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Adequate bacterial respiration (BR) estimations are necessary to understand the flow of carbon through marine plankton food webs. A considerably higher bacterial contribution to total microbial plankton community respiration (CR) has been observed in oligotrophic systems compared to more productive systems. Classical BR estimation procedures, typically oxygen consumption mea- surements, comprise pre-incubation filtration to separate bacteria from the rest of the plankton community and long incubation times (24 h). The resulting disruption of the community linkages over long time periods might affect BR measure- ments, especially in oligotrophic systems charac- terized by tightly coupled microbial food webs. In this study, BR data were compiled from 2 con - trasting environments: the highly productive Ria de Vigo (NW Spain) and the North Atlantic oligotrophic gyre. Standard procedures or in vivo electron transport system (ETS) activity (non pre- filtered and short-time incubation) procedures were used to obtain a total of 209 BR estimations. Bacterial contribution to plankton CR was signifi- cantly higher in oligotrophic than in highly pro- ductive conditions (155% and 42%, respectively) when using standard procedures, while similar values were observed (31% and 30%) with in vivo ETS procedures. The relation between plankton CR and BR along the studied productivity gradient suggests that bacterial contribution to total CR varies less than previously assumed with an aver- age value approximating 30% through different trophic situations.

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 CO2 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 CO2 concentration (1000 ppmv) on the biomass and meta- bolic rates (leucine incorporation, CO2 fixation and respiration) of 2 isolates belonging to 2 rele- vant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high pCO2 did not nega- tively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO2 (pCO2) caused, in both cases, higher rates of CO2 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 CO2 concentrations and ocean acidification.

17 páginas, 7 figuras, 2 tablas, The short-termeffects of inorganic N and P (nitrate, ammonium, phosphate) and organic C and N (glucose, amino acids) input, added separately as well as jointly, on microplankton community structure and metabolism were studied in 6 microcosm experiments conducted from February to July 2008 in a eutrophic coastal embayment in NW Spain under contrasting hydrographic conditions. The responses of planktonic communities were highly variable. Strong positive effects of combined inorganic and organic nutrient enrichments for phytoplankton and heterotrophic bacteria were found. In most experiments enhanced phytoplankton biomass and productivity (primary production increased up to 5.5-fold) was measured after mixed (inorganic plus organic) additions but phytoplankton responded only in one of the experiments after inorganic additions. Heterotrophic bacteria responded faster than phytoplankton to the additions but were never affected by inorganic nutrient additions, suggesting a C limitation of bacterial activity in this coastal environment. Heterotrophic bacteria biomass, productivity, and respiration responded to mixed additions (containing C, N, and P) in all the experiments (bacterial production increased from 14- to 34-fold) whereas its response to organic (containing C and N) additions greatly differed among experiments (bacterial production increased from 0.6- to 21-fold), which could be partially related to P availability. A general increase in bacterial growth efficiency occurred only after mixed additions (up to 2-fold). The tendency towards heterotrophy of the microbial plankton community was weaker after the mixed (Production/Respiration (P/R) ratio changes from 0.34- to 1.6-fold) than after the organic additions (P/R changes from 0.16- to 0.8-fold). Our results suggest that phytoplankton requirememnts for organic nutrients for growth and P deficiency of bacterial populations intermittently occur in this coastal area., This research was supported by the Xunta de Galicia through projects AddEx (grant PGIDIT06PXIB312222PR) and IMAN (grant 07MMA002402PR). S.M.-G. was funded by an FPU MEC fellowship. E.T. was funded by a Juan de la Cierva and a Ramón y Cajal-MEC contract. C.L. was funded by a fellowship from the Early Stage Training site ECOSUMMER (MEST-CT-2004-0205019) and the Carlsberg Foundation.