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Dissolved organic carbon (DOC) production by microbial populations was measured at 19 stations in the Atlantic Ocean to quantify the fraction of photoassimilated carbon that flows through the dissolved organic pool at basin scale and to assess the relationship between the percentage of DOC production, phytoplankton size structure, and rates of net community production. Experiments were conducted during four cruises carried out between May 1998 and October 1999, covering three upwelling regions: Benguela (SW Africa), Mauritania (NW Africa) and NW Spain, and the oligotrophic North Atlantic subtropical gyre between 30ЊN and 36ЊN. Photic zone integrated particulate organic carbon (POC) production rates ranged from 10 to 1,178 mg C m Ϫ2 h Ϫ1, thus covering a wide productivity spectrum. The percentage of DOC production with respect to total integrated primary production ranged from 4 to 42%, being larger in oligotrophic, picoplankton-dominated waters, where a balanced metabolism of the microbial community was observed, than in productive, net autotrophic waters, where large-sized cells formed the bulk of the phytoplankton biomass. A highly significant relationship was calculated between DOC and POC production rates in upwelling conditions. By contrast, the relationship between these variables in oligotrophic environments was weak, which suggests that different processes could be controlling the release of dissolved organic matter in productive and unproductive waters.Dissolved organic matter (DOM) is one of the least understood pools of marine matter and represents a major reservoir of organic carbon in the ocean. A large fraction of the dissolved organic carbon (DOC) present in the ocean ultimately derives from primary producers. However, a great deal of controversy still exists on the ecological significance and the ultimate control of DOC production in the ocean.The magnitude of DOC-related fluxes remains still largely uncertain, especially in oligotrophic regions. The high rates of DOC uptake by heterotrophic bacteria in relation to primary production recently measured in unproductive waters (e.g., Hansell et al. 1995;del Giorgio et al. 1997), largely justifies the growing biogeochemical interest of measuring and modeling DOC production in planktonic ecosystems.Initially, the radioactive carbon method for primary production estimation was modified for the measurement of direct excretion of dissolved organic compounds from algal cells. More recently, it has been recognized that several processes, besides direct excretion from intact algal cells, are 1 Corresponding author (eteira@uvigo.es). AcknowledgmentsWe thank G. Tilstone, B. Mouriño, C. Cariño, and C. Robinson for their contributions to the collection of data. Thanks to P. J. le B. Williams for the generous loan of analytical equipment used on the AMT-6 cruise. We are indebted to the captain and crew of research vessels, as well as to all the colleagues on board during the four cruises. We appreciate the comments of two anonymous referees, which improv...

A knowledge of the balance between plankton gross primary production (GPP) and community respiration (CR) in the open ocean is vital to the accurate determination of the global carbon cycle, yet the paucity of open ocean measurements severely limits our understanding. This study measured GPP, net community production, dark CR, and size-fractionated primary production in the upper 200 m of a 12,100 km latitudinal (32ЊS-48ЊN) transect in the Eastern Atlantic Ocean during May and June 1998. This comprehensive data set, which spans five contrasting plankton regimes, including two open ocean oligotrophic provinces, is used to derive a GPP : CR relationship, which suggests that net heterotrophy (GPP Ͻ CR) prevails in the eastern Atlantic when primary production falls below ϳ100 mmol O 2 m Ϫ2 d Ϫ1 . The predictive capability of this relationship is compared with that of the only other published relationship based on similar methodologies and is found to give a more representative description of the autotrophic (GPP Ͼ CR) to heterotrophic seasonal cycle in the Bay of Biscay. This improved predictive power is attributed to the increased representativeness of the current data set. Specifically, the interpretation suggests that the influence of community structure on net ecosystem metabolism implies that prediction of GPP : CR balances in pelagic ecosystems can be best achieved by use of a data set that covers a wide range of community structure and not only a wide range in the magnitude of primary production.

Gross oxygen production (GP), dark respiration (DR) and net community production (NCP) were studied for 16 mo in the euphotic layer of 3 stations through the coastal transitional zone of the southern Bay of Biscay, and related to hydrographic and nutrient conditions, phytoplankton biomass and C incorporation. Microbial O2 fluxes exhibited seasonal patterns linked to the seasonal cycle of water column stratification and mixing, with positive NCP during the spring, negative throughout the summer and close to zero in winter. This pattern was altered at coastal regions, where productive periods were linked to coastal upwelling, whereas in winter persistent net heterotrophy was measured, presumably in relation to increases in organic matter discharge of continental origin. The comparison of NCP with O2 anomaly and No3 concentration in the euphotic zone, the spatial and temporal scales studied and the prevalence of steady-state conditions offshore support the conclusion that the maintenance of summer heterotrophy in the region was based upon the consumption of the surplus of organic matter produced in spring. The uncoupling in the microbial auto-and heterotrophic metabolisms, based on the accumulation and delayed consumption of dissolved organic matter as a consequence of the processes controlling phytoplankton growth and microbial heterotrophic activity in temperate seas, would explain such a pattern. The close relationshp observed between the seasonal variability in NCP and the magnitude of spring net production and predictions derived from the seasonal cycles of O2 anomaly in middle latitudes and atmospheric O2 led us to conclude that the seasonal compensation of production and respiration processes is a characteristic of the dynamics of the pelagic ecosystem, at least in coastal temperate seas. The implications of this conclusion are of great relevance for the interpretation of new production and the estimation of the trophic status of the ocean from direct measurements of plankton net production.

Contrasting hydrographic regimes were studied in the NW Iberian coastal transition zone in order to gain understanding of the relationship between planktonic community structure, production and loss rates, by concurrently measuring size-fractionated phytoplankton biomass and carbon incorporation as well as dissolved organic carbon production and dark respiration by microbial communities. Sampling was carried out in August 1998 and October 1999 at a series of stations representing vertical stratification and coastal upwelling conditions in summer, and vertical mixing and shelf-break poleward flow situations during the autumn. A close relationship was found between size-fractionated phytoplankton biomass and production, the relative allocation of total photosynthesis to dissolved and particulate organic carbon fractions, and the balance between production and respiration. Picoplankton-dominated communities showed net heterotrophic metabolism and were associated with relatively high rates of DOC production with respect to total carbon incorporation (>15%). In contrast, in coastal upwelling stations, where > 2 µm cells dominated, less than 10% of total fixed carbon flowed to DOC, and the net metabolism of the microbial plankton community was autotrophic.

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Experimental results related to the effects of ocean acidification on planktonic marine microbes are still rather inconsistent and occasionally contradictory. Moreover, laboratory or field experiments that address the effects of changes in CO 2 concentrations on heterotrophic microbes are very scarce, despite the major role of these organisms in the marine carbon cycle. We tested the direct effect of an elevated CO 2 concentration (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO 2 fixation and respiration) of 2 isolates belonging to 2 relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high p CO 2 did not negatively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO 2 (pCO 2 ) caused, in both cases, higher rates of CO 2 fixation in the dissolved fraction and, in the case of MED217, lower respiration rates. Both responses would tend to increase the pH of seawater acting as a negative feedback between elevated atmospheric CO 2 concentrations and ocean acidification. KEY WORDS: Bacterial metabolism · Flavobacteriaceae · Ocean acidification · RhodobacteraceaeResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 453: 27-36, 2012 on marine bacterial isolates (Takeuchi et al. 1997, Labare et al. 2010. The latter studies found a decrease in the production and growth rates at pH < 7 -values far from the usual pH observed in ocean waters under present or future scenarios of elevated p CO 2 .Most microorganisms, particularly heterotrophic bacteria, are able to assimilate CO 2 as part of their metabolism through anaplerotic reactions (Roslev et al. 2004). Although light-independent or dark CO 2 assimilation has been usually assumed to be insignificant in oxygenated marine waters, a recent work by Alonso-Sáez et al. (2010) suggests that the global relevance of this process could have been underestimated. Those results show for the first time that high ambient CO 2 concentrations could stimulate CO 2 fixation rates by increasing the CO 2 flux into the cells.A comprehensive understanding of the effect of elevated CO 2 concentration on carbon cycling in the ocean requires the analysis of both production and respiration rates to provide a total carbon budget. However, to the best of our knowledge none of the published studies have simultaneously addressed the effect of CO 2 on BP and respiration, which are essential variables for bacterial growth efficiency (BGE) calculations. Allgaier et al. (2008) did find changes in bacterial taxonomic composition in response to high CO 2 concentrations, which suggest that the effects of elevated p CO 2 are likely to vary among species. Therefore, the aim of the present study was to test the direct effect of elevated CO 2 concentrations (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO 2 fixation and respiration...