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Due to the scarcity of organic matter (OM) sources in the bathypelagic (1000–4000 m depth), prokaryotic metabolism is believed to be concentrated on particles originating from the surface. However, the structure of active bathypelagic prokaryotic communities and how it changes across environmental gradients remains unexplored. Using a combination of 16S rRNA gene and transcripts sequencing, metagenomics, and substrate uptake potential measurements, here we aimed to explore how water masses aging and the quality of OM influence the structure of the active microbiome, and the potential implications for community function. We found that the relative contribution of taxa with a free-living lifestyle to the active microbiome increased in older water masses that were enriched in recalcitrant OM, suggesting that these prokaryotes may also play a substantial role in the bathypelagic metabolism of vast areas of the ocean. In comparison to particle-associated prokaryotes, free-living prokaryotes exhibited lower potential metabolic rates, and harbored a limited number of two-component sensory systems, suggesting they have less ability to sense and respond to environmental cues. In contrast, particle-associated prokaryotes carried genes for particle colonization and carbohydrate utilization that were absent in prokaryotes with a free-living lifestyle. Consistently, we observed that prokaryotic communities inhabiting older waters displayed reduced abilities to colonize particles, and higher capabilities to use complex carbon sources, compared to communities in waters with a higher proportion of labile OM. Our results provide evidence of regionalization of the bathypelagic active prokaryotic microbiome, unveiling a niche partitioning based on the quality of OM

[ES] Este proyecto pionero reúne a 18 socios de 20 instituciones y 11 países europeos. El objetivo es avanzar en la comprensión de los efectos combinados de los contaminantes y el cambio climático en diversos entornos marinos, [CAT] Aquest projecte pioner reuneix 18 socis de 20 institucions i 11 països europeus. L’objectiu és avançar en la comprensió dels efectes combinats dels contaminants i el canvi climàtic a diversos entorns marins

The Ocean microbiome has a crucial role in Earth’s biogeochemical cycles. During the last decade, global cruises such as Tara Oceans and the Malaspina Expedition have expanded our understanding of the diversity and genetic repertoire of marine microbes. Nevertheless, there are still knowledge gaps regarding their diversity patterns throughout depth gradients ranging from the surface to the deep ocean. Here we present a dataset of 76 microbial metagenomes (MProfile) of the picoplankton size fraction (0.2–3.0 µm) collected in 11 vertical profiles covering contrasting ocean regions sampled during the Malaspina Expedition circumnavigation (7 depths, from surface to 4,000 m deep). The MProfile dataset produced 1.66 Tbp of raw DNA sequences from which we derived: 17.4 million genes clustered at 95% sequence similarity (M-GeneDB-VP), 2,672 metagenome-assembled genomes (MAGs) of Archaea and Bacteria (Malaspina-VP-MAGs), and over 100,000 viral genomic sequences. This dataset will be a valuable resource for exploring the functional and taxonomic connectivity between the photic and bathypelagic tropical and sub-tropical ocean, while increasing our general knowledge of the Ocean microbiome

María Montserrat Sala es investigadora del Instituto de Ciencias del Mar y una de las coordinadoras del proyecto

The DOGMA cruise is a single day campaign to collect water and particles for subsequent organic matter degradation experiments

There is a notable underrepresentation of female scientists in STEM research fields. Pursuing an academic career in the sciences requires persistence, perseverance, and courage. Despite awareness and implementation of measures to counteract them, women in STEM often face persistent challenges, including gender bias, stereotypes, unequal access to opportunities, and limited mentorship (Shen, 2013; Charlesworth and Banaji, 2019; Avolio et al., 2020; Jebsen et al., 2022; Freedman et al., 2023; Lathifa, 2023). Women juggle multiple roles in the workplace as researchers, teachers, and mentors, on top of administrative duties, and at home, women often shoulder much of the caregiving (Cech and Blair-Loy, 2019; Allen et al., 2023). The COVID-19 pandemic intensified these difficulties, adversely impacting work productivity, mental health, the pursuit of leadership positions, and an essential aspect for conducting outstanding research: achieving a healthy work-life balance (Gewin, 2020; Krukowski et al., 2021; National Academies of Sciences and Medicine, 2021; Lawson et al., 2023). [...]

There is a gap in knowledge on broad-scale diversity and functional patterns along the continuum between the photic and the dark ocean microbes, including trends in genome size (GS) and functional diversity. To address this gap, we present the Malaspina Microbial Vertical Profiles Metagenomes dataset, comprising 76 microbial metagenomes (0.2-3um) collected in 11 stations in the tropical and subtropical open oceans from 3 to 4,000 m deep, a 46.3 million gene catalog and 1,228 Metagenome-Assembled Genomes (MAGs). Our results show increased functional richness in the bathypelagic and a strong positive association between GS and depth for the most abundant phyla in samples up to 1,000 m, while this association is absent in the bathypelagic. Conversely, lower abundance taxa displayed a bimodal distribution of GS, with a smaller peak in the mesopelagic and a larger peak in the bathypelagic. Opposing trends were observed for protein-encoding gene density and functional diversity across epipelagic and deep ocean MAGs. Specifically, protein-encoding gene density was higher at the epipelagic and decreased towards the bathypelagic for most of the taxa, pointing out that genome streamlining generally holds true, but it is not universal for all microbes. Larger prokaryotic GS in the bathypelagic was associated with a higher prevalence of prophages and defense systems. We suggest that a combination of factors, including genome streamlining, metabolic diversification, viral infection, and molecular defense systems, drive the genomic diversity of marine prokaryotes across ocean layers

Dissolved organic matter (DOM) remineralization is limited to a greater or lesser degree at different layers in the ocean. In order to investigate the key abiotic and biotic factors that may enhance DOC remineralization in the deep ocean, we studied the changes in function and structure of deep prokaryotic communities after additions of the same amount of dissolved organic compounds with different quality. We performed experiments with deep sea prokaryotic assemblages subjected to four different treatments: CL (addition of labile DOC), CM (addition of a mix of organic compounds with different lability) and CR (addition of recalcitrant humic acids) and K (control). Prokaryotic biomass markedly increased when the labile compounds or the diverse chemical mixture were added (CL and CM treatments), whereas the prokaryotic response was lower under either the control K or CR treatments. Accordingly, we found lower activity and similar patterns of extracellular enzymatic activities in CR and K compared to CL and CM treatments. In all treatments a decrease in prokaryotic richness consistently occurred along the incubations, but interestingly, the decrease was lower in the control and in the CR treatments. Our results indicate that the chemical diversity of organic matter is a key factor determining microbial structure and activity in the deep ocean

Phytoplankton-derived particles represent one of the main sources of organic matter (OM) for marine systems, fuelling heterotrophic life and sequestering C in the ocean interior. The solubilization of sinking particles and the production of recalcitrant dissolved organic matter (RDOM) linked to microbial activities are two of the main processes influencing the ocean C storage capacity. These processes depend on the seasonally-variable makeup of phytoplankton assemblages, however, our understanding of this relationship is hindered by the difficulty of relating the processing of OM at depth with its phytoplanktonic source. To better understand the effects of the phytoplanktonic origin of particles on their degradation process, we performed three experiments using particles from the surface NW Mediterranean covering a wide range of phytoplankton assemblage compositions. The particulate material was added to mesopelagic water containing natural microbial communities and incubated in the dark during 25 days while monitoring particulate and dissolved organic carbon, and microbial concentrations. In addition, using fluorescence spectroscopy, we were able to track the labile and recalcitrant fluorescent fractions of both the dissolved and total (particulate + dissolved) OM pools. We will describe the fluorescent OM dynamics during the degradation process and discuss the results in relation to the source phytoplankton composition, with special focus on the processes of particle solubilization and production of RDOM within the context of the biological and microbial pump efficiencies

Phytoplankton-derived organic matter sustains heterotrophic marine life in regions away from terrestrial inputs such as the Southern Ocean. Fluorescence spectroscopy has long been used to characterize the fluorescent organic matter (FOM) pool. However, most studies focus only in the dissolved FOM fraction (FDOM) disregarding the contribution of particles. In order to assess the dynamics and drivers of the dissolved and particulate fractions of FOM, we used a Lagrangian approach to follow the time evolution of phytoplankton proliferations at four different sites in the Southern Ocean and compared the FOM in filtered and unfiltered seawater aliquots. We found that filtration had little effects on FOM visible spectrum fluorescence intensities, implying that most of this signal was due to dissolved fluorophores. On the other hand, protein-like fluorescence was strongly supressed by filtration, with fluorescence of particles accounting for up to 90 % of the total protein-like FOM. Photobleaching was identified as the main driver of visible FDOM composition, which was better described by indices of phytoplankton photoacclimation than by measurements of the incident solar radiation dose. In contrast, protein-like FOM intensity and fractionation were primarily related to abundance, composition and physiological state of phytoplankton proliferations. The chlorophyll a concentration from non-diatom phytoplankton explained 91 % of the particulate protein-like FOM variability. The proportion of protein-like fluorescence found in the dissolved phase was predicted by the combination of potential viral and grazing pressures, which accounted for 51 and 29 % of its variability, respectively. Our results show that comparing FOM measurements from filtered and unfiltered seawater provides relevant information on the taxonomic composition and cell integrity of phytoplankton communities. A better understanding of the commonly overlooked FOM fractionation process is essential for the impl

[EN] Marine phytoplankton, including cyanobacteria and microalgae, dominates primary production across two thirds of the earth’s surface, sustaining virtually all marine life and exerting a fundamental control over global climate through carbon sequestration into the deep ocean. These unicellular photoautotrophs are responsible for roughly 50% of global net primary production, which is equivalent to producing 50 gigatons of organic carbon (C) per year (about 140 million t per day). […], [ES] El fitoplancton marino, que incluye tanto a las cianobacterias como a las microalgas, domina la producción primaria en dos tercios de la superficie de la Tierra, sustentando prácticamente toda la vida marina y ejerciendo un control fundamental sobre el clima global mediante el secuestro de carbono en las profundidades del océano. Estos productores primarios unicelulares son responsables de aproximadamente el 50% de la producción primaria neta mundial, lo que equivale a producir 50 gigatoneladas de carbono orgánico (C) al año (alrededor de 140 millones de toneladas al día). […], [CAT] El fitoplàncton marí, que inclou tant als cianobacteris com a les microalgues, domina la producció primària en dos terços de la superfície de la Terra, sustentant pràcticament tota la vida marina i exercint un control fonamental sobre el clima global mitjançant el segrest de carboni en les profunditats de l’oceà. Aquests productors primaris unicel·lulars són responsables d’aproximadament el 50% de la producció primària neta mundial, la qual cosa equival a produir 50 gigatones de carboni orgànic (C) l’any (al voltant de 140 milions de tones al dia). […]

[EN] The best known story about biotechnology and biodiversity discovery is probably the one that links the scientists Thomas D. Brock and Kary Mullis to the death row convict Kirk Bloodsworth. Back in the 1960s, Thomas Brock studied hyperthermophiles (prokaryotes that develop at very high temperatures) in Yellowstone. He found a bacterium that grows and divides at temperatures of up to 70°C, and named it Thermus aquaticus. He worked for the sake of discovery, to understand the limits of life, and nobody asked him to do applied research. Many years later, Kary Mullis remembered that this prokaryote replicates its DNA at high temperature, and he developed a method for copying and multiplying any DNA molecules using the enzyme isolated from Thermus. This technique, named polymerase chain reaction (PCR), has become a buzzword for most of us and led him to win the Nobel Prize for Chemistry and to earn a great deal of money. When PCR was first used in a court case, it demonstrated that Kirk Bloodsworth was not responsible for the murder he had been charged with. Brock never imagined that his basic research could make some people rich, lead to others being acquitted and allow many of us to be tested for COVID19. His work led to significant biotechnological advances and developments. […], [ES] Probablemente es la historia sobre biotecnología y descubrimiento de biodiversidad que más gente conoce: la que relaciona los científicos Thomas D. Brock y Kary Mullis con el condenado a muerte Kirk Bloodsworth. En los años 60, Thomas Brock estudió en Yellowstone los microorganismos hipertermófilos (procariotas que se desarrollan a temperaturas muy altas). Encontró una bacteria que crece y se divide a temperaturas de hasta 70°C, y la llamó Thermus aquaticus. Trabajó por el placer del conocimiento, para comprender los límites de la vida. Nadie le pidió nunca que hiciera investigaciones “aplicadas”. Muchos años después, Kary Mullis recordó que este procariota replica su ADN a altas temperaturas y desarrolló un método para copiar y multiplicar cualquier molécula de ADN mediante una enzima aislada de Thermus. Esta técnica, llamada reacción en cadena de la polimerasa (PCR), se ha convertido en una palabra de moda habitual para la mayoría de nosotros y le hizo ganar el premio Nobel de Química, a la vez que él y su empresa ganaban mucho dinero. Y he aquí que unos años después, la primera vez que se utilizó la PCR en un caso judicial, permitió demostrar que Kirk Bloodsworth no era responsable del asesinato del que le habían acusado. Brock, el descubridor de la biodiversidad, nunca pensó que su investigación básica podría llegar a hacer que algunas personas fueran ricas, otras absueltas de asesinato y la mayor parte de nosotros nos hiciéramos pruebas para detectar la COVID19. La investigación básica en diversidad permitió avances y desarrollos biotecnológicos muy significativos. […], [CAT] És probablement la història sobre biotecnologia i descoberta de biodiversitat que més gent coneix: la que relaciona els científics Thomas D. Brock i Kary Mullis amb el condemnat a mort Kirk Bloodsworth: als anys 60, Thomas Brock va estudiar a Yellowstone els microorganismes hipertermòfils (procariotes que es desenvolupen a temperatures molt altes). Va trobar un bacteri que creix i es divideix a temperatures de fins a 70°C, i el va anomenar Thermus aquaticus. Va treballar pel plaer del coneixement, per comprendre els límits de la vida. Ningú li va demanar que fes investigacions “aplicades”. Molts anys després, Kary Mullis va recordar que aquest procariota replica el seu ADN a altes temperatures i va desenvolupar un mètode per copiar i multiplicar qualsevol molècula d’ADN mitjançant un enzim aïllat de Thermus. Aquesta tècnica, anomenada reacció en cadena de la polimerasa (PCR), s’ha convertit en una paraula de moda habitual per a la majoria de nosaltres i li va fer guanyar el premi Nobel de Química, alhora que ell i la seva empresa guanyaven molts diners. I vet aquí que uns anys després, la primera vegada que es va utilitzar la PCR en un cas judicial, va permetre demostrar que Kirk Bloodsworth no era responsable de l’assassinat del qual l’havien acusat. Brock, el descobridor de la biodiversitat, mai no va pensar que la seva investigació bàsica podria arribar a fer que algunes persones fossin riques, altres absoltes d’assassinat i la major part de nosaltres ens féssim proves per detectar la COVID19. La recerca bàsica en diversitat va permetre avenços i desenvolupaments biotecnològics molt significatius. […]

[EN] The biosphere has always undergone changes brought about by alterations in the geological substrate and the climate, and by the emergence of species with new functions and interactions. However, today we find ourselves in a unique scenario, in which a single species, humankind, is drastically and rapidly threatening all ecosystems. To deal with this situation, we need tools to track natural systems, understand changes at different time scales, document the current status and apply models to predict the future. This knowledge will be essential for proposing strategies aimed at mitigating anthropogenic changes. Because of its extent and its volume of water, the sea is a crucial component of the Earth system. It was where life originated and diversified and today it contains an enormous biodiversity. Human societies have established a close relationship with the sea, which provides food, hosts maritime transport, and has become an attractive leisure space. The sea is also under the influence of global change, with challenges such as warming, acidification and pollution. Marine microorganisms, invisible but ubiquitous, are also affected (Hutchins and Fu 2017). Since they divide very quickly, they can act as sentinels of these changes, and this justifies the need for microbial observatories. […], [ES] La biosfera siempre ha experimentado cambios, forzados por modificaciones del sustrato geológico, alteraciones climáticas, y la aparición de especies con nuevas funciones e interacciones. Sin embargo, hoy en día nos encontramos en un escenario único, donde una sola especie amenaza de manera drástica y rápida todos los ecosistemas. Más que nunca, necesitamos herramientas para hacer un seguimiento de los sistemas naturales, entender los cambios a diferentes escalas temporales, documentar el estado actual, y aplicar modelos para hacer predicciones de futuro. Este conocimiento será fundamental para proponer estrategias de mitigación de los cambios antropogénicos. El mar, por su extensión y volumen de agua, representa un componente crucial del sistema Tierra. Ha sido donde la vida se originó y se diversificó, y contiene hoy en día una biodiversidad enorme. Las sociedades humanas han establecido un vínculo estrecho con el mar, de donde extraen alimentos, les sirve de medio de transporte y se ha convertido en un atractivo espacio de ocio. El mar está también bajo la influencia del cambio global, con retos como el calentamiento, la acidificación, y la contaminación. Los microorganismos marinos, invisibles pero omnipresentes, se ven también afectados (Hutchins y Fu 2017). Dado que se dividen muy rápido, pueden actuar como centinelas de estos cambios, y esto justifica la necesidad de observatorios microbianos. […], [CAT] La biosfera sempre ha experimentat canvis, forçats per modificacions del substrat geològic, alteracions climàtiques, i l’aparició d’espècies amb noves funcions i interaccions. Tanmateix, avui en dia ens trobem en un escenari únic, on una sola espècie amenaça de manera dràstica i ràpida tots els ecosistemes. Més que mai, necessitem eines per a fer un seguiment dels sistemes naturals, entendre els canvis a diferents escales temporals, documentar l’estat actual, i aplicar models per a fer prediccions de futur. Aquest coneixement serà fonamental per proposar estratègies de mitigació dels canvis antropogènics. El mar, per la seva extensió i volum d’aigua, representa un component crucial del sistema Terra. Ha estat on la vida s’originà i es diversificà, i conté avui dia una biodiversitat enorme. Les societats humanes han establert un vincle estret amb el mar, d’on extreuen aliments, els serveix de medi de transport i ha esdevingut un atractiu espai de lleure. El mar està també sota la influència del canvi global, amb reptes com l’escalfament, l’acidificació, i la contaminació. Els microorganismes marins, invisibles però omnipresents, es veuen també afectats (Hutchins i Fu 2017). Atès que es divideixen molt ràpid, poden actuar com a sentinelles d’aquests canvis, i això justifica la necessitat d’observatoris microbians. […]

[EN] Life came to earth almost four million years ago, when primitive microorganisms appeared in the middle of sulphurous oceans and toxic skies. That the air we have today is breathable is mainly due to microorganisms, which have been evolving and changing our planet for billions of years until we now have enough oxygen for our lives (Figure 1) (Vila and Balagué 2021). Because of their enormous genetic variability and rapid growth, microorganisms have a great capacity to respond and adapt to new environmental conditions. Lately, however, marine microorganisms are facing a series of unprecedented environmental changes, many of them associated with human impacts, and their responsiveness will be key to sustaining life in marine ecosystems (Hutchins and Fu 2017). […], [ES] La vida surgió en la tierra hace casi 4 millones de años, cuando microorganismos primitivos aparecieron en medio de océanos sulfurosos y cielos tóxicos. Que el aire que tenemos hoy en día sea respirable lo debemos a microorganismos, que han ido evolucionando y modificando nuestro planeta durante miles de millones de años hasta conseguir que ahora haya suficiente oxígeno para nuestra vida (figura 1) (Vila y Balagué 2021). Su enorme variabilidad genética y rápido crecimiento hace que los microorganismos tengan una gran capacidad de respuesta y adaptación a las nuevas condiciones ambientales. Últimamente, sin embargo, los microorganismos marinos se están enfrentando a toda una serie de cambios ambientales sin precedentes, muchos de ellos asociados a impactos humanos, y su capacidad de respuesta será clave para mantener la vida en los ecosistemas marinos (Hutchins y Fu 2017). […], [CAT] La vida va sorgir a la terra fa gairebé 4 milions d’anys, quan microorganismes primitius van aparèixer en mig d’oceans sulfurosos i cels tòxics. Que l’aire que tenim avui en dia sigui respirable ho devem sobretot a microorganismes, que han anat evolucionant i modificant el nostre planeta durant milers de milions d’anys fins a aconseguir que ara hi hagi suficient oxigen per la nostra vida (figura 1) (Vila i Balagué 2021). La seva enorme variabilitat genètica i ràpid creixement fa que els microorganismes tinguin una gran capacitat de resposta i adaptació a les noves condicions ambientals. Darrerament, però, els microorganismes marins s’estan enfrontant a tota una sèrie de canvis ambientals sense precedents, molts d’ells associats a impactes humans, i la seva capacitat de resposta serà clau per mantenir la vida als ecosistemes marins (Hutchins i Fu 2017). […]

Anthropogenic carbon emissions are causing changes in seawater carbonate chemistry including a decline in the pH of the oceans. While its aftermath for calcifying microbes has been widely studied, the effect of ocean acidification (OA) on marine viruses and their microbial hosts is controversial, and even more in combination with another anthropogenic stressor, i.e., human-induced nutrient loads. In this study, two mesocosm acidification experiments with Mediterranean waters from different seasons revealed distinct effects of OA on viruses and viral-mediated prokaryotic mortality depending on the trophic state and the successional stage of the plankton community. In the winter bloom situation, low fluorescence viruses, the most abundant virus-like particle (VLP) subpopulation comprising mostly bacteriophages, were negatively affected by lowered pH with nutrient addition, while the bacterial host abundance was stimulated. High fluorescence viruses, containing cyanophages, were stimulated by OA regardless of the nutrient conditions, while cyanobacteria of the genus Synechococcus were negatively affected by OA. Moreover, the abundance of very high fluorescence viruses infecting small haptophytes tended to be lower under acidification while their putative hosts' abundance was enhanced, suggesting a direct and negative effect of OA on viral–host interactions. In the oligotrophic summer situation, we found a stimulating effect of OA on total viral abundance and the viral populations, suggesting a cascading effect of the elevated pCO2 stimulating autotrophic and heterotrophic production. In winter, viral lysis accounted for 30 ± 16% of the loss of bacterial standing stock per day (VMMBSS) under increased pCO2 compared to 53 ± 35% in the control treatments, without effects of nutrient additions while in summer, OA had no significant effects on VMMBSS (35 ± 20% and 38 ± 5% per day in the OA and control treatments, respectively). We found that phage production and resulting o

Així ho destaca un estudi liderat per l’Institut de Ciències del Mar que ha comparat mostres de la badia de Blanes de fa 15 anys amb les del 2020

El confinamiento por la epidemia de covid-19 del año pasado mejoró la calidad del agua del Mediterráneo, según un estudio liderado por el Instituto de Ciencias del Mar (ICM-CSIC), que ha comparado muestras de la bahía de Blanes (Girona) recogidas durante la primavera de 2020 con otras del mismo enclave recopiladas a lo largo de los 15 años anteriores

The ocean surface microlayer (SML), with physicochemical characteristics different from those of subsurface waters (SSW), results in dense and active viral and microbial communities that may favor virus¿host interactions. Conversely, wind speed and/or UV radiation could adversely affect virus infection. Furthermore, in polar regions, organic and inorganic nutrient inputs from melting ice may increase microbial activity in the SML. Since the role of viruses in the microbial food web of the SML is poorly understood in polar oceans, we aimed to study the impact of viruses on prokaryotic communities in the SML and in the SSW in Arctic and Antarctic waters. We hypothesized that a higher viral activity in the SML than in the SSW in both polar systems would be observed. We measured viral and prokaryote abundances, virus-mediated mortality on prokaryotes, heterotrophic and phototrophic nanoflagellate abundance, and environmental factors. In both polar zones, we found small differences in environmental factors between the SML and the SSW. In contrast, despite the adverse effect of wind, viral and prokaryote abundances and virus-mediated mortality on prokaryotes were higher in the SML than in the SSW. As a consequence, the higher carbon flux released by lysed cells in the SML than in the SSW would increase the pool of dissolved organic carbon (DOC) and be rapidly used by other prokaryotes to grow (the viral shunt). Thus, our results suggest that viral activity greatly contributes to the functioning of the microbial

Particulate organic matter (POM) lability is one of the key factors determining the residence time of organic carbon (OC) in the marine system. Phytoplankton community composition can influence the rate at which heterotrophic microorganisms decompose phytoplankton detrital particles and thus, it controls the fraction of OC that reaches the ocean depths, where it can be sequestered for climate-relevant spans of time. Here, we compared the degradation dynamics of POM from phytoplankton assemblages of contrasting diatom dominance in the presence of mesopelagic prokaryotic communities during a 19-day degradation experiment. We found that diatom-derived POM exhibited an exponential decay rate approximately three times lower than that derived from a community dominated by flagellated phytoplankton (mainly coccolithophores and nanoflagellates). Additionally, dissolved organic matter (DOM) released during the degradation of diatom particles accumulated over the experiment, whereas only residual increases in DOM were detected during the degradation of non-diatom materials. These results suggest that diatom-dominance enhances the efficiencies of the biological carbon pump and microbial carbon pump through the relatively reduced labilities of diatom particles and of the dissolved materials that arise from their microbial processing

The ocean surface microlayer (SML), with physicochemical characteristics different from those of subsurface waters (SSW), results in dense and active viral and microbial communities that may favor virus–host interactions. Conversely, wind speed and/or UV radiation could adversely affect virus infection. Furthermore, in polar regions, organic and inorganic nutrient inputs from melting ice may increase microbial activity in the SML. Since the role of viruses in the microbial food web of the SML is poorly understood in polar oceans, we aimed to study the impact of viruses on prokaryotic communities in the SML and in the SSW in Arctic and Antarctic waters. We hypothesized that a higher viral activity in the SML than in the SSW in both polar systems would be observed. We measured viral and prokaryote abundances, virus-mediated mortality on prokaryotes, heterotrophic and phototrophic nanoflagellate abundance, and environmental factors. In both polar zones, we found small differences in environmental factors between the SML and the SSW. In contrast, despite the adverse effect of wind, viral and prokaryote abundances and virus-mediated mortality on prokaryotes were higher in the SML than in the SSW. As a consequence, the higher carbon flux released by lysed cells in the SML than in the SSW would increase the pool of dissolved organic carbon (DOC) and be rapidly used by other prokaryotes to grow (the viral shunt). Thus, our results suggest that viral activity greatly contributes to the functioning of the microbial food web in the SML, which could influence the biogeochemical cycles of the water column

Different factors affect the way dissolved organic matter (DOM) is processed in the ocean water column, including environmental conditions and the functional capabilities of the communities. Recent studies have shown that bathypelagic prokaryotes are metabolically flexible, but whether this versatility translates into a higher ability to process DOM has been barely explored. Here we performed a multifactorial transplant experiment to compare the growth, activity and changes in DOM quality in surface and bathypelagic waters inoculated with either surface or bathypelagic prokaryotic communities. The effect of nutrient additions to surface waters was also explored. Despite no differences in the cell abundance of surface and deep ocean prokaryotes were observed in any of the treatments, in surface waters with nutrients the heterotrophic production of surface prokaryotes rapidly decreased. Conversely, bathypelagic communities displayed a sustained production throughout the experiment. Incubations with surface prokaryotes always led to a significant accumulation of recalcitrant compounds, which did not occur with bathypelagic prokaryotes, suggesting they have a higher ability to process DOM. These contrasting abilities could be explained by the recruitment of a comparatively larger number of opportunistic taxa within the bathypelagic assemblages, which likely resulted in a broader community capability of substrate utilization