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16 pages, 4 figures, 3 tables, The Mediterranean coast receives large inputs of dust with a potential fertilization effect. We evaluated the effect of a wet dust deposition event on microbial abundance and activity, community structure and metabolic balance. Dust collected during a dust storm event was added to a series of microcosms. We added a realistic concentration (0.05 g L–1) based on the phosphorus concentration contained in the dust and, in addition, we included a P amended tank to distinguish the effect of dust from that of the phosphorus in the dust. We also included a higher dust concentration tank (0.5 g L–1). Dust increased the initial water–phosphorus concentration by 0.3 µM and the dissolved organic carbon (DOC) concentration by 14 µM, and increased bacterial abundance (1.8-fold) and bacterial production (5-fold). At the end of the experiment, primary production and community respiration were stimulated by dust and by P, but the net result of the addition of low amounts of dust was an initial switch towards heterotrophy, whereas the net result of the high-dust (DH) additions and the P addition was a shift towards autotrophy. Bacterial community structure changed little between P and low dust, but these were very different from the control and the DH communities, This work was supported by Spanish projects MODIVUS (CTM2005-04795/MAR), VARITEC (CTM2004-0442-C02) and STORM (CTM2009-09352/MAR), and is a contribution to the European Network of Excellence EUROCEANS and MARBEF

Suggett, David J. et. al.-- Contribution to AME Special 2 ‘Progress and perspectives in aquatic primary productivity’.-- 13 pages, 5 figures, Bioassay experiments were performed to identify how growth of key groups within the microbial community was simultaneously limited by nutrient (nitrogen and phosphorus) availability during spring in the Gulf of Aqaba’s oceanic waters. Measurements of chlorophyll a (chl a) concentration and fast repetition rate (FRR) fluorescence generally demonstrated that growth of obligate phototrophic phytoplankton was co-limited by N and P and growth of facultative aerobic anoxygenic photoheterotropic (AAP) bacteria was limited by N. Phytoplankton exhibited an increase in chl a biomass over 24 to 48 h upon relief of nutrient limitation. This response coincided with an increase in photosystem II (PSII) photochemical efficiency (Fv/Fm), but was preceded (within 24 h) by a decrease in effective absorption crosssection (σPSII) and electron turnover time (τ). A similar response for τ and bacterio-chl a was observed for the AAPs. Consistent with the up-regulation of PSII activity with FRR fluorescence were observations of newly synthesized PSII reaction centers via low temperature (77K) fluorescence spectroscopy for addition of N (and N + P). Flow cytometry revealed that the chl a and thus FRR fluorescence responses were partly driven by the picophytoplankton (, We thank the Batsheva de Rothschild Foundation, Bar-Ilan University, the Moshe Shilo Center for Marine Biogeochemistry, and the staff of the Interuniversity Institute for funding and logistic support. Attendance at the workshop was also supported by an international travel grant from the Texas Institute of Oceanography (A.Q.) and EUROCEANS (D.J.S., E.V.D. and T.L.)

12 pages, 4 figures, 2 tables, Growing evidence of inorganic nutrient limitation on oceanic bacteria suggests a global dependence of bacterial activity and production on rates of nutrient supply. The present study examined whether surface bacterial abundance is significantly related to water column stability, and whether bacterial activity and growth rate are related to the rate of diffusive supply of inorganic nutrients to the mixed layer in the Central Atlantic during 2 meridional cruises. The 2 cruises were run under very different oceanic conditions, with relatively low values of bacterial activity in spring 1995 and relatively higher values in fall 1995. We obtained depth-resolved data in the second cruise and found that the integrated value of bacterial production was also related to the rate of nutrient supply, while integrated particulate primary production and chlorophyll concentration were not. There was also no relationship between particulate primary production and bacterial production. The relationship between nutrient supply and integrated bacterial production was tested with data from a mesocosm experiment showing a good fit to the pattern obtained in the Atlantic. Average bacterial production was ~21% of primary production in the Central Atlantic, with values ranging between 5 and 100%, and higher values in the tropical areas. The demonstration of a direct relationship between nutrient supply and bacterial activity helps to explain a relatively large bacterial biomass as compared to phytoplankton biomass, a low bacterial growth efficiency, and a high bacterial carbon demand relative to contemporaneous primary production often measured in the open ocean, as well as the accumulation of dissolved organic carbon (DOC) observed in nutrient-limited oligotrophic seas, This research was part of the LATITUD project (AMB94-0739), funded by the Spanish Interministerial Commission for Science and Technology, while writing was supported by projects COCA (REN2000-1471-CO2-01/MAR) and MODIVUS (CTM2005-04795/MAR). We thank the crew of RV ‘Hespérides’ for assistance

Sala, M. Montserrat ...et al.-- Special issue: Towards a Broader Perspective on Ocean Acidification Research.-- 10 pages, 2 figures, 3 tables, supplementary data http://icesjms.oxfordjournals.org/content/73/3/670/suppl/DC1, We investigated the effects of an increase in dissolved CO2 on the microbial communities of the Mediterranean Sea during two mesocosm experiments in two contrasting seasons: winter, at the peak of the annual phytoplankton bloom, and summer, under low nutrient conditions. The experiments included treatments with acidification and nutrient addition, and combinations of the two. We followed the effects of ocean acidification (OA) on the abundance of the main groups of microorganisms (diatoms, dinoflagellates, nanoeukaryotes, picoeukaryotes, cyanobacteria, and heterotrophic bacteria) and on bacterial activity, leucine incorporation, and extracellular enzyme activity. Our results showed a clear stimulation effect of OA on the abundance of small phytoplankton (pico- and nanoeukaryotes), independently of the season and nutrient availability. A large number of the measured variables showed significant positive effects of acidification in summer compared with winter, when the effects were sometimes negative. Effects of OA were more conspicuous when nutrient concentrations were low. Our results therefore suggest that microbial communities in oligotrophic waters are considerably affected by OA, whereas microbes in more productive waters are less affected. The overall enhancing effect of acidification on eukaryotic pico- and nanophytoplankton, in comparison with the non-significant or even negative response to nutrient-rich conditions of larger groups and autotrophic prokaryotes, suggests a shift towards medium-sized producers in a future acidified ocean, Financial support from the Spanish Ministry of Economy and Competitiveness is acknowledged, specifically through the projects STORM(CTM2009–09352), DOREMI (CTM2012-34294), MANIFEST (CTM2012-32017), and PROTOS (CTM2009-08783) of the Spanish Ministry of Economy and Competitiveness. We are also grateful for the financial support of the Generalitat de Catalunya to the “Grup de Diversitat Microbiana en Ecosistemes Acuàtics” (2014SGR/1591) and the “Grup d’Estructura i Funció de Xarxes Tròfiques Microbianes Planctòniques” (2014SGR/1179)

8 pages, 6 figures, 5 tables, We have measured both prokaryotic heterotrophic production (PHP) and respiration (R), then providing direct estimates of prokaryotic growth efficiencies (PGE), in the upper mesopelagic zone (300-600m) of the South Atlantic and Indian Oceans. Our results show that in situ R ranged 3-fold, from 87 to 238μmolCm-3d-1. In situ PHP rates were much lower but also more variable than R (ranging from 0.3 to 9.1μmolCm-3d-1). The derived in situ PGE values were on average ~1.4% (from 0.3% to 3.7%), indicating that most of the organic substrates incorporated by prokaryotes were respired instead of being used for growth. Together with the few previous studies on PGE published before for the Atlantic Ocean and Mediterranean Sea, our findings support the hypothesis that the global mesopelagic zone represents a key remineralization site for export production in the open ocean. We also found a strong correlation between R and PGE with temperature across a gradient ranging from 8.7 to 14.9°C. The derived Q10 value of 3.7 suggests that temperature variability in the mesopelagic zone plays a significant role in the remineralization of organic matter. © 2014 Elsevier Ltd., This paper is a contribution to the INGENIO 2010 CONSOLIDER program (CDS2008-00077). IPM was supported by a JAE Pre-doc fellowship, from the Spanish National Research Council (CSIC) and the BBVA Foundation, Spain. IPM, JA and JMG were partly supported by Project HotMix (CTM2011-30010-C02/MAR)

Photoacclimation of picophytoplankton was studied in the mixed layer of 3 stations in the central Cantabrian Sea (southern Bay of Biscay). Picophytoplankton chl a:carbon ratios (θ) presented minimum values during summer, when irradiance, temperature, and biomass of prokaryotes reached maximum values and inorganic nutrient concentrations were low. Conversely, the maximum θ were reached during winter, coincident with lowest annual irradiance but maximum concentration of inorganic nutrients and higher relative biomass of eukaryotes. Changes in θ were modeled using irradiance as an independent variable. Exponentially decreasing functions of θ with irradiance were significant only when the mean temperatures in the mixed layer were above 14°C. These functions presented light-saturated minimum ratios (θmin) that decreased linearly with temperature and low-light maximum ratios (θmax) that increased exponentially with temperature. Such relationships were used to establish an empirical model that reproduced the seasonality of picophytoplankton θ in the mixed layer, with minima in summer and maxima in winter. A maximum potential θ, θN,T-max, was determined to estimate picophytoplankton growth rates in the central Cantabrian Sea. Combinations of picophytoplankton growth rates and biomass in the mixed layer were used to estimate areal picophytoplankton primary production rates in the euphotic zone that presented a bimodal seasonal cycle, with maxima in late winter (ca. 100 mg C m-2 d-1) and in late autumn (>200 mg C m-2 d-1) and mean annual values around 120 mg C m-2 d-1. En prensa

16 pages, 6 figures, 3 tables, supplementary data http://plankt.oxfordjournals.org/content/36/1/198/suppl/DC1, We estimated the bacterial production and losses to predators along an open ocean trophic gradient from coastal upwelling waters to oligotrophic waters in the Subtropical Northern Atlantic Ocean. Two zonal sections (21 and 26°N) extending from the NW African shelf to the Open Atlantic Ocean at 26°W were sampled during September-October 2002 (autumn), and May-June 2003 (spring). The main goal was to elucidate whether the impacts of bacterial losses were more important in upwelling rather than in offshore waters. Whereas temperature and salinity decreased and nutrient concentrations increased from offshore to the coastal upwelling regions, phytoplankton, ciliate and bacterial biomass followed a similar trend increasing towards the upwelling zone. In addition, heterotrophic nanoflagellate biomass, bacterial production and grazing rates on bacteria did not follow this pattern, although the highest activities were recorded at upwelling stations. However, at the stations not affected by upwelling the average impact on bacteria expressed as a percentage of bacterial production consumed by predators in autumn and spring (values that varied between 70% ± 6% and 129% ± 15%, respectively) was significantly higher than at the upwelling stations (where it ranged between 49% ± 7% and 68% ± 5%, respectively). Our results suggest that in the upwelling areas bacteria escape from predators and growth cannot be balanced by grazing, while it is at the oligotrophic open ocean stations when, on occasions, grazing can overcome bacterial production. © The Author 2013, This work was supported by projects COCA (REN2000 1471-CO2-O1-MAR) to S.A. and J.A., DEBACOCA (REN2001-4211-E) to J.M.G. and FLAME (CGL2010-16304) to R.M. funded by the former Spanish Ministry of Sciences and Innovation

The biomass and production of phytoplankton and bacterioplankton was investigated in relation to the mesoscale structures found in the Algerian Current during the ALGERS'96 cruise (October 1996). Biological determinations were carried out in three transects between 0°and 2°E aimed at crossing a so-called event, formed by a coastal anticyclonic eddy associated with an offshore cyclonic eddy to the west. The concentration of chlorophyll a (Chl) was maximum (> 1.2 mg m-3) within the Cyclonic eddy and at the frontal zones between the Modified Atlantic Water (MAW) of the Algerian Current and the Mediterranean waters further north. Chl (total and > 2 μm) was significantly correlated with proxies of nutrient flux into the upper layers. Autotrophic picoplankton and heterotrophic bacterial abundance and production presented clear differences between MAW and Mediterranean water, with higher values at those stations under the influence of the Algerian Current. In general, greater differences were observed in production than in biomass variables. The photosynthetic parameters (derived from P-E relationships) and integrated primary production (range 189-645 mg m-2 d-1) responded greatly to the different hydrological conditions. The mesoscale phenomena inducing fertilization caused a 2 to 3-fold increase in primary production rates. The relatively high Values found within the cyclonic eddy suggest that, although short-lived in comparison with anticyclonic eddies, these eddies may produce episodic increases of biological production not accounted for in previous surveys in the region, We thank the people on board the R/V Hespérides for their help during the cruise. We are also grateful to Dolors Vaqué, Jordi Font, Belén Martı́n-Mı́guez and two anonymous reviewers for helpful criticism and comments on a previous version of the manuscript. X.A.G.M. acknowledges the receipt of a FPI pre-doctoral fellowship from the Spanish Ministry of Education and Culture. This work was supported by grants AMB95-0901-C02 (Spanish CYCIT) and MAS3-CT96-0051 (European Union MAST Programme, MATER Project)

Special volume: Advances in Spanish physical oceanography. Scientia Marina 76(Suppl.1) 2012.-- 14 pages, 10 figures, [EN] We use hydrographic, velocity and drifter data from a cruise carried out in November 2008 to describe the continental slope current system in the upper thermocline (down to 600 m) between Cape Verde and the Canary Islands. The major feature in the region is the Cape Verde Frontal Zone (CVFZ), separating waters from tropical (southern) and subtropical (northern) origin. The CVFZ is found to intersect the slope north of Cape Blanc, between 22°N and 23°N, but we find that southern waters are predominant over the slope as far north as 24°N. South of Cape Blanc (21.25°N) the Poleward Undercurrent (PUC) is a prominent northward jet (50 km wide), reaching down to 300 m and indistinguishable from the surface Mauritanian Current. North of Cape Blanc the upwelling front is found far offshore, opening a near-slope northward path to the PUC. Nevertheless, the northward PUC transport decreases from 2.8 Sv at 18°N to 1.7 Sv at 24°N, with about 1 Sv recirculating ofshore just south of Cape Blanc, in agreement with the trajectory of subsurface drifters. South of the CVFZ there is an abrupt thermohaline transition at sq=26.85 kg m–3, which indicates the lower limit of the relatively pure (low salt and high oxygen content) South Atlantic Central Water (SACW) variety that coexists with the dominant locally-diluted (salinity increases through mixing with North Atlantic Central Water but oxygen diminishes because of enhanced remineralization) Cape Verde (SACWcv) variety. At 16°N about 70% of the PUC transport corresponds to the SACW variety but but this is transformed into 40% SACWcv at 24°N. However, between Cape Verde and Cape Blanc and in the 26.85, [ES] En este trabajo describimos el sistema de corrientes en la termoclina superior (hasta los 600 m) a lo largo del talud continental entre Cabo Verde y las Islas Canarias utilizando datos hidrográficos, de velocidad y de boyas de deriva obtenidos durante una campaña realizada en noviembre de 2008. La principal característica de la región es el Frente de Cabo Verde (Cape Verde Frontal Zone, CVFZ), que separa aguas de origen tropical (meridional) y subtropical (septentrional). A lo largo del talud el CVFZ suele situarse al norte de Cabo Blanco, entre 22 y 23°N, pero durante la campaña observamos sobre el talud y hasta 24°N una predominancia de aguas de origen meridional. Al sur de Cabo Blanco (21.25°N) la Corriente Subsuperficial hacia el Polo (Poleward Undercurrent, PUC) se observa como un flujo bien pronunciado (de 50 km de anchura), alcanzando los 300 m de profundidad e indistinguible de la superficial Corriente de Mauritania (Mauritanian Current, MC). Al norte de Cabo Blanco, el frente de afloramiento se encuentra alejado mar adentro, lo que facilita a la PUC seguir hacia el norte a lo largo del talud. Aún así, este transporte hacia el norte de la PUC se reduce de 2.8 Sv en 18°N hasta 1.7 Sv en 24°N, con alrededor de 1 Sv recirculando mar adentro justo al sur de Cabo Blanco, en coincidencia con las trayectorias de los derivadores subsuperficiales. Al sur del CVFZ se observa una marcada transición termohalina en sq=26.85. Esta transición indica el límite inferior de la relativamente pura (poco salada y rica en oxígeno) variedad de masa de agua Central Suratlántica (South Atlantic Central Water, SACW), la cual coexiste con la localmente dominante y diluida (más salada debido a la mezcla con NACW pero pobre en oxígeno por una intensificación de la remineralización) variedad de Cabo Verde (SACWcv). A 16°N alrededor del 70% del agua transportada por la PUC corresponde a SACW mientras que a 24°N se ha transformado en un 40% de SACWcv. En cambio, entre Cabo Verde y Cabo Blanco y en la capa 26.85, This research has been funded by the Spanish Ministerio de Ciencia e Innovación through projects MOC2 (CTM2008-06438-C02-01), ARGO-CANARIAS (CTM2009-08462-E) and TIC-MOC (CTM2011-28867). JP-I has been supported through an FPI doctoral grant linked to MOC2. We are grateful to the technicians and crew of the R/V Sarmiento de Gamboa for their field support and to our cruise colleagues for many fruitful discussions

15 pages, 5 figures, 2 tables, Prokaryotes and protists are important players in the carbon biogeochemistry of marine ecosystems, and temperature is one of the physical factors most influential in the metabolism and composition of plankton communities. Small changes in temperature can change the flow of carbon and the community structure of planktonic ecosystems, and climatological models predict a rise in temperature of 2 to 5°C toward the end of the century in the NW Mediterranean. Laboratory culture experiments have shown that warming can increase the transfer of carbon between bacteria and protists and alter the community composition of microbial top predators (i.e. ciliates and dinoflagellates) by increasing the abundance of bacterivores and producing the extinction of herbivores. Here, we tested whether a small rise in temperature would produce these effects in the coastal Mediterranean. Between March 2003 and February 2004, we established 12 microcosm experiments with water from the Bay of Blanes. The samples were incubated for 48 h at ambient and warmer temperatures (~2.7°C higher than in situ values), and the net and gross growth rates of bacteria and heterotrophic nanoflagellates (HNF) were measured using unfiltered and 0.8 μm filtered treatments. Warming increased the rates of bacterial gross production and bacterial losses to grazing with a clear seasonality; the largest increments in the rates were observed during the cooler months of the year. Warming did not change the net growth rates of dinoflagellates. It decreased the net production of HNF and the net growth rates of ciliates but did not promote the extinction of herbivorous protists. Temperature changed the microbial food web function in NW Mediterranean waters, with small alterations in the community composition of microbial top predators. © Inter-Research 2012, This research is a contribution to the IMBER and SOLAS and was supported by projects MALASPINA (Consolider), MOC2, FISOCEAN, FLAME, and MODIVUS (MICIIN, Spain); and BASICS, MEECE and ATP (UE). [...] Writing of the manuscript was supported by grants SUMMER and STORM (MICIIN, Spain)

13 pages, 3 figures, 4 tables, The contribution of dimethylsulfoniopropionate (DMSP) to the fluxes of carbon and sulfur through phytoplankton and bacterioplankton was investigated throughout an annual cycle in the Blanes Bay Microbial Observatory (coastal NW Mediterranean). DMSP accounted for 0.3-7% of biovolume-estimated phytoplankton carbon and 4-93% of calculated phytoplankton sulfur, with higher contributions in 'summer' (highly irradiated, oligotrophic waters, May-September) and lower in 'winter' (October-April). DMSP biosynthesis rates accounted for 0.8-7% of carbon fixation and 11-88% of sulfur assimilation through primary production, with slightly higher shares in summer. Upon release from the algal cells, DMSP supplied 0.5-6% of the total carbon demand of heterotrophic bacteria, and 3->100% of the sulfur demand over the year. Uncertainties associated with these calculations are due to a scarce knowledge of C:S ratios in marine bacteria. Bacterial DMSP-sulfur assimilation (measured with 35S-DMSP) was positively correlated with bacterial heterotrophic production rates (measured with 3H-leucine). In summer waters, characterized by higher ratios of particulate DMSP to chlorophyll a (DMSPp:Chl a), DMSP-sulfur assimilation by bacteria was higher and contributed a larger share of the bacterial sulfur demand. We propose the DMSP:Chl a ratio as a good indicator of the relative role of DMSP in the carbon and sulfur fluxes through the first levels of the planktonic food-web, M.V.-C. was supported by a PhD fellowship from the Spanish Ministry of Education and Science. The work was funded by Spanish projects REN2001-2120/MAR, CTM2004-20022-E and CTM2005-04795/M

12 pages, 4 tables, 6 figures.-- Printed version published Oct 2008.-- Supporting information (Suppl. tables S1-S2 + references) available at: https://doi.org/10.1016/j.pocean.2008.08.002, The role of microorganisms in the transfer of carbon of marine systems is very important in open oligotrophic oceans. Here, we analyze the picoplankton structure, the heterotrophic bacterioplankton activity, and the predator–prey relationships between heterotrophic bacteria and nanoflagellates during two large scale cruises in the Central Atlantic Ocean (~29°N to ~40°S). Latitud cruises were performed in 1995 between March–April and October–November. During both cruises we crossed the regions of different trophic statuses; where we measured different biological variables both at the surface and at the deep chlorophyll maximum (DCM). The concentration of chlorophyll a varied between 0.1 and 0.8 mg m−3, the abundance of heterotrophic bacteria varied between 1.0 × 106 cells ml−1, and that of heterotrophic nanoflagellates between 1.0 × 104 cells ml−1. The production of heterotrophic bacteria varied more than three orders of magnitude between 3 × 105 cells ml−1, Synechococcus between 1.0 × 104 cells ml−1, and picoeukaryotes between 104 cells ml−1., Two empirical models were used to learn more about the relationship between heterotrophic bacteria and nanoflagellates. Most bacterial production was ingested when this production was low, the heterotrophic nanoflagellates could be controlled by preys during Latitud-I cruise at the DCM, and by predators in the surface and in the Latitud-II cruise. Our results were placed in context with others about the structure and function of auto- and heterotrophic picoplankton and heterotrophic nanoplankton in the Central Atlantic Ocean, This work was supported by grant AMB94-0739 to S. Agustí. Final writing of the manuscipt has been supported by an I3P felow (CSIC) to E.V.-D.

12 pages, 5 figures, 2 tables.-- Corrigendum published in J.Plankt.Res. 28(9):879(2006); doi:10.1093/plankt/fbl026, The trophic relation between prokaryotes and heterotrophic nanoflagellates was studied during two latitudinal cruises in the central Atlantic Ocean. The losses to predation on prokaryotes were determined in 12 locations covering a wide range of trophic situations, from ultraoligotrophic [1 mg Chl a m–3). In these locations, the abundance of prokaryotes (P) covaries with that of heterotrophic nanoflagellates, thus suggesting that resources controlled the abundance of heterotrophic nanoflagellates (HNF). Besides, the losses to predation were positively related to prokaryotic and heterotrophic nanoflagellate biomass, which points toward higher consumption rates associated with larger concentrations of preys and predators. Conversely, declining trends between prokaryotic production (PP) and the fraction of this production lost to predation revealed higher relative losses in the environments with lower productions. Our study shows for the central Atlantic that 35% of prokaryotic biomass (BP), equating to between 40 and 83% of PP can be ingested daily and that 55% of the variability observed in the rate of prokaryotic loss to predation was related with the HNF. As predators graze on many prey types, in an oligotrophic system containing many prey species but little numeric loading, there will still be prey for predators but not enough hosts for viruses. In this sense, our study confirms the importance of the prey–predator relationship between prokaryotes and heterotrophic nanoflagellates in the flow of carbon of the less productive regions of the ocean, This is a contribution of the ‘Latitud’ project (AMB94-0739, Spanish Education and Science Ministry). E.V.-D. was supported by grants FPI (PF3698574900, Spanish Education and Science Ministry), C-RED 2002–2003, Generalitat de Catalunya, and ‘Microflux’ project (CMT, 2004-04404-CO2-01)

The hypothesis that increasing nutrient supply increases the biomass of autotrophs proportionately more than the biomass of heterotrophs was tested by increasing (0, 1-, 2-, 4-, 8-, and 16-fold over the background loading of 5 mmol N m-2 d-1, 1.6 mmol Si m-2 d-1, and 0.25 mmol P m-2 d-1) the addition of nutrients to large (33 000 l) mesocosm units enclosing an oligotrophic coastal Mediterranean planktonic community. Autotrophic plankton biomass increased 50-fold along the range of nutrient inputs, whereas heterotrophic biomass increased only 10-fold. Heterotrophic biomass increased as the 1/5 power of the increase in the biomass of autotrophs, implying that the ratio of heterotroph to autotroph biomass (HB/AB ratio) declined rapidly as the biomass of autotrophs increases with increasing nutrient inputs. The biomass distribution within the community shifted from an 'inverted pyramid' distribution, involving greater biomass of heterotrophs than that of autotrophs, at low nutrient inputs, to the conventional 'upward' pyramid pattern, where the biomass of autotrophs exceeds that of consumers, at the highest nutrient inputs. This shift stabilized after 4 d, and the pyramids remained quite constant for the rest of the experiment. The experimental test presented supports the hypothesis that the relative biomass distribution between heterotrophs and autotrophs is regulated by nutrient supply., This is a contribution to the project COMWEB, funded by the ELOISE programme of the European Commission (contract number MAS3-CT96-0045), and a grant from the Spanish Commission for Science and Technology (CICYT MAR96-1715-CE).

10 pages, 6 figures, Fluorescently labeled tracers (FLT) are often used to estimate the loss rates of picoplankton to grazers. These tracers are commonly enumerated by epifluorescence microscopy, although flow cytometry is a viable alternative in the detection of FITC (fluorescein-5-isothiocyanate)- or DTAF (5-([4,6-dichlorotriazin-2-yl)amino]-fluorescein)-stained bacterial tracers. However, the bacterivory measured with FLT has hardly been applied to routine monitoring of oceanic waters, partly because of the time-consuming preparation of the tracers and other problems associated with the long-term incubations needed to generate detectable rates of tracer change. In addition, these long-term incubations make samples especially sensitive to the unwanted addition of nutrients carried over with the tracers. Here we present some experiments designed to ease the estimation of grazing rates on bacteria with this technique. Two bacterial strains and 2 fluorescent dyes were tested: Escherichia coli minicells (0.065 μm3) and Pseudomonas diminuta (0.064 μm3), stained with DTAF or with FITC. In addition, instead of the common use of pyrophosphate buffer during the staining protocol, the use of carbonate-bicarbonate buffer and cells scraped directly from solid media is suggested to avoid the problems associated with phosphorus enrichment of the sample that at times can occur in oligotrophic water samples. The FITC- or DTAF-stained tracers can be observed with either epifluorescence microscopy or flow cytometry. However, FITC- or DTAF-stained P. diminuta were more easily resolved with the flow cytometer than stained minicells. Flow cytometric detection of P. diminuta tracers, prepared in bicarbonate-buffer and stained with FITC, is a fast protocol for the estimation of the grazing loss rates of bacteria in oceanic environments, This work was part of the project AMB94-0746, financed by CYCIT which included a research grant to E.V.-D. F.P. and J.M.G. also acknowledge the EU project MEDEA (MAS3-CT95-0016)