show that virioplankton do not necessarily terminate algal and bacterial blooms but that they keep the host populations at non-blooming levels.KEY WORDS: Bacteria · DGGE · Diversity · Flow cytometry · Light microscopy · PFGE · Phytoplankton · VirusResale or republication not permitted without written consent of the publisher

Abstract. The ocean's influence on volatile organic compounds (VOCs) in the atmosphere is poorly understood. This work characterises the oceanic emission and/or uptake of methanol, acetone, acetaldehyde, isoprene and dimethyl sulphide (DMS) as a function of photosynthetically active radiation (PAR) and a suite of biological parameters. The measurements were taken following a phytoplankton bloom, in May/June 2005 with a proton transfer reaction mass spectrometer (PTR-MS), from mesocosm enclosures anchored in the Raunefjord, Southern Norway. The net flux of methanol was always into the ocean, and was stronger at night. Isoprene and acetaldehyde were emitted from the ocean, correlating with light (r avcorr,isoprene =0.49; r avcorr,acetaldehyde =0.70) and phytoplankton abundance. DMS was also emitted to the air but did not correlate significantly with light (r avcorr,dms =0.01). Under conditions of high biological activity and a PAR of ∼450 µmol photons m −2 s −1 , acetone was emitted from the ocean, otherwise it was uptaken. The inter-VOC correlations were highest between the day time emission fluxes of acetone and acetaldehyde (r av =0.96), acetaldehyde and isoprene (r av =0.88) and acetone and isoprene (r av =0.85). The mean fluxes for methanol, acetone, acetaldehyde, isoprene and DMS were −0.26 ng m −2 s −1 , 0.21 ng m −2 s −1 , 0.23 ng m −2 s −1 , 0.12 ng m −2 s −1 and 0.3 ng m −2 s −1 , respectively. This work shows that compound specific PAR and biological dependency should be used for estimating the influence of the global ocean on atmospheric VOC budgets.

Abstract. We report the transient population dynamic response of the osmotrophic community initiated by a nutrient pulse in mesocosms exposed to different pCO2 levels as well as quantitative variations in phytoplankton and heterotrophic bacteria created by the difference in CO2 exposure. Coastal seawater was enclosed in floating mesocosms (27 m3) and nutrients were supplied initially in order to stimulate growth of microbial organisms, including the coccolitophorid Emiliania huxleyi. The mesocosms were modified to achieve 350 μatm (1×CO2), 700 μatm (2×CO2) and 1050 µatm (3×CO2) CO2 pressure. The temporal dynamics was related to the nutrient conditions in the enclosures. Numerically small osmotrophs (picoeukaryotes and Synechoccocus sp.) dominated initially and towards the end of the experiment, whereas intermediate sized osmotrophs bloomed as the initial bloom of small sized osmotrophs ceased. Maximum concentrations of E. huxleyi were approximately 4.6×103 cells ml−1 whereas other intermediate sized osmotrophs reached approximately twice as high concentrations. Osmotrophic succession pattern did not change, and we were not able to detect differences with regard to presence or absence of specific osmotrophic taxa as a consequence of altered atmospheric CO2 concentration. Quantitative effects on the microbial communities associated with the CO2 treatment were, however, observed towards the end of the experiment.

Abstract. The potential impact of seawater acidification on the concentrations of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP), and the activity of the enzyme DMSP-lyase was investigated during a pelagic ecosystem CO 2 enrichment experiment (PeECE III) in spring 2005. Natural phytoplankton blooms were studied for 24 days under present, double and triple partial pressures of CO 2 (pCO 2 ; pH=8.3, 8.0, 7.8) in triplicate 25 m 3 enclosures. The results indicate similar DMSP concentrations and DMSPlyase activity (DLA) patterns for all treatments. Hence, DMSP and DLA do not seem to have been affected by the CO 2 treatment. In contrast, DMS concentrations showed small but statistically significant differences in the temporal development of the low versus the high CO 2 treatments. The low pCO 2 enclosures had higher DMS concentrations during the first 10 days, after which the levels decreased earlier and more rapidly than in the other treatments. Integrated over the whole study period, DMS concentrations were not significantly different from those of the double and triple pCO 2 treatments. Pigment and flow-cytometric data indicate that phytoplanktonic populations were generally similar between the treatments, suggesting a certain resilience of the marine ecosystem under study to the induced pH changes, which is reflected in DMSP and DLA. However, there were significant differences in bacterial community structure and the abundance of one group of viruses infecting nanoeukaryotic algae. The amount of DMS accumulated per total DMSP or chlorophyll-a differed significantly between the present and future scenarios, suggesting that the pathways for DMS production or bacterial DMS consumption were affected by sea-

Abstract. The ocean's influence on volatile organic compounds (VOCs) in the atmosphere is poorly understood. This work characterises the oceanic emission and / or uptake of methanol, acetone, acetaldehyde, isoprene and dimethyl sulphide (DMS) as a function of photosynthetically active radiation (PAR) and a suite of biological parameters. The measurements were taken following a phytoplankton bloom, in May/June 2005 with a proton transfer reaction mass spectrometer (PTR-MS), from mesocosm enclosures anchored in the Raunefjord, Southern Norway. The net flux of methanol was always into the ocean, and was stronger at night. Isoprene and acetaldehyde were emitted from the ocean, correlating strongly with light (ravcorr, isoprene=0.49; ravcorr, acetaldehyde=0.70) and phytoplankton abundance. DMS was also emitted to the air but did not correlate significantly with light (r avcorr, dms= 0.01). Under conditions of high biological activity and a PAR of ~450 µmol photons m−2 s−1, acetone was emitted from the ocean, otherwise it was uptaken. The mean fluxes for methanol, acetone, acetaldehyde, isoprene and DMS were −0.26 ng m−2 s−1, 0.21 ng m−2 s−1, 0.23 ng m −2 s−1, 0.12 ng m−2 s−1 and 0.3 ng m−2 s−1 respectively. This work shows that compound specific PAR and biological dependency should be used for estimating the influence of the global ocean on atmospheric VOC budgets.