Description: With the accumulation of anthropogenic carbon dioxide (CO2), a proceeding decline in seawater pH has been induced that is referred to as ocean acidification. The ocean's capacity for CO2 storage is strongly affected by biological processes, whose feedback potential is difficult to evaluate. The main source of CO2 in the ocean is the decomposition and subsequent respiration of organic molecules by heterotrophic bacteria. However, very little is known about potential effects of ocean acidification on bacterial degradation activity. This study reveals that the degradation of polysaccharides, a major component of marine organic matter, by bacterial extracellular enzymes was significantly accelerated during experimental simulation of ocean acidification. Results were obtained from pH perturbation experiments, where rates of extracellular alpha- and beta-glucosidase were measured and the loss of neutral and acidic sugars from phytoplankton-derived polysaccharides was determined. Our study suggests that a faster bacterial turnover of polysaccharides at lowered ocean pH has the potential to reduce carbon export and to enhance the respiratory CO2 production in the future ocean.

Description: As the atmospheric CO2 concentration rises, more CO2 will dissolve in the oceans, leading to a reduction in pH. Effects of ocean acidification on bacterial communities have mainly been studied in biologically complex systems, in which indirect effects, mediated through food web interactions, come into play. These approaches come close to nature but suffer from low replication and neglect seasonality. To comprehensively investigate direct pH effects, we conducted highly-replicated laboratory acidification experiments with the natural bacterial community from Helgoland Roads (North Sea). Seasonal variability was accounted for by repeating the experiment four times (spring, summer, autumn, winter). Three dilution approaches were used to select for different ecological strategies, i.e. fast-growing or low-nutrient adapted bacteria. The pH levels investigated were in situ seawater pH (8.15–8.22), pH 7.82 and pH 7.67, representing the present-day situation and two acidification scenarios projected for the North Sea for the year 2100. In all seasons, both automated ribosomal intergenic spacer analysis and 16S ribosomal amplicon pyrosequencing revealed pH-dependent community shifts for two of the dilution approaches. Bacteria susceptible to changes in pH were different members of Gammaproteobacteria, Flavobacteriaceae, Rhodobacteraceae, Campylobacteraceae and further less abundant groups. Their specific response to reduced pH was often context-dependent. Bacterial abundance was not influenced by pH. Our findings suggest that already moderate changes in pH have the potential to cause compositional shifts, depending on the community assembly and environmental factors. By identifying pH-susceptible groups, this study provides insights for more directed, in-depth community analyses in large-scale and long-term experiments.

Description: Marine picophytoplankton, in particular cyanobacteria and prochlorophytes are ubiquitously distributed in the worlds oceans and are often especially in the more oligotrophic regions among the most important primary producers. The present work shows the results of in-situ measurements taken on meridional transects across the Atlantic Ocean. Data for phytoplankton pigment composition and particulate absorption give information about the functional groups found in the study area. Flow cytometry and an analysis of phycobilins were employed to encompass genera of prochlorophytes and cyanobacteria. These in situ measurements are used as ground-truth for the validation of hyper-spectral remote sensing data. High spectrally resolved satellite data from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) on ENVISAT and GOME-2 (Global Ozone Measurement Experiment 2) on Meteosat are analysed with Differential Optical Absorption Spectroscopy (DOAS) and combined with the in situ data to improve determinations of phytoplankton functional groups distribution. http://www.sgmeet.com/aslo/nice2009/viewabstract2.asp?AbstractID=5210

Description: Marine picophytoplankton, in particular cyanobacteria and prochlorophytes are ubiquitously distributed in the worlds oceans and are often especially in the more oligotrophic regions among the most important primary producers. The present work shows the results of in-situ measurements taken on three meridional transects across the Atlantic Ocean. Data for phytoplankton pigment composition and particulate absorption give information about the functional groups found in the study area. Flow cytometry and an analysis of phycobilins were employed to encompass genera of cyanobacteria such as Synechococcus and Trichodesmium. The aim is to improve 1) estimates of global marine primary production and 2) determinations of the distribution of major phytoplankton functional groups, by using hyper-spectral remote sensing data in combination with those in-situ measurements. http://oceanopticsconference.org/abstracts/all

Description: Microbiological processes play a major role in biogeochemical cycling of organic matter in the ocean. However, little is known about the direct effects of ocean acidification on heterotrophic activity and, hence organic matter cycling through the microbial loop. To elucidate whether future ocean acidification will directly affect microbial organic matter degradation, a study was conducted examining the impact of decreasing pH on bacterial hydrolytic enzyme activities. Different acid treatments were used to induce different pCO2 concentrations in offshore mesocosms during a series of short time experiments. The study was carried out in the Baltic Sea in July 2007 in the frame of the SOPRAN (Surface Ocean PRocesses in the ANthropocene) project. Based on kinetic measurements, maximum turn-over rates (Vmax) and half-saturation constant (Km) were calculated for glucosidases, peptidases and phosphatase. A decrease in pH led to a significant increase in Vmax of carbohydrates accompanied by a decrease of peptide degradation. Possible implications of this short time changes in heterotrophic organic matter turnover due to acidification will be discussed.

Description: Transparent exopolymer particles (TEP) form from dissolved precursors and significantly contribute to the pool of particulate organic carbon in the ocean. In addition, TEP are an important structural component since they provide attachment sites for microbes on a nanometer to micrometer scale. To investigate the effect of ocean acidification on the concentration and dynamics of TEP, we conducted a series of experimental studies in the Baltic Sea in summer 2007 within the frame of the SOPRAN (Surface Ocean PRocesses in the ANthropocene) project. At this time diazotrophs were the main primary producers. Thus, the relation between autotrophic N2-fixation and heterotrophic activity (uptake of radiolabeld Leucine) was determined and compared with TEP concentration measurements to elucidate how production and fate of TEP may be altered due to short term responses to acidification.We observed that the amount of TEP as well as microbiological activities were sensitive to changes in pCO2. Our results indicate a decrease of TEP concentration with increasing pCO2 under net heterotrophic conditions. Furthermore, significant correlations between TEP concentration and bacterial abundance suggest a tight coupling between the dynamics of acidic carbohydrates and bacteria dynamics.Our results imply that ocean acidification could potentially affect microbial carbon turn-over and, hence, organic matter cycling in the ocean.

Description: In July 2007 free floating mesocosm with app. 60m³ of volume were deployed in the southern Baltic Proper for app. two weeks. A gradient of increasing pCO2 concentrations was adjusted starting with the present concentration of 312 µatm up to 1600µatm as highest concentration. The response of the autotrophic plankton community was recorded in terms of nitrogen fixation rates. Heterotrophic activity was calculated based on the uptake of radiolabeled Thymidine. There was no clear increase of nitrogen fixation rates with higher pCO2. It rather seemed that the colonial species 〉10µm suffered under decreasing pH while the size fraction 〈10µm did not show any significant response In contrast to that cell specific bacterial biomass production sharply decreased within the first 24 hours after acidification but showed increasing activity with increasing CO2 concentration for the rest of the experiment. . A daily cycle of N-fix rates was only visible in the mesocosm with no pCO2 changes which also had highest N-fixation rates of up to 1.4nmol N/L/h.Our results indicate decoupling of primary and secondary production under increasing CO2 concentrations.