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13 páginas, 5 figuras ; [EN] An increase in the ratio of carotenoids/chlorophylls has been widely used as an indicator of light or nutrient stress in phytoplankton. An experiment was carried out to examine the changes of pigment composition in a dinoflagellate, Heterocapsa sp., and a diatom, Thalassiosira weissflogii grown in batch cultures under high (15 0 mmol m-2 s-1) and low (50 mmol m-2 s-1) irradiances. During the exponential growth phase, the cell content of the pigments involved in light harvesting functions such as fucoxanthin, peridinin, chlorophyll c, and chlorophyll a, was higher in cells cultured under lower irradiances. Diadinoxanthin concentration, however, was higher in cells cultured under higher irradiances, which supports a photoprotective role for this pigment. The shift from the exponential to the stationary phase (likely to be a result of nutrient starvation) produced a more drastic pigment loss in cells growing in the high irradiance cultures. As an exception, diatoxanthin showed an important increase, independent of irradiance and especially in T. weissflogii, when cells progressed from the exponential to the stationary phase. This result suggests that this pigment could be used as an indicator of nutrient starvation. In general, pigment ratios were more affected by the nutrient condition of the cells than by the different irradiances ; [ES] El incremento del cociente carotenoides/clorofilas ha sido comúnmente usado como un indicador del grado de estrés de luz o nutrientes del fitoplancton. Se ha llevado a cabo un experimento para examinar los cambios que sufre la composición pigmentaria de un dinoflagelado. Heterocapsa sp., y una diatomea. Thalassiosira weissflogii, creciendo en cultivos cerrados bajo altas (15 0 mmol m-2 s-1) y bajas (50 mmol m-2 s-1) condiciones de irradiancia. La concentración celular de los pigmentos recolectores de luz (fucoxantina. peridinina, clorofila c. y clorofila a) fue más alta en las células sometidas a baja irradiancia. Por el contrario, la concentración de diadinoxantina fue mayor en las células que crecíán a alta irradiancia, lo cual confirma su papel fotoprotector. El paso de fase exponencial a fase estacionaria (probablemente como consecuencia de un agotamiento de nutrientes) produjo una mayor pérdida de pigmentos en las células que crecían bajo alta irradiancia ; This study was supported by a FPI fellowship from the Spanish Government (Ministerio de Educación y Ciencia) and by the CYCIT MAR88-0252 project ; Peer reviewed

21 pages, 7 figures, 3 tables, supporting information https://dx.doi.org/10.1002/lno.10452 ; The fine vertical distribution of phytoplankton groups within the deep chlorophyll maximum (DCM) was studied in the NE Atlantic during summer stratification. A simple but unconventional sampling strategy allowed examining the vertical structure with ca. 2 m resolution. The distribution of Prochlorococcus, Synechococcus, chlorophytes, pelagophytes, small prymnesiophytes, coccolithophores, diatoms, and dinoflagellates was investigated with a combination of pigment-markers, flow cytometry and optical and FISH microscopy. All groups presented minimum abundances at the surface and a maximum in the DCM layer. The cell distribution was not vertically symmetrical around the DCM peak and cells tended to accumulate in the upper part of the DCM layer. The more symmetrical distribution of chlorophyll than cells around the DCM peak was due to the increase of pigment per cell with depth. We found a vertical alignment of phytoplankton groups within the DCM layer indicating preferences for different ecological niches in a layer with strong gradients of light and nutrients. Prochlorococcus occupied the shallowest and diatoms the deepest layers. Dinoflagellates, Synechococcus and small prymnesiophytes preferred shallow DCM layers, and coccolithophores, chlorophytes and pelagophytes showed a preference for deep layers. Cell size within groups changed with depth in a pattern related to their mean size: the cell volume of the smallest group increased the most with depth while the cell volume of the largest group decreased the most. The vertical alignment of phytoplankton groups confirms that the DCM is not a homogeneous entity and indicates groups' preferences for different ecological niches within this layer ; This study has been financed by projects FAMOSO (CTM2008-062621-C03/MAR, Ministerio de Economia y Competitividad, Spanish Government) and GIDEP (GRUPIN14-144, Principado de Asturias). A. M. Cabello was supported by a Spanish FPI grant (BES-2009-0277194) ; Peer Reviewed

23 pages, 10 figures, 5 tables, supplementary material https://doi.org/10.1016/j.pocean.2021.102737 ; A detailed study of phytoplankton composition and dynamics was carried out during three contrasting situations (cruises F1, F2, and F3) in the northwestern (NW) Mediterranean Sea. Haptophytes, diatoms, and green algae dominated in F1, during the spring bloom, with high nutrients and high phytoplankton biomass. In F2, the post-bloom situation with a still weak stratification and lower nutrient concentrations, we found a high spatial variability. Stations were clearly dominated by either Synechococcus, haptophytes or cryptophytes; with Synechococcus reaching the highest abundance (4 × 105 cells mL−1, 60% of the integrated chlorophyll a) reported to date for the open Mediterranean Sea. Cryptophytes accumulated close to the surface in very shallow mixed layer stations. In late summer, F3 revealed a fully developed stratification with low nutrients and a marked deep chlorophyll maximum (DCM). Prochlorococcus was present only during this cruise, mainly in deep layers together with haptophytes and pelagophytes, while haptophytes and Synechococcus dominated the upper mixed layer. Flow cytometry (FCM) and pigment-based abundance estimates for Prochlorococcus, Synechococcus and cryptophytes were well correlated, as happened also between small picoeukaryotes (FCM) and green algae (pigments), and between large picoeukaryotes (FCM) and haptophytes (pigments). Dinoflagellate abundance by microscopy and by pigments did not agree well, probably due to the presence of heterotrophic forms or because they contained pigments other than peridinin, the standard dinoflagellate marker. The decrease in size of the FCM large picoeukaryotes group with depth was presumably related to the increasing contribution of pelagophytes, with smaller cells than haptophytes, the other main component of this fraction. Cell size increase of Prochlorococcus and Synechococcus with depth suggests vertical segregation of genotypes or photoadaptation. The groups' ecological preferences are presented with respect to depth and nutrient concentrations. Synechococcus and cryptophytes occupied shallow layers; diatoms, green algae and Prochlorococcus showed a tendency for deep layers and pelagophytes for even deeper layers, while haptophyte and dinoflagellate allocations were less clear. As for nutrients, the maximum relative contributions of green algae and especially diatoms occurred when dissolved inorganic phosphorus (DIP) concentrations were highest, of Prochlorococcus, dinoflagellates and pelagophytes when lowest, and of Synechococcus and cryptophytes when DIP concentrations were low but not minimal. The contribution of haptophytes did not show a relationship with DIP concentration. These results from individual groups stand as significant exceptions to the general relationship between phytoplankton cell size and nutrient availability ; This work was funded by Spanish Government, Ministerio de Ciencia e Innovación (project FAMOSO, grant CTM2008-06261-C03/MAR). AGR was also supported by the Generalitat de Catalunya (Grup de Recerca 2061) ; With the institutional support of the 'Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S) ; Peer reviewed

A detailed study of phytoplankton composition and dynamics was carried out during three contrasting situations (cruises F1, F2, and F3) in the northwestern (NW) Mediterranean Sea. Haptophytes, diatoms, and green algae dominated in F1, during the spring bloom, with high nutrients and high phytoplankton biomass. In F2, the post-bloom situation with a still weak stratification and lower nutrient concentrations, we found a high spatial variability. Stations were clearly dominated by either Synechococcus, haptophytes or cryptophytes; with Synechococcus reaching the highest abundance (4 × 105 cells mL−1, 60% of the integrated chlorophyll a) reported to date for the open Mediterranean Sea. Cryptophytes accumulated close to the surface in very shallow mixed layer stations. In late summer, F3 revealed a fully developed stratification with low nutrients and a marked deep chlorophyll maximum (DCM). Prochlorococcus was present only during this cruise, mainly in deep layers together with haptophytes and pelagophytes, while haptophytes and Synechococcus dominated the upper mixed layer. Flow cytometry (FCM) and pigment-based abundance estimates for Prochlorococcus, Synechococcus and cryptophytes were well correlated, as happened also between small picoeukaryotes (FCM) and green algae (pigments), and between large picoeukaryotes (FCM) and haptophytes (pigments). Dinoflagellate abundance by microscopy and by pigments did not agree well, probably due to the presence of heterotrophic forms or because they contained pigments other than peridinin, the standard dinoflagellate marker. The decrease in size of the FCM large picoeukaryotes group with depth was presumably related to the increasing contribution of pelagophytes, with smaller cells than haptophytes, the other main component of this fraction. Cell size increase of Prochlorococcus and Synechococcus with depth suggests vertical segregation of genotypes or photoadaptation. The groups' ecological preferences are presented with respect to depth and nutrient concentrations. Synechococcus and cryptophytes occupied shallow layers; diatoms, green algae and Prochlorococcus showed a tendency for deep layers and pelagophytes for even deeper layers, while haptophyte and dinoflagellate allocations were less clear. As for nutrients, the maximum relative contributions of green algae and especially diatoms occurred when dissolved inorganic phosphorus (DIP) concentrations were highest, of Prochlorococcus, dinoflagellates and pelagophytes when lowest, and of Synechococcus and cryptophytes when DIP concentrations were low but not minimal. The contribution of haptophytes did not show a relationship with DIP concentration. These results from individual groups stand as significant exceptions to the general relationship between phytoplankton cell size and nutrient availability. ; We thank the captain and crew of R/V Sarmiento de Gamboa and the Marine Technology Unit (UTM) for their co-operation and support during the cruises. We thank Carmen Cabeza and Felipe González for their help with HPLC pigment and microscopy analyses, and Alejandro Isla for his help with figure preparation and data processing. The thoughtful comments of two anonymous reviewers helped to improve the manuscript. This work was funded by Spanish Government, Ministerio de Ciencia e Innovación (project FAMOSO, grant CTM2008-06261-C03/MAR). AGR was also supported by the Generalitat de Catalunya (Grup de Recerca 2061).

19 pages, 9 figures, 3 tables ; We investigated the influence of ocean mixing and nutrient supply dynamics on picoplankton community composition in the context of Margalef's Mandala (Margalef 1978). Simultaneous measurements of microturbulence, nutrient concentration, and autotrophic and heterotrophic picoplankton properties, were collected during 3 cruises carried out in the northwestern Mediterranean Sea in March (F1), April/May (F2) and September (F3) 2009. The 3 cruises sampled different oceanographic conditions, starting with early stages of the late winter-early spring bloom, followed by the late stage of the bloom, and finally summer stratification. As a result of the variability in vertical diffusivity and the nitrate gradient across the nitracline, nitrate vertical fluxes were higher during F1 (23 ± 35 mmol m d), compared to F2 (0.4 ± 0.2 mmol m d) and F3 (0.09 ± 0.09 mmol m d). Prochlorococcus abundance was low when nitrate supply was high, Synechococcus exhibited the highest abundances at intermediate levels of nitrate supply and highest irradiance during F2, and large and small picoeukaryotic groups increased their abundance under high nutrient supply in F1. No significant relationships between the abundance of high and low nucleic acid heterotrophic bacteria and nitrate supply were found. In agreement with Margalef's model, our results show different responses of picophytoplankton groups to nitrate supply (probably reflecting differences in nutrient uptake abilities), and that the ratio of prokaryotic to picoeukaryotic photoautotrophic biomass decreases with increasing nitrate supply ; This work was funded by the Spanish projects TRYNITROP (CTM2004-05174-C02), FAMOSO (CTM2008-06261-C03), TURBIMOC (CTM2009-06712-E/MAR), and CHAOS (CTM 2012-30680). P.C. was supported by a Ramon y Cajal fellow-ship. B.F.-C. thanks the Spanish Government for support through a FPU fellowship (AP2010-5594) ; Peer Reviewed

6 pages, 4 figures, 1 table.-- PMID: 18552178 [PubMed].-- Supporting information available at: http://www.pnas.org/content/105/25/8724/suppl/DCSupplemental ; Analysis of marine cyanobacteria and proteobacteria genomes has provided a profound understanding of the life strategies of these organisms and their ecotype differentiation and metabolisms. However, a comparable analysis of the Bacteroidetes, the third major bacterioplankton group, is still lacking. In the present paper, we report on the genome of Polaribacter sp. strain MED152. On the one hand, MED152 contains a substantial number of genes for attachment to surfaces or particles, gliding motility, and polymer degradation. This agrees with the currently assumed life strategy of marine Bacteroidetes. On the other hand, it contains the proteorhodopsin gene, together with a remarkable suite of genes to sense and respond to light, which may provide a survival advantage in the nutrient-poor sun-lit ocean surface when in search of fresh particles to colonize. Furthermore, an increase in CO2 fixation in the light suggests that the limited central metabolism is complemented by anaplerotic inorganic carbon fixation. This is mediated by a unique combination of membrane transporters and carboxylases. This suggests a dual life strategy that, if confirmed experimentally, would be notably different from what is known of the two other main bacterial groups (the autotrophic cyanobacteria and the heterotrophic proteobacteria) in the surface oceans. The Polaribacter genome provides insights into the physiological capabilities of proteorhodopsin-containing bacteria. The genome will serve as a model to study the cellular and molecular processes in bacteria that express proteorhodopsin, their adaptation to the oceanic environment, and their role in carbon-cycling. Copyright © 2008 National Academy of Sciences ; This work was supported by the European Union (contract EU-FP6-505403NoE Marine Genomics Europe), the Spanish Ministry of Education and Science (Grant CTM2004-02586/MAR), and the Swedish Research Council (Grant 621-2003-2692) ; Peer reviewed