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14 pages, 4 figures, 2 tables.-- This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, Prokaryotes represent a major fraction of marine biomass and play a key role in the global carbon cycle. We studied the vertical profiles (0–3500 m) of abundance, viability, and activity of prokaryotic communities along a productivity gradient in the subtropical and tropical Atlantic to assess whether there is a vertical linkage between surface productivity regimes and deep ocean prokaryotic communities. We found that latitudinal changes in the vertical patterns of cytometric variables were coupled with surface productivity: higher prokaryotic abundances and viabilities, and smaller cell sizes were observed below highly productive surface waters, an effect reaching down to the bathypelagic layer. Leucine uptake rates in deep waters showed no clear relationship with surface productivity. Changes in resource and energy allocation to growth vs. maintenance in hostile environments, cell-size-dependent metabolic requirements, and variability in leucine-to-carbon conversion may all be part of the array of factors involved in controlling the prokaryotic activity patterns that were measured. Our work adds to the recent findings that highlight the importance of vertical connectivity for prokaryotic communities in the dark ocean and unveils a remarkable impact of surface conditions in the viability of deep ocean prokaryotes. This is a key aspect when considering metabolic rates of prokaryotic communities in the bathypelagic realm, This work is a contribution to projects MAFIA (grant number CTM2012-39587-C04-01), FLUXES (grant number CTM2015-69392-C3), e-IMPACT (grant number PID2019-109084RB-C21 and -C22), and INTERES (CTM2017-83362-R) funded by the Spanish “Plan Nacional/Estatal de I+D” and cofounded with FEDER funds, and to project SUMMER (grant number AMD-817806-5) funded by the European Union's Horizon 2020 research and innovation program. Markel Gómez-Letona is supported by the Ministerio de Ciencia, Innovación y Universidades, Gobierno de España (grant number FPU17-01435) during his PhD. Marta Sebastián is supported by the Project MIAU (grant number RTI2018-101025-B-I00) and the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S)

18 pages, 8 figures, 1 table.-- This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY), Marine microorganisms are involved in a variety of biogeochemical cycles and live in diverse ecological communities where they interact with each other and with other organisms to guarantee ecosystem functions. The present study focused on a shallow marine environment located in Ría de Vigo (NW, Spain), where sediment and size-fractionated plankton samples were collected from 2016 to 2018. DNA metabarcoding was used to describe the eukaryote and prokaryote composition and diversity in sediments and plankton and to depict possible associations among the most frequent and abundant organisms by co-occurrence network analysis. High eukaryote and prokaryote diversity indices were obtained in all compartments. Significant differences among eukaryote and prokaryote communities were found between sediment and plankton samples, with a high percentage of exclusive operational taxonomic units (OTUs) associated with each compartment, especially from sediment. Despite these differences, shared taxa between water and sediment were also obtained, suggesting a relatively meaningful exchange of organisms between both environmental compartments. Significant co-occurrences were mainly obtained between prokaryotes (41%), followed by eukaryotes–prokaryotes (32%) and between eukaryotes (27%). The abundant and strong positive correlations between organisms, including representatives from the sediment and the water column, suggested an essential role of biotic interactions as community-structuring factors in shallow waters where beneficial associations likely prevail. This study provides a novel approach for the detailed description of the eukaryote and prokaryote diversity and co-occurrence patterns in a shallow marine area, including both the sediment and different water-size fractions. The high diversity obtained and the detection of predominantly coexisting interactions among organisms from sediment and the overlying water column suggest a movement of species between both habitats and therefore confirm the importance of integratively studying shallow marine ecosystems, This work was supported by the Ministerio de Economía y Competitividad, Spain (CTM2017-83362-R), Consellería de Economía, Emprego e Industria–GAIN, Xunta de Galicia (IN607B 2019/01), Fondo Europeo de Desarrollo Regional FEDER en el marco del programa Interreg V A España – Portugal (POCTEP) 2014-(20200474_BLUEBIOLAB), VIVALDI [678589] (EU H2020), and Controlling Microbiomes Circulations for Better Food Systems” (CIRCLES) [818290] (EU H2020). RR-C wishes to thank the Axencia Galega de Innovación (GAIN, Xunta de Galicia) for her predoctoral contract (IN606A-2018/020)

The responses of coastal microbial communities to nutrient inputs depend on the magnitude and nature of those inputs. To understand the response of prokaryote communities to contrasting levels of nutrient inputs from different sources, three nutrient addition experiments were performed in the Ría de Vigo in spring, summer, and autumn. The experiments consisted of controlled inorganic (nitrate, ammonium and phosphate), organic (glucose and a mix of 18 amino acids), and mixed (inorganic and organic); as well as natural (atmospheric and riverine) nutrient additions to surface water samples taken from the middle of the embayment and incubated for 48h, under in situ temperature and irradiance conditions. Prokaryote community DNA was collected and analyzed using high-throughput amplicon sequencing of the 16S rRNA gene. The joint addition of inorganic and organic nutrients provoked the most pronounced changes in community composition, systematically leading to great increases in the proportion of sequence reads belonging to Vibrionales, while natural amendments only caused subtle alterations. The proportion of reads assigned to the Planktomarina genus, bacteria belonging to the order Rhodobacterales, tended to increase when natural atmospheric nutrients were added. Overall, the fraction of reads belonging to the order Flavobacteriales did not show relevant changes, although the contribution of sequences affiliated to Polaribacter and Aurantivirga genera increased after atmospheric and riverine inputs in summer, and autumn. The proportion of Vibrionales reads also increased after controlled organic amendments in autumn, and to a lesser extent after riverine additions in spring and summer. In general, our findings suggest that an increasing input of nitrogenous organic matter from allochthonous sources into this coastal environment might promote the growth of potentially pathogenic bacteria belonging to the order Vibrionales. Financiado para publicación en acceso aberto: Universidade de Vigo/CISUG Ministerio de Ciencia e Innovación | Ref. CTM2011-30155- C03-01 Xunta de Galicia | Ref. EM2013/023

11 pages, 5 figures, 2 tables, supplementary data https://doi.org/10.1016/j.ecss.2019.106565, Organic nutrient loading in coastal waters has considerably increased due to anthropogenic activities over the last decades. In particular, the increase in the proportion of organic vs. inorganic nitrogen may have an impact on microbial plankton communities, particularly within the small size classes, where mixotrophy is likely to be widespread. Here we explore the change in taxonomic composition of small eukaryotes (, This research was supported by the Xunta de Galicia through DIMENSION project (grant EM2013/023) and by Spanish Ministry of Economy, Industry and Competitivity (MINECO) trough ENVISION project (grant CTM2014-59031-P) in collaboration with the “Instituto Español de Oceanografía” (IEO) RADIALES project. Esther Barber-Lluch was funded by an F.P.I. MINECO fellowship. [...] R. Logares was supported by a Ramón y Cajal fellowship (RYC-2013-12554, MINECO, Spain), With the funding support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S), of the Spanish Research Agency (AEI)

10 pages, 7 figures, 2 tables, The chemical composition of the seawater soluble fraction (WSF) of yellow-legged gulls and harbour seal faeces and their impact on microbial plankton communities from an eutrophic coastal area have been tested. After characterisation of the C:N:P stoichiometry, trace metals content and organic molecular composition of the faeces, significant differences between species have been observed in all parameters. Seagull faeces present about three times larger N content than seal faeces and are also richer in trace elements except for Cu and Zn. Organic nitrogen in seagull faeces is dominated by uric acid, while the proteins are the main N source in seal faeces. It is remarkable that seagull faeces are five times more soluble in seawater than seal faeces and present a much higher N content (48.0 versus 3.5 mg N in the WSF per gram of dry faeces), >85% of which as dissolved organic nitrogen, with C:N molar ratios of 2.4 and 13 for seagull and seal faeces, respectively. Based on this contrasting N content, large differences were expected in their impact on microbial populations. To test this hypothesis, a 3-day microcosm incubation experiment was performed, in which coastal seawater was amended with realistic concentrations of the WSF of seagull or seal faeces. A significant and similar increase in bacterial biomass occurred in response to both treatments. In the case of phytoplankton, the impact of the treatment with seagull faeces was significantly larger that the effect of the treatment with seal faeces. Our data suggest that the distinct competitive abilities of phytoplankton and bacteria largely influence the potential impact of distinct animal faeces on primary productivity in coastal ecosystems. Impacts on the microbial plankton communities do not affect only this trophic level, but the whole trophic chain, contributing to nutrient recycling in coastal areas where large populations of these species are settled, This research was partially supported by the Spanish Ministry of Economy and Competitiveness through the project INTERES (CTM2017-83362-R)

Knowledge of the magnitude and variability of plankton respiration is a crucial gap in our understanding of marine carbon cycling. In order to validate the INT reduction method as a proxy for plankton respiration, we have compiled and analyzed a dataset (n = 376) of concurrent measurements of dissolved oxygen consumption (CRO2) and in vivo reduction of 2-para (iodophenyl)-3(nitrophenyl)-5(phenyl) tetrazolium chloride tetrazolium salt (INT) spanning a wide range of oceanic regions and physicochemical conditions. Data were randomly divided into two independent subgroups: two thirds of the data were used to derive a regression conversion between dissolved oxygen consumption and INT reduction (“training” dataset) and one third of the data was used to validate the regression (“test” dataset). There was a significant relationship between the log-transformed dissolved oxygen consumption rates and the log-transformed INT reduction rates (INTT) with the “training” dataset (logCRO2 = 0.72logINTT + 0.44, R2 = 0.69, n = 249, p < 0.001). The estimated oxygen consumption rates obtained applying the former equation were compared to the measured dissolved oxygen consumption rates from the “test” dataset which spans over three orders of magnitude range of respiration rates. There was no significant difference between the measured and estimated dissolved oxygen consumption rates, indicating that the INT reduction method can be used as a proxy for respiration in natural plankton communities. Further analysis dividing the data by temperature and chlorophyll-a concentration, indicated that the predictive capacity of the empirical conversion equation is increased at temperatures >8°C and chlorophyll-a concentrations >0.2 μg L-1 and reduced at lower temperatures and chlorophyll-a concentrations. The results of this study endorse the adequate performance and reliability of the INT method for natural plankton communities.

10 pages, 6 figures, 5 tables, Over the past decades, as a consequence of human activity, there was an increase in nutrient inputs to the ocean and they are expected to enhance even more in the future. Coastal areas, accounting for a significant proportion of marine primary productivity, are the most vulnerable zones to anthropogenic impacts. The response of phytoplankton communities to an increase in organic and inorganic nutrients levels from natural allochthonous sources was assessed in microcosm experiments conducted in a coastal system affected by intermittent upwelling events (Ría de Vigo, NW Iberia). Three nutrient addition experiments were performed in spring, summer and autumn, when surface water was supplemented with 5 and 10% of atmospheric and riverine matter. Pico-, nano- and microphytoplankton abundances, chlorophyll a concentration (Chl a) and primary production rates (PP) were measured and compared with those in the control seawater sample (without additions) after 48 h of incubation. Simultaneous experiments with controlled additions of inorganic and organic nutrients were also performed in order to describe the limiting nutrient for phytoplankton growth at each experiment. The composition of the matter inputs and the structure of the phytoplankton communities determined the type of response observed. Phytoplankton responses varied among seasons, being positively correlated with dissolved inorganic nitrogen (DIN) concentrations. As expected, the phytoplankton responses to external nutrient inputs were stronger under low nutrient levels (summer) than when phytoplankton was already growing in nutrient replete conditions (spring). Null and negative responses to the natural inputs were observed in autumn, which suggests that the oceanic phytoplankton advected to this coastal system during downwelling events could be occasionally inhibited by these nutrient inputs. In a future global change scenario, characterized by enhanced nutrient inputs from riverine and atmospheric origin, the response of phytoplankton communities will strongly depend on the concentration and chemical composition of these inputs and on the structure of phytoplankton communities able to respond to them., This work was funded by the Spanish Ministry of Science and Innovation under the project REIMAGE (CTM2011-30155-C03-03) and the Spanish Ministry of Economy and Competitivity under the project iSmall (CTM2014-56119-R). I.G.T was funded by a Post-doctoral grant by the Portuguese Foundation for Science and Technology (SFRH/BPD/108485/2015) co-financed by national and European funds (POCH programme) and M.N.-C. was supported by the CSIC Program "Junta para la Ampliación de Estudios" co-financed by the ESF (reference JAE DOC 040) and the project FERMIO (MINECO, CTM2014-57334-JIN)

Analysis of seasonal patterns of marine bacterial community structure along horizontal and vertical spatial scales can help to predict long-term responses to climate change. Several recent studies have shown predictable seasonal reoccurrence of bacterial assemblages. However, only a few have assessed temporal variability over both horizontal and vertical spatial scales. Here, we simultaneously studied the bacterial community structure at two different locations and depths in shelf waters of a coastal upwelling system during an annual cycle. The most noticeable biogeographic patterns observed were seasonality, horizontal homogeneity, and spatial synchrony in bacterial diversity and community structure related with regional upwelling–downwelling dynamics. Water column mixing eventually disrupted bacterial community structure vertical heterogeneity. Our results are consistent with previous temporal studies of marine bacterioplankton in other temperate regions and also suggest a marked influence of regional factors on the bacterial communities inhabiting this coastal upwelling system. Bacterial-mediated carbon fluxes in this productive region appear to be mainly controlled by community structure dynamics in surface waters, and local environmental factors at the base of the euphotic zone.

11 páginas, 5 tablas, 5 figuras, The processes mediated by microbial planktonic communities occur along the entire water column, yet the microbial activity and composition have been studied mainly in surface waters. This research examined the vertical variation in bacterial abundance, activity and community composition and structure from surface down to 5000 m depth following a longitudinal transect off the Galician coast (NW Iberian margin, from 43°N, 9°W to 43°N, 15°W). Community activity and composition changed with depth. The leucine incorporation rates decreased from the euphotic layer to the bathypelagic waters by three orders of magnitude, whereas prokaryotic abundance decreased only by one order of magnitude. The relative abundance of SAR11 and Alteromonas, determined by catalyzed reported deposition fluorescence in situ hybridization (CARD-FISH), decreased with depth. Meanwhile, the contribution of SAR 202 and SAR324 was significantly higher in the deeper layers (i.e. NEADW, North East Atlantic Deep Water and LDW, Lower Deep Water) than in the euphotic zone. Bacterial community structure, assessed by Automated Ribosomal Intergenic Spacer Analysis (ARISA), was depth-specific. A distance based linear model (DistLM) revealed that the variability found in bacterial community structure was mainly explained by temperature nitrate, phosphate, dissolved organic matter (DOM) fluorescence, prokaryotic abundance, leucine incorporation and to a lesser extent salinity, oxygen, CDOM absorbance and dissolved organic carbon concentration. Our results displayed a bacterial community structure shaped not only by depth-related physicochemical features but also by DOM quality, indicating that different prokaryotic taxa have the potential to metabolize particular DOM sources, Funding for the sampling and analysis was supported by the Local Research Program of Xunta de Galicia grant “Biodiversidade Funcional do Microplancton nas Profundidades Mariñas de Galicia” (BIO-PROF, Ref. 10MMA604024PR, 2010–2012) and “Relación Entre la Diversidad Funcional del Bacterioplancton y la Circulación de Carbono en el Océano Costero en el Contexto del Cambio Global” (DIFUNCAR, Ref. CTM2008–03790). V. D-A. was funded by a Campus do Mar Master Scholarship. M. N.-C. was funded by the project FERMIO (MINECO, CTM2014-57334-JIN). Hernando-Morales V. was supported by the MICINN program “Formación de Personal Investigador” (FPI), Ref. grant BES-2009–028186

12 páginas, 5 figuras, 2 tablas, Anthropogenic pressures are changing the magnitude and nature of matter inputs into the ocean. The Ría de Vigo (NW Spain) is a highly productive and dynamic coastal system that is likely affected by such alterations. Previous nutrient-addition microcosm experiments conducted during contrasting hydrographic conditions suggested that heterotrophic bacteria are limited by organic carbon (C) and occasionally co-limited by inorganic nutrients in this coastal area. In order to assess short-term responses in biomass, production, and respiration of heterotrophic bacteria from the Ría de Vigo to increasing amounts of natural inputs of matter, we conducted 6 microcosm experiments, wherein surface seawater collected in spring, summer, and autumn was mixed with increasing amounts of dissolved natural matter concentrates from riverine and atmospheric origin. Simultaneous experiments with controlled inorganic and/or organic additions indicated that bacteria were co-limited by inorganic nutrients and C in spring and summer and primarily limited by C in autumn. Production responded more than biomass to increasing inputs of matter, whereas respiration did not change. The bacterial production response to increasing dissolved organic C load associated with riverine and atmospheric inputs was strongly related to the relative phosphorus (P) content of the dissolved matter concentrates. Our data suggest that bacterial production might decrease with the increase of P-deficient allochthonous matter inputs, which would have important biogeochemical consequences for C cycling in coastal areas, This research was supported by the Spanish Ministry of Science and Innovation through project REIMAGE (grants CTM2011-30155-C03-01 and CTM2011-30155-C03-02) and by the Xunta the Galicia trough project DIMENSION (grant EM2013/023). E. B-L was funded by a F.P.I. MICINN fellowship. M.N.-C. was funded by the CSIC Program "Junta para la Ampliación de Estudios" co-financed by the ESF. S.M.G. was funded by a Marie Curie fellowship

Phytoplankton and bacterioplankton dynamics were studied around South Shetland Islands (Antarctica) with special emphasis on the Drake Passage region, during austral summer, in order to expand our knowledge on the coupling between the autotrophic and heterotrophic microbial plankton compartments in polar ecosystems. In addition, we directly estimated bacterial growth efficiency in the Drake Passage with the aim of better constraining total bacterial carbon utilization in this important polar ecosystem. Integrated chlorophyll-a concentration (21–86 mg m � 2 ), primary production rates (0.7– 19.3 mg C m � 3 d � 1 ) and mean water-column photochemical efficiency (0.24–0.60) were significantly correlated with Si n tracer (r 2 ¼ 0.55, 0.46 and 0.64, respectively), which indirectly points to iron as the

The input of allochthonous matter of continental origin to coastal zones globally may cause changes in the activity (function) as well as the taxonomic composition (structure) of the micro- bial plankton community. The goal of the present study was the simultaneous analysis of microbial plankton community structure (size-fractionated phytoplankton biomass, bacterial community com- position) and function (particulate and dissolved primary production, bacterial production, microbial plankton community respiration) in the northwest Iberian coastal transition zone during a dry (Feb- ruary 2005) and a rainy (October 2005) period. An influence of freshwater input was observed in October, even at an offshore site, but not in February. We found an autotrophic community domi- nated by picophytoplankton during both sampling periods. In contrast, the bacterial groups Beta- and Gammaproteobacteria were significantly more abundant during the high precipitation period. Pri- mary production rates were low and similar during both sampling periods; however, bacterial pro- duction was 8-fold and community respiration was 3-fold higher in October than in February. Conse- quently, the microbial community metabolism was net autotrophic in February and net heterotrophic in October. The high precipitation and the significant presence of bacteria belonging to the Betapro- teobacteria, typical for freshwater systems, in October compared to February, strongly suggest an influence of material of continental origin on microbial metabolism in this coastal transition zone.

17 páginas, 8 figuras, 2 tablas, We simultaneously studied microbial-mediated carbon fluxes at 2 contrasting sites within the coastal upwelling system off Galicia, near A Coruña and Vigo, Spain, over an annual cycle in order to compare the fraction of primary production released as dissolved organic carbon (DOC) and the degree of coupling between bacteria and phytoplankton. A significant fraction of primary production was released as DOC at both sites, averaging ~30%. DOC release accounted for, on average, 30% of the total bacterial carbon demand, which is indicative of a moderate trophic dependence of bacteria on phytoplankton in these coastal ecosystems. Nevertheless, differences in hydrographic conditions associated with stronger upwelling pulses off Vigo, and deeper upper mixed layers during the downwelling period off A Coruña, led to significant differences in phytoplankton dynamics and the subsequent direct coupling with heterotrophic bacteria. Strong direct coupling between phytoplankton extracellular release and bacterial production (BP) was found off Vigo, which could be related to the quality of the DOC produced by actively growing phytoplankton. By contrast, DOC release and BP rates were decoupled off A Coruña, likely due to unaccounted DOC associated with indirect trophic processes or to the low availability of freshly produced exudates associated with overflow or photoinhibition mechanisms, This research was supported in part by the MEC project DIFUNCAR (CTM2008-03790/MAR) and by IEO project RADIALES. S.M-G. was funded by a FPU– MEC fellowship. V.H-M. was funded by a FPI– MEC fellowship. E.T. was funded by a Ramón y Cajal-MEC con- tract

11 páginas, 3 tablas, 5 figuras, The impact of rain water collected at marine, urban and rural sites on coastal phytoplankton biomass, primary production and community composition as well as the effect on microbial plankton metabolism was studied in 3 microcosm experiments conducted under contrasting spring, autumn and winter conditions. The measured responses were highly variable. Rainwater additions increased chlorophyll a (Chl a) concentration (5–68% difference between rainwater treatments relative to the control) in all experiments and reduced or stimulated primary production (PP) depending on the treatment and the experiment (from −10 to +169% relative to the control). Autotrophic stimulation was highest in spring, probably related to the low initial natural nutrient concentrations. Under winter nutrient replete conditions, rainwater inputs changed the phytoplankton community although this change did not promote increases in primary production. Enhancement of net autotrophy (increase of net oxygen production up to 227%) after rainwater inputs were only found during the period of low nutrient availability. Inputs of dissolved organic nitrogen (DON) explained a large fraction of the variability in the response of PP, Chl a, community respiration (CR) and net community production (NCP). Our results suggest that differences in the initial environmental conditions (i.e. nutrient availability), rainwater composition and the ability of the present autotrophic communities to utilize the new nutrients result in substantial changes in the microbial responses and associated biologically-mediated carbon fluxes. As atmospheric nutrient inputs into coastal oceans are increasing rapidly, our results help to understand the effects of different inputs on the metabolism of distinct microbial communities, This research was supported by the Galician Government (Xunta de Galicia) through the grants 07MMA002402PR (IMAN) and PGIDIT06PXIB312222PR (AddEx). S.M-G. and E.E.G-M. were funded by F.P.U. fellowships and E.T. by a Ramón y Cajal contract of the Spanish Ministry of Science and Innovation

We determined the distribution and activity of the major prokaryotic groups (Bacteria, Cren-, and Euryarchaeota) inhabiting the deep water masses of the North Atlantic by following the path of the North Atlantic Deep Water (NADW) from its formation in the Greenland-Iceland-Norwegian (GIN) Sea along its two major branches covering approximately the first 50 yr of the NADW in the oceanic conveyor belt system. The relative abundance of Eury- and Crenarchaeota, assessed by catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH), was significantly higher in the western branch (17% and 24% of 49,69-diamidino-2- phenylindole (DAPI)-stained cells, respectively) than in the eastern (9% and 17%, respectively) branch of the NADW. In contrast, the relative abundance of Bacteria (30% of DAPI-stained cells) did not differ between the western and the eastern basin. Prokaryotic production and turnover rates, however, were higher in the western than the eastern basin. Generally, the contribution of Euryarchaeota to total picoplankton was correlated positively with oxygen concentrations ( p < 0.001) and negatively with salinity ( p < 0.001) and temperature ( p < 0.001). The contribution of Crenarchaeota to total picoplankton correlated positively with oxygen ( p < 0.05) and negatively with salinity ( p < 0.001). There was a positive correlation between the crenarchaeotal contribution to picoplankton and nitrite concentration ( p < 0.001), especially in the oxygen minimum layer, suggesting their potential involvement in the marine nitrogen cycle as nitrifiers. The observed variability in archaeal abundance in relation to bulk prokaryotic activity supports the emerging notion that Archaea are a highly dynamic and metabolically active component of the deep ocean prokaryotic community.

We determined the contribution of the three major prokaryotic groups (Bacteria, Crenarchaeota, and Euryarchaeota) on the uptake of D- and L-aspartic acid (Asp) in the major water masses of the North Atlantic (from 100to 4,000-m depth) with the use of microautoradiography combined with catalyzed reporter deposition fluorescence in situ hybridization (MICRO-CARD-FISH). The percentage of prokaryotic cells that assimilated D- and L-Asp ranged from ,5% to 25%. In the meso- and bathypelagic waters of the North Atlantic, Archaea are more abundant (42% 6 2% of 49,69-diamino-2-phenylindole [DAPI]‐stained cells) than Bacteria (30% 6 1% of DAPI-stained cells), and more archaeal than bacterial cells are actively incorporating D-Asp (62% 6 2% vs. 38% 6 2% of total D-Asp active cells). In contrast, Bacteria and Archaea almost equally contribute to L-Asp use in the deep waters of the North Atlantic (47% 6 2% vs. 53% 6 2% of total L-Asp active cells). The increase in the D-Asp : L-Asp uptake ratio in the prokaryotic community with depth appears to be driven by the efficient uptake of D-Asp by, especially, the Crenarchaeota in the deep waters. Because Archaea, and particularly Crenarchaeota, commonly dominate the prokaryotic communities in the ocean’s interior, we suggest that they represent a previously unrecognized sink of D‐amino acids in the deep ocean.

Size-fractionated chlorophyll-a and carbon incorporation rates were determined on a series of 13 cruises carried out from 1992 to 2001with the aim of investigating the patterns and causes of variability in phytoplankton chlorophyll and production in the Eastern North Atlantic Subtropical Gyral Province (NASE). Averaged (±SE) integrated chlorophyll-a concentration and primary production rate were 17±1 mg m−2 and 253±22 mg C m−2 d−1. Small-sized cells (

Slope Water anticyclonic Oceanic eDDIES (SWODDIES) are typical mesoscale features of open-ocean waters of the southern Bay of Biscay which usually develop in winter by shedding from the seasonal poleward current flowing along the northern Spanish slope. These eddies have been intensively studied from the physical perspective. However, their effect on the distribution of biological properties and on the functioning of the pelagic ecosystem has not been assessed so far. To this aim, a sea-truth, multidisciplinary and comprehensive study of a SWODDY was carried out in summer 1998. The eddy, radius of ≈50 km, was initially centred at 45·5°N 6·0°W, being characterized by a relatively homogeneous core of water in the centre of the eddy extending from 80 to about 200 dbar. In the central region of the core, temperature (12·55–12·75°C) and salinity (≈35·70) values were higher than outside the eddy. The optical properties of the eddy also differed from those of the surrounding waters. A distinct biological signature was found associated with the eddy. Depth-integrated chlorophyll-a concentrations were 25% higher at the eddy centre where upward doming of the seasonal pycnocline (up to 30 dbar) occurred. Enhanced phytoplankton biomass was related to a higher contribution of >10 μm cells, mainly represented by diatoms and chrysophyceans. Phytoplankton and mesozooplankton species composition in and outside the eddy differed significantly reflecting the coastal origin of the water parcel trapped by the eddy. The sharp modification of the planktonic community composition, biomass and associated size-structure caused by slope water oceanic eddies are likely to exert a significant effect upon the upper trophic levels of the pelagic ecosystem of the southern Bay of Biscay.

Ammonium regeneration and dissolved organic nitrogen (DON) release were studied experimentally in the euphotic zone of shelf and oceanic waters of NW Spain in relation to coastal upwelling dynamics and the size-structure of phytoplankton communities. Incubations of plankton labelled with [ 15 N]ammonium were made during four cruises, two of which also included size-fractionated determinations of chlorophyll a and primary production, and experimental determinations of production rates of dissolved organic carbon (DOC) using 14 C. Inorganic nitrogen concentrations were mainly related to nitrate enrichment by upwelling pulses, while ammonium concentrations were generally low in all situations. Ammonium did not accumulate in the study area, suggesting a daily time scale coupling between regeneration and uptake. In contrast, DON largely exceeded inorganic nitrogen in all situations and generally increased from spring to autumn. Ammonium regeneration was positively correlated with DON release and both rates showed the largest variation in summer, with minimum values during active upwelling and maximum values when upwelling relaxed. Comparison of DON stocks and rates in different shelf areas suggests that DON release near the coast during summer was more persistent in the water than DON release in off-shelf and oceanic areas. The carbon:nitrogen ratio of DOC and DON release rates was highly variable, revealing a large excess of DOC compared with DON. This excess can be attributed to either an underestimate of total DON release (as only release from ammonium was measured) or to an enhanced production of carbon-rich organic substances by diatoms in coastal areas. By considering a broad range of trophic situations, this study reveals a fundamental difference between short term release of DOC and DON by plankton. Physiological processes (such as carbohydrate exudation by diatoms) seem to be the cause of large DOC excess, whereas trophic processes (such as grazing) are more likely the cause of enhanced DON release.

27 páginas, 3 tablas, 5 figuras, Concurrent measurements of dark community respiration (DCR), gross production (GP), size fractionated primary production (14C PP), nitrogen uptake, nutrients, chlorophyll a concentration, and heterotrophic and autotrophic bacterial abundance were collected from the upper 200m of a latitudinal (321S–481N) transect in the Eastern Atlantic Ocean during May/June 1998. The mean mixed layer respiration rate was 2.572.1 mmol O2m 3 d 1 (n ¼ 119) for the whole transect, 2.271.1 mmol O2m 3 d 1 (n ¼ 32) in areas where chlorophyll a was o0.5 mgm 3 and 1.570.7 mmol O2m 3 d 1 (n ¼ 10) where chlorophyll a waso0.2 mgm 3. These values lie within the range of published data collected in comparable waters, they co-vary with indicators of heterotrophic and autotrophic biomass (heterotrophic bacterial abundance, chlorophyll a concentration, beam attenuation and particulate organic carbon concentration) and they can be reconciled with accepted estimates of total respiratory activity. The mean and median respiratory quotient (RQ), calculated as the ratio of dissolved inorganic carbon production to dissolved oxygen consumption, was 0.8 (n ¼ 11). At the time of the study, plankton community respiration exceeded GP in the picoautotroph dominated oligotrophic regions (Eastern Tropical Atlantic [15.51S–14.21N] and North Atlantic Subtropical Gyre [21.5–42.51N]), which amounted to 50% of the stations sampled along the 12,100km transect. These regions also exhibited high heterotrophic: autotrophic biomass ratios, higher turnover rates of phytoplankton than of bacteria and low f ratios. However, the carbon supply mechanisms required to sustain the rates of respiration higher than GP could not be fully quantified. Future research should aim to determine the temporal balance of respiration and GP together with substrate supply mechanisms in these ocean regions, CR was funded by a NERC Advanced Research Fellowship (GT5/96/8/MS), PSI was funded by an EU Marie Curie Research Training Grant (ERBFMBICT972700), E.T. and E.F. were funded by MEC grant MAR98-1417E and the EU contract CANIGO (MAS3CT960060). Funding for equipment and training for CR from the Royal Society is gratefully appreciated. This study was supported by the UK Natural Environmental Research Council through the Plymouth Marine Laboratory Core Strategic Research Programmes ‘‘Ocean Carbon Cycle Changes in the Coastal Zone’’ (OC4Z: 1999–2001) and ‘‘Microbially Driven Biogeochemical Cycles’’ (MDB: 2001–2006) and is MDB contribution number 56.

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 308N and 368N. Photic zone integrated particulate organic carbon (POC) production rates ranged from 10 to 1,178 mg C m 22 h 21 , 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

8 páginas, 2 tablas, 1 figura, The effect of solar radiation on dissolved organic matter (DOM) and the subsequent impact of photo-altered DOM on bacterial activity and community structure were assessed during two experiments in the coastal system of the Ría de Vigo (NW Iberian Peninsula). After exposure of 0.2 μm filtered seawater for 3.5 days to dark and full sunlight, an inoculum of the bacterial community collected at the same time as the exposed water was added and the mixture was incubated for 4 days in the dark at 15 °C. Changes in bacterial production (BP), diversity (assessed by Fluorescence in situ hybridization) and electron transport system (ETS) activity, dissolved organic carbon (DOC) and nitrogen (DON) and DOM humic-like absorption and fluorescence were followed. The exposure to sunlight had no effect on DOM concentrations while an average (±SD) decrease in DOM humic fluorescence of 45 ± 10% was found. The incubations with photo-altered DOM had lower BP (57 ± 11%), ETS (42 ± 9%) and bacterial carbon demand (BCD) (42 ± 8%) compared with the dark incubations, while bacterial growth efficiency (BGE) was unaffected. This suggests that DOM photo-alteration had a negative effect on bacterial metabolism in the study system. The bacterial growth on irradiated DOM resulted in a significant enrichment of the Gammaproteobacteria group compared with the dark control, indicating that solar exposure of DOM led to rapid changes in the bacterial community composition of the Ría de Vigo., This study was funded by fellowships to C.L from the early stage training site ECOSUMMER (MEST-CT-2004-020501) and the Carlsberg Foundation

10 pages, 5 figures, 2 tables, Although recognized as a potentially important source of both inorganic and organic nutrients, the impact of rainwater on microbial populations from marine planktonic systems has been poorly assessed. The effect of rainwater additions on bacterioplankton metabolism and community composition was evaluated in microcosm experiments enclosing natural marine plankton populations from the Ría de Vigo (NW Spain). The experiments were conducted during three different seasons (spring, autumn and winter) using rainwater collected at three different locations: marine, urban and rural sites. Bacterial abundance and production significantly increased up to 1.3 and 1.8-fold, respectively, after urban rainwater additions in spring, when ambient nutrient concentration was very low. Overall, the increments in bacterial production were higher than those in bacterial respiration, which implies that a higher proportion of carbon consumed by bacteria would be available to higher trophic levels. The response of the different bacterial groups to distinct rainwater types differed between seasons. The most responsive bacterial groups were Betaproteobacteria which significantly increased their abundance after urban (in spring and winter) and marine (in spring) rainwater additions, and Bacteroidetes which positively responded to all rainwater treatments in spring and to urban rainwater in autumn. Gammaproteobacteria and Roseobacter responded only to urban (in spring) and marine (in winter) rainwater treatment, respectively. The responses to rainwater additions were moderate and transient, and the resulting bacterial community structure was not importantly altered, This research was supported by the Xunta de Galicia through projects AddEx (grant PGIDIT06PXIB312222PR) and IMAN (grant 07MMA002402PR). S.M-G. was funded by a F. P. U. MEC fellowship. V.H-M. was funded by a F. P. I. MEC fellowship. E.T. was founded by a Ramón y Cajal-MEC contract

13 pages, 4 figures, 2 tables, The effects of inorganic and/or organic (glucose+AAs) inputs on phytoplankton and heterotrophic bacteria were assessed, using a microcosm approach, in two contrasting marine environments: an open ocean oligotrophic site (North Atlantic Subtropical Gyre) and a highly productive coastal embayment (Ría de Vigo, NW Spain). Overall, changes in microbial plankton biomass were smaller than those of metabolic rates. The largest increases in primary production, bacterial production and community respiration were measured in response to mixed () nutrient additions in both sites. Primary production responded to additions only in oligotrophic waters. The distinct autotrophic responses to nutrient additions measured in these environments were related to the different initial composition of phytoplankton populations and, presumably, also to differences in grazing pressures in both marine ecosystems. Heterotrophic bacteria were limited by organic substrates in both ecosystems, although mixed additions further enhanced bacterial growth in the subtropical gyre. The differences detected in bacterial responses to nutrient additions may be related to differences in nutrient limitations and to the prevalence of different relationships between components of the microbial food web (e.g. coupling between heterotrophic bacteria and phytoplankton and predation pressure) in both environments. We found a more relevant role of inorganic nutrients in controlling the efficiency of bacterial growth in oligotrophic regions as compared with highly productive systems. Our results suggest that organic matter inputs into both ecosystems might result in a tendency towards heterotrophy and in increases in bacterial growth efficiency. © 2013 Copyright Taylor and Francis Group, LLC., This research was supported by the Xunta de Galicia AddEx contract (PGIDIT06PXIB312222PR). S.M-G. was funded by a F. P. U. MEC fellowship. E.T. was funded by a Juan de la Cierva and a Ramo´n y Cajal-MEC contract

Proyectos RADIALES (IEO), REFORZA (PGIDIT06RMA60401PR)y HERCULES (09MMA027604PR)de la Xunta de Galicia y ANILE (CTM2009-08396) del Plan nacional de I+D+i, Long-term variability of phytoplankton biomass (chlorophyll a) and primary production (carbon fixation) in relation to changes in climatic and oceanographic factors was analysed for oceanic and shelf waters of the north and northwest Iberian Peninsula. Changes related to stratification in the water column and upwelling intensity were specifically addressed. The time series include measurements of primary production and biomass over 19 yr. Seasonally, the main mode of temporal variability of the series is the occurrence of phytoplankton blooms during the winter-spring and summer- autumn transition periods. On the northwestern coast (Galicia), the blooms are more frequent and attain higher biomass and production values than on the northern coast (Mar Cantábrico) due to the longer duration and higher intensity of upwelling in the former region. Even for the longest series there is no significant long-term trend in biomass, but primary production increased in Galicia and decreased in the Mar Cantábrico at nearly equivalent rates when considering monthly series. The lagged relationships between external factors and phytoplankton biomass and production suggest non-linear interactions between phytoplankton and climate. Neither the increase in surface stratification nor the decrease in upwelling intensity has negatively affected primary production in this region following consistent and systematic patterns. Changes in the amount and proportions of nutrients during winter water-mass formation, and an increased use of nutrients remineralised in situ, are proposed as the main causes of decadal variability in primary production in this region., Plan Nacional I+D+i, Xunta de Galicia, IEO

10 páginas, 4 tablas, 5 figuras, The degradation of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) collected in the coastal upwelling system of the Ría de Vigo (NW Iberian Peninsula) was assessed following the time-course of DOC, DON and DOP concentrations in laboratory incubations. Initial concentrations varied from 73 to 94 µM for DOC, 4.5 to 7.2 µM for DON and 0.12 to 0.32 µM for DOP. The bioavailable fraction (BDOM) represented 17 ± 6% (average ± SE) of DOC, 38 ± 6% of DON and 65 ± 9% of DOP. BDOM was significantly correlated with temperature (R2 > 0.3, p < 0.05) and chlorophyll a (R2 > 0.5, p < 0.05), indicating that the differences in DOM bioavailability were associated with the seasonal variations in plankton biomass and activity. The C:N:P stoichiometry of BDOM, 111 (±38):18 (±6):1, was not significantly different from the Redfield ratio (106:16:1), pointing to a phytoplankton origin of BDOM. Accordingly, exponential rate constants of BDOM suggest that this pool is very labile. Despite the reduced flushing times of the Ría de Vigo, from 3 to 8 d, as much as 70 ± 13% of BDOC (average ± SD), 81 ± 8% of BDON and 88 ± 5% of BDOP are mineralized within the embayment. The remaining C-rich BDOM is exported to the adjacent oligotrophic waters, suggesting that the offshore transport of labile DOM must be accounted for when considering the carbon balance of ocean surface waters adjacent to productive coastal areas, This study was funded by a fellowship to C.L from the Early stage Training site ECOSUMMER (MEST-CT-2004-020501). Access to vessel time was provided by the RAFTING project (Impact of the mussel raft culture on the benthic-pelagic coupling in a Galician Ria, grant number: CTM2007-61983/MAR).

13 pages, 7 figures, 2 tables.-- Available online Nov 17, 2008.-- Issue title: "The Atlantic Meridional Transect Progamme"., We present, test and implement two contrasting models to predict euphotic zone net community production (NCP), which are based on 14C primary production (PO14CP) to NCP relationships over two latitudinal (ca. 30°S–45°N) transects traversing highly productive and oligotrophic provinces of the Atlantic Ocean (NADR, CNRY, BENG, NAST-E, ETRA and SATL, Longhurst et al., 1995 [An estimation of global primary production in the ocean from satellite radiometer data. Journal of Plankton Research 17, 1245–1271]). The two models include similar ranges of PO14CP and community structure, but differ in the relative influence of allochthonous organic matter in the oligotrophic provinces. Both models were used to predict NCP from PO14CP measurements obtained during 11 local and three seasonal studies in the Atlantic, Pacific and Indian Oceans, and from satellite-derived estimates of PO14CP. Comparison of these NCP predictions with concurrent in situ measurements and geochemical estimates of NCP showed that geographic and annual patterns of NCP can only be predicted when the relative trophic importance of local vs. distant processes is similar in both modeled and predicted ecosystems. The system-dependent ability of our models to predict NCP seasonality suggests that trophic-level dynamics are stronger than differences in hydrodynamic regime, taxonomic composition and phytoplankton growth. The regional differences in the predictive power of both models confirm the existence of biogeographic differences in the scale of trophic dynamics, which impede the use of a single generalized equation to estimate global marine plankton NCP., This paper shows the potential of a systematic empirical approach to predict plankton NCP from local and satellite-derived P estimates., This work was supported by the NERC AMT programme (NER/O/S/2001/00680; this is contribution number 168. http://www.amt-uk.org) and the CYCIT project CIRCANA (MAR1999-1072-C03-01). P.S. was funded by EU Marie Curie Research Training and Return Grants and a MECD Contrato de Reincorporación, C.R. by a NERC Advanced Research Fellowship, E.T. and E.F. by MECD grants and the EU contract CANIGO (MAS3CT960060), and G.H.T. by an EU MAST fellowship.

Bacterial abundance, biomass and heterotrophic activity, along with hydrographic and phytoplankton variables, were measured from February 1998 to September 1999 at two coastal stations off A Coruña (Galicia, NW Spain). Bacterial abundance varied from 0.1 to 1.8 x 106 cells ml-1 and water-column integrated bacterial production ranged from 34 to 1132 mg C m-2 d-1. Two annual maxima of bacterial abundance and production were observed, in spring and summer, although relatively high abundance values remained until autumn. In autumn and winter production values were low with the exception of the results obtained in November 1998, when the highest values of bacterial production of the whole study were observed at both stations. Bacterial biomass constituted a significant, but low, fraction of particulate organic carbon (4-21 %) and was always lower than phytoplankton biomass (up to 40% of phytoplankton carbon). Also, bacterial production was generally < 30% of primary production, but in some situations (as in early spring or in autumn) bacterial production exceeded primary production, suggesting a delayed response of bacterial activity after a phytoplankton bloom., Se midió la abundancia, biomasa y actividad heterotrófica bacteriana, junto con variables hidrográficas y fitoplanctónicas, entre febrero de 1998 y septiembre de 1999 en dos estaciones costeras de A Coruña (Galicia, NO España). La abundancia bacteriana varió entre 0.1 y 1.8 x 106 células ml-1 y la producción bacteriana integrada en la columna de agua tomó valores entre 34 y 1132 mg C m-2 d-1. Se observaron dos máximos de abundancia y producción bacteriana en primavera y verano, aunque se mantuvieron valores relativamente altos de abundancia durante el otoño. En otoño e invierno los valores de producción fueron bajos con la excepción de los resultados obtenidos en noviembre de 1998, donde se observaron los mayores valores de producción de todo el estudio en ambas estaciones. La biomasa bacteriana constituyó una fracción significativa, aunque baja, del carbono orgánico particulado (4-21%) y fue siempre menor que la biomasa del fitoplancton (hasta un 40% del carbono fitoplanctónico). Del mismo modo, la producción bacteriana fue generalmente < 30% de la producción primaria, aunque en ciertas situaciones (como a principios de primavera o en otoño) la producción bacteriana excedió a la producción primaria, lo que sugiere un retraso en la respuesta de la actividad bacteriana a la proliferación del fitoplancton., Xunta de Galicia (Xuga-10302B98). Instituto Español de Oceanografía (Proyecto IEO-1007)

Seasonal variations in phytoplankton and bacterial biomass along with rates of primary production, dissolved organic carbon (DOC) release, bacterial production, dark respiration and vertical particle flux were determined from December 1998 to September 1999 at a coastal station off A Coruña (NW Spain) affected by episodes of wind-driven upwelling. Maximum phytoplankton biomass and production was associated with upwelling events. Bacterial biomass remained relatively constant throughout the study and was always lower than phytoplankton biomass. Depth-integrated rates of DOC release, which ranged from 0.2 to 3.1 gC m–2 d–1, were not a constant fraction of the total amount of carbon incorporated by primary producers. On average, DOC release accounted for 37 ± 7% of total primary production, and the highest values were measured during low-productivity periods. The supply of DOC by microplankton was globally sufficient to support the estimated bacterial activity, although a temporal shift between DOC release and bacterial consumption was observed. Primary production exceeded respiration during most of the seasonal study, with production/respiration ratios ranging from 1 to 7. The annual averaged daily net community production was 253 (±105) mmol O2m–2 d–1. The amount of particulate organic carbon exported from the euphotic layer was 1 to 86% of total primary production. The results of this study indicate that the coastal system is autotrophic, and suggest that most of the organic matter produced photosynthetically was channelled through the microbial food web. Xunta de Galicia; XUGA-10302B98 Instituto Español de Oceanografía; IEO-1007

11 pages, 9 figures, 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 oceanmeasurements 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 (328S–488N) transect in theEastern Atlantic Ocean during May and June 1998. This comprehensive data set, which spans five contrastingplankton regimes, including two open ocean oligotrophic provinces, is used to derive a GPP : CR relationship, whichsuggests that net heterotrophy (GPP,CR) prevails in the eastern Atlantic when primary production falls below;100 mmol O2m22d21. The predictive capability of this relationship is compared with that of the only otherpublished relationship based on similar methodologies and is found to give a more representative description of theautotrophic (GPP.CR) to heterotrophic seasonal cycle in the Bay of Biscay. This improved predictive power isattributed to the increased representativeness of the current data set. Specifically, the interpretation suggests that theinfluence of community structure on net ecosystem metabolism implies that prediction of GPP : CR balances inpelagic ecosystems can be best achieved by use of a data set that covers a wide range of community structure andnot only a wide range in the magnitude of primary production

We used data collected at > 60 stations over a 10 yr period to build the carbon budget of the plankton community in the euphotic layer of the Eastern North Atlantic Subtropical Gyre (NASE). Autotrophic biomass exceeded microbial heterotrophic biomass by a factor of 1.7. Mean ( SE), integrated chlorophyll a concentration and net particulate primary production (PP) were 17 +/- 1 Mg m(2) and 271 +/- 29 mg C m(-2) d(-1), respectively. Protist grazing on phytoplankton represented > 90% of PP. Bacterial production (BP) was 17 +/- 3 mg C m(-2) d(-1). In vitro O-2-evolution experiments indicated that net community production was -65 +/- 16 mmolO(2) m(-2) d(-1), while community respiration (CR) averaged 124 +/- 13 mmolO(2) m(-2) d-1, equivalent to 1324 +/- 142 mg C m(-2) d(-1). However, the sum of the respiration rates by each microbial group, estimated from their biomass and metabolic rates, ranged from 402 to 848 Mg C m(-2) d-1. Therefore, CR could not be reconciled with the respiratory fluxes sustained by each microbial group. Comparison between estimated gross photosynthesis by phytoplankton (481 to 616 mg C m(-2) d-1) and the sum of respiration by each group suggests that the microbial community in the NASE province is close to metabolic balance, which would agree with the observed O-2 supersaturation in the euphotic layer. Taking into account the mean open-ocean values for PP, BP, CR and bacterial growth efficiency, we show that bacteria account for approximately 20% of CR. Our results suggest that the view that bacteria dominate carbon cycling in the unproductive ocean must be reconsidered, or else that in vitro incubations misrepresent the real metabolic rates of one or several microbial groups.

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 micro- bial 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 photosynthe- sis 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.

Rates of particulate organic carbon (POC) production, dissolved organic carbon (DOC) production and bacterial production were measured at 8 stations in the eastern North Atlantic Sub- tropical Gyre during August 1998. The euphotic-depth-integrated POC rate was on average 27 mgC m -2 h -1 . The corresponding averaged integrated DOC production rate was 5 mgC m -2 h -1 , i.e. about 20% of the total primary production. No statistically significant relationship was found between the rates of POC and DOC production, suggesting that other processes besides phytoplankton exudation, such as cell lysis or protist grazing, could be implied in the release of dissolved organic materials to the oceanic environment. Euphotic-depth-integrated bacterial biomass and production were on aver- age 214 mgC m -2 and 1.4 mgC m -2 h -1 , respectively. The lack of correlation between the rates of DOC release and bacterial activity and a bacterial carbon demand (BCD, calculated using an esti- mated bacterial growth efficiency ranging from 11 to 18%) in excess of DOC production suggested the existence of additional organic carbon sources (both allochthonous and/or autochthonous reser- voirs), apart from in situ phytoplankton-derived DOC production, for the maintenance of bacterial activity in this region during summer 1998. This uncoupling between phytoplankton and bacterio- plankton could partially explain the heterotrophic metabolic balance in this oligotrophic region during summer 1998.