Patterns of prokaryotic activity along the marine planktonic matter continuum.

Prokaryotic abundance and activity are commonly assessed by dividing them into two size-fractions: free-living and attached to particles. Nevertheless, organic matter, essential for the growth of heterotrophic prokaryotes, is present in the environment in a continuum of sizes, from purely dissolved to large particles. Therefore, defining the activity of the prokaryotic community would be more accurate by considering all the distinct size fractions. To achieve this, we measured prokaryotic abundance (PA), heterotrophic prokaryotic activity (as leucine incorporation) and extracellular enzyme activities at a coastal site in the NW Mediterranean Sea. We conducted measurements in both bulk seawater and size fractionated samples sequentially passing through 5 different filter types: 0.2-0.8-3-5-10 µm pore size. Our results indicate that the fraction <0.8 µm contained the highest percentage of cells (91.6 ± 1.1 %) and leucine incorporation rates (72.2 ± 3.5 %). Most of the extracellular enzyme activity appeared in the dissolved fraction (<0.2 µm; 19.8-79.4 %), yet the specific activity of the enzymes (per cell activity) was 100-1000 times higher in the particulate (>0.8 µm) than in the free-living (0.2-0.8 µm) fraction. The size fraction with highest specific activities for leucine incorporation and most of the enzyme activities (ß-glucosidase, esterase, Leu-aminopeptidase and alkaline phosphatase) was the 5-10 µm fraction. In contrast, the higher specific chitobiase activity in the >10 µm fraction, suggests that the prokaryotic community colonizing large particles might be more specialized in the hydrolysis of organic matter of zooplanktonic origin than the community colonizing smaller particles.

Prokaryotic Capability to Use Organic Substrates Across the Global Tropical and Subtropical Ocean.

Prokaryotes play a fundamental role in decomposing organic matter in the ocean, but little is known about how microbial metabolic capabilities vary at the global ocean scale and what are the drivers causing this variation. We aimed at obtaining the first global exploration of the functional capabilities of prokaryotes in the ocean, with emphasis on the under-sampled meso- and bathypelagic layers. We explored the potential utilization of 95 carbon sources with Biolog GN2 plates® in 441 prokaryotic communities sampled from surface to bathypelagic waters (down to 4,000 m) at 111 stations distributed across the tropical and subtropical Atlantic, Indian, and Pacific oceans. The resulting metabolic profiles were compared with biological and physico-chemical properties such as fluorescent dissolved organic matter (DOM) or temperature. The relative use of the individual substrates was remarkably consistent across oceanic regions and layers, and only the Equatorial Pacific Ocean showed a different metabolic structure. When grouping substrates by categories, we observed some vertical variations, such as an increased relative utilization of polymers in bathypelagic layers or a higher relative use of P-compounds or amino acids in the surface ocean. The increased relative use of polymers with depth, together with the increases in humic DOM, suggest that deep ocean communities have the capability to process complex DOM. Overall, the main identified driver of the metabolic structure of ocean prokaryotic communities was temperature. Our results represent the first global depiction of the potential use of a variety of carbon sources by prokaryotic communities across the tropical and the subtropical ocean and show that acetic acid clearly emerges as one of the most widely potentially used carbon sources in the ocean.

Spatial variability of marine bacterial and archaeal communities along the particulate matter continuum.

Biotic and abiotic particles shape the microspatial architecture that defines the microbial aquatic habitat, being particles highly variable in size and quality along oceanic horizontal and vertical gradients. We analysed the prokaryotic (bacterial and archaeal) diversity and community composition present in six distinct particle size classes ranging from the pico- to the microscale (0.2 to 200 µm). Further, we studied their variations along oceanographic horizontal (from the coast to open oceanic waters) and vertical (from the ocean surface into the meso- and bathypelagic ocean) gradients. In general, prokaryotic community composition was more variable with depth than in the transition from the coast to the open ocean. Comparing the six size-fractions, distinct prokaryotic communities were detected in each size-fraction, and whereas bacteria were more diverse in the larger size-fractions, archaea were more diverse in the smaller size-fractions. Comparison of prokaryotic community composition among particle size-fractions showed that most, but not all, taxonomic groups have a preference for a certain size-fraction sustained with depth. Species sorting, or the presence of diverse ecotypes with distinct size-fraction preferences, may explain why this trend is not conserved in all taxa.

Unveiling the role and life strategies of viruses from the surface to the dark ocean.

Viruses are a key component of marine ecosystems, but the assessment of their global role in regulating microbial communities and the flux of carbon is precluded by a paucity of data, particularly in the deep ocean. We assessed patterns in viral abundance and production and the role of viral lysis as a driver of prokaryote mortality, from surface to bathypelagic layers, across the tropical and subtropical oceans. Viral abundance showed significant differences between oceans in the epipelagic and mesopelagic, but not in the bathypelagic, and decreased with depth, with an average power-law scaling exponent of -1.03 km-1 from an average of 7.76 × 106 viruses ml-1 in the epipelagic to 0.62 × 106 viruses ml-1 in the bathypelagic layer with an average integrated (0 to 4000 m) viral stock of about 0.004 to 0.044 g C m-2, half of which is found below 775 m. Lysogenic viral production was higher than lytic viral production in surface waters, whereas the opposite was found in the bathypelagic, where prokaryotic mortality due to viruses was estimated to be 60 times higher than grazing. Free viruses had turnover times of 0.1 days in the bathypelagic, revealing that viruses in the bathypelagic are highly dynamic. On the basis of the rates of lysed prokaryotic cells, we estimated that viruses release 145 Gt C year-1 in the global tropical and subtropical oceans. The active viral processes reported here demonstrate the importance of viruses in the production of dissolved organic carbon in the dark ocean, a major pathway in carbon cycling.

Patterns of bacterial diversity in the marine planktonic particulate matter continuum.

Depending on their relationship with the pelagic particulate matter, planktonic prokaryotes have traditionally been classified into two types of communities: free-living (FL) or attached (ATT) to particles, and are generally separated using only one pore-size filter in a differential filtration. Nonetheless, particulate matter in the oceans appears in a continuum of sizes. Here we separated this continuum into six discrete size-fractions, from 0.2 to 200 µm, and described the prokaryotes associated to each of them. Each size-fraction presented different bacterial communities, with a range of 23-42% of unique (OTUs) in each size-fraction, supporting the idea that they contained distinct types of particles. An increase in richness was observed from the smallest to the largest size-fractions, suggesting that increasingly larger particles contributed new niches. Our results show that a multiple size-fractionation provides a more exhaustive description of the bacterial diversity and community structure than the use of only one filter. In addition, and based on our results, we propose an alternative to the dichotomy of FL or ATT lifestyles, in which we differentiate the taxonomic groups with preference for the smaller fractions, those that do not show preferences for small or large fractions, and those that preferentially appear in larger fractions.

Eutrophication and acidification: Do they induce changes in the dissolved organic matter dynamics in the coastal Mediterranean Sea?

Two mesocosms experiments were conducted in winter 2010 and summer 2011 to examine how increased pCO2 and/or nutrient concentrations potentially perturbate dissolved organic matter dynamics in natural microbial assemblages. The fluorescence signals of protein- and humic-like compounds were used as a proxy for labile and non-labile material, respectively, while the evolution of bacterial populations, chlorophyll a (Chl a) and dissolved organic carbon (DOC) concentrations were used as a proxy for biological activity. For both seasons, the presence of elevated pCO2 did not cause any significant change in the DOC dynamics (p-value<0.05). The conditions that showed the greatest changes in prokaryote abundances and Chl a content were those amended with nutrients, regardless of the change in pH. The temporal evolution of fluorophores and optical indices revealed that the degree of humification of the organic molecules and their molecular weight changed significantly in the nutrient-amended treatment. The generation of protein-like compounds was paired to increases in the prokaryote abundance, being higher in the nutrient-amended tanks than in the control. Different patterns in the magnitude and direction of the generation of humic-like molecules suggested that these changes depended on initial microbial populations and the availability of extra nutrient inputs. Based on our results, it is expected that in the future projected coastal scenarios the eutrophication processes will favor the transformations of labile and recalcitrant carbon regardless of changes in pCO2.

Horizontal and Vertical Distributions of Transparent Exopolymer Particles (TEP) in the NW Mediterranean Sea Are Linked to Chlorophyll a and O2 Variability.

Transparent Exopolymer Particles (TEP) are relevant in particle and carbon fluxes in the ocean, and have economic impact in the desalination industry affecting reverse osmosis membrane fouling. However, general models of their occurrence and dynamics are not yet possible because of the poorly known co-variations with other physical and biological variables. Here, we describe TEP distributions in the NW Mediterranean Sea during late spring 2012, along perpendicular and parallel transects to the Catalan coast. The stations in the parallel transect were sampled at the surface, while the stations in the perpendicular transect were sampled from the surface to the bathypelagic, including the bottom nepheloid layers. We also followed the short-term TEP dynamics along a 2-day cycle in offshore waters. TEP concentrations in the area ranged from 4.9 to 122.8 and averaged 31.4 ± 12.0 µg XG eq L-1. The distribution of TEP measured in transects parallel to the Catalan Coast correlated those of chlorophyll a (Chla) in May but not in June, when higher TEP-values with respect to Chla were observed. TEP horizontal variability in epipelagic waters from the coast to the open sea also correlated to that of Chla, O2 (that we interpret as a proxy of primary production) and bacterial production (BP). In contrast, the TEP vertical distributions in epipelagic waters were uncoupled from those of Chla, as TEP maxima were located above the deep chlorophyll maxima. The vertical distribution of TEP in the epipelagic zone was correlated with O2 and BP, suggesting combined phytoplankton (through primary production) and bacterial (through carbon reprocessing) TEP sources. However, no clear temporal patterns arose during the 2-day cycle. In meso- and bathypelagic waters, where phytoplanktonic sources are minor, TEP concentrations (10.1 ± 4.3 µg XG eq l-1) were half those in the epipelagic, but we observed relative TEP increments coinciding with the presence of nepheloid layers. These TEP increases were not paralleled by increases in particulate organic carbon, indicating that TEP are likely to act as aggregating agents of the mostly inorganic particles present in these bottom nepheloid layers.

Particle-association lifestyle is a phylogenetically conserved trait in bathypelagic prokaryotes.

The free-living (FL) and particle-attached (PA) marine microbial communities have repeatedly been proved to differ in their diversity and composition in the photic ocean and also recently in the bathypelagic ocean at a global scale. However, although high taxonomic ranks exhibit preferences for a PA or FL mode of life, it remains poorly understood whether two clear lifestyles do exist and how these are distributed across the prokaryotic phylogeny. We studied the FL (<0.8 µm) and PA (0.8-20 µm) prokaryotes at 30 stations distributed worldwide within the bathypelagic oceanic realm (2150-4000 m depth) using high-throughput sequencing of the small subunit ribosomal RNA gene (16S rRNA). A high proportion of the bathypelagic prokaryotes were mostly found either attached to particles or freely in the surrounding water but rarely in both types of environments. In particular, this trait was deeply conserved through their phylogeny, suggesting that the deep-ocean particles and the surrounding water constitute two highly distinct niches and that transitions from one to the other have been rare at an evolutionary timescale. As a consequence, PA and FL communities had clear alpha- and beta-diversity differences that exceeded the global-scale geographical variation. Our study organizes the bathypelagic prokaryotic diversity into a reasonable number of ecologically coherent taxa regarding their association with particles, a first step for understanding which are the microbes responsible for the processing of the dissolved and particulate pools of organic matter that have a very different biogeochemical role in the deep ocean.