Vertical and Seasonal Patterns Control Bacterioplankton Communities at Two Horizontally Coherent Coastal Upwelling Sites off Galicia (NW Spain).

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

Water mass mixing shapes bacterial biogeography in a highly hydrodynamic region of the Southern Ocean.

Even though compelling evidences indicate that marine microbes show biogeographic patterns, very little is known on the mechanisms driving those patterns in aquatic ecosystems. In the present study, bacterial community structure was examined in epipelagic waters of a highly hydrodynamic area of the Southern Ocean to gain insight into the role that biogeochemical factors and water mass mixing (a proxy of dispersal) have on microbial biogeography. Four water masses that converge and mix around the South Shetland Islands (northern tip of the Antarctic Peninsula) were investigated. Bacterioplankton communities were water-mass specific, and were best explained by dispersal rather than by biogeochemical factors, which is attributed to the relatively reduced environmental gradients found in these cold and nutrient rich waters. These results support the notion that currents and water mixing may have a considerable effect in connecting and transforming different water bodies, and consequently, in shaping communities of microorganisms. Considering the multidimensional and dynamic nature of the ocean, analysis of water mass mixing is a more suitable approach to investigate the role of dispersal on the biogeography of planktonic microorganisms rather than geographical distance.

Optical properties of dissolved organic matter relate to different depth-specific patterns of archaeal and bacterial community structure in the North Atlantic Ocean.

Prokaryotic abundance, activity and community composition were studied in the euphotic, intermediate and deep waters off the Galician coast (NW Iberian margin) in relation to the optical characterization of dissolved organic matter (DOM). Microbial (archaeal and bacterial) community structure was vertically stratified. Among the Archaea, Euryarchaeota, especially Thermoplasmata, was dominant in the intermediate waters and decreased with depth, whereas marine Thaumarchaeota, especially Marine Group I, was the most abundant archaeal phylum in the deeper layers. The bacterial community was dominated by Proteobacteria through the whole water column. However, Cyanobacteria and Bacteroidetes occurrence was considerable in the upper layer and SAR202 was dominant in deep waters. Microbial composition and abundance were not shaped by the quantity of dissolved organic carbon, but instead they revealed a strong connection with the DOM quality. Archaeal communities were mainly related to the fluorescence of DOM (which indicates respiration of labile DOM and generation of refractory subproducts), while bacterial communities were mainly linked to the aromaticity/age of the DOM produced along the water column. Taken together, our results indicate that the microbial community composition is associated with the DOM composition of the water masses, suggesting that distinct microbial taxa have the potential to use and/or produce specific DOM compounds.

Photochemical alteration of dissolved organic matter and the subsequent effects on bacterial carbon cycling and diversity.

The impact of solar radiation on dissolved organic matter (DOM) derived from 3 different sources (seawater, eelgrass leaves and river water) and the effect on the bacterial carbon cycling and diversity were investigated. Seawater with DOM from the sources was first either kept in the dark or exposed to sunlight (4 days), after which a bacterial inoculum was added and incubated for 4 additional days. Sunlight exposure reduced the coloured DOM and carbon signals, which was followed by a production of inorganic nutrients. Bacterial carbon cycling was higher in the dark compared with the light treatment in seawater and river samples, while higher levels were found in the sunlight-exposed eelgrass experiment. Sunlight pre-exposure stimulated the bacterial growth efficiency in the seawater experiments, while no impact was found in the other experiments. We suggest that these responses are connected to differences in substrate composition and the production of free radicals. The bacterial community that developed in the dark and sunlight pre-treated samples differed in the seawater and river experiments. Our findings suggest that impact of sunlight exposure on the bacterial carbon transfer and diversity depends on the DOM source and on the sunlight-induced production of inorganic nutrients.

Sample dilution and bacterial community composition influence empirical leucine-to-carbon conversion factors in surface waters of the world's oceans.

The transformation of leucine incorporation rates to prokaryotic carbon production rates requires the use of either theoretical or empirically determined conversion factors. Empirical leucine-to-carbon conversion factors (eCFs) vary widely across environments, and little is known about their potential controlling factors. We conducted 10 surface seawater manipulation experiments across the world's oceans, where the growth of the natural prokaryotic assemblages was promoted by filtration (i.e., removal of grazers [F treatment]) or filtration combined with dilution (i.e., also relieving resource competition [FD treatment]). The impact of sunlight exposure was also evaluated in the FD treatments, and we did not find a significant effect on the eCFs. The eCFs varied from 0.09 to 1.47 kg C mol Leu(-1) and were significantly lower in the FD than in the F samples. Also, changes in bacterial community composition during the incubations, as assessed by automated ribosomal intergenic spacer analysis (ARISA), were more pronounced in the FD than in the F treatments, compared to unmanipulated controls. Thus, we discourage the common procedure of diluting samples (in addition to filtration) for eCF determination. The eCFs in the filtered treatment were negatively correlated with the initial chlorophyll a concentration, picocyanobacterial abundance (mostly Prochlorococcus), and the percentage of heterotrophic prokaryotes with high nucleic acid content (%HNA). The latter two variables explained 80% of the eCF variability in the F treatment, supporting the view that both Prochlorococcus and HNA prokaryotes incorporate leucine in substantial amounts, although this results in relatively low carbon production rates in the oligotrophic ocean.

Global abundance of planktonic heterotrophic protists in the deep ocean.

The dark ocean is one of the largest biomes on Earth, with critical roles in organic matter remineralization and global carbon sequestration. Despite its recognized importance, little is known about some key microbial players, such as the community of heterotrophic protists (HP), which are likely the main consumers of prokaryotic biomass. To investigate this microbial component at a global scale, we determined their abundance and biomass in deepwater column samples from the Malaspina 2010 circumnavigation using a combination of epifluorescence microscopy and flow cytometry. HP were ubiquitously found at all depths investigated down to 4000 m. HP abundances decreased with depth, from an average of 72±19 cells ml(-1) in mesopelagic waters down to 11±1 cells ml(-1) in bathypelagic waters, whereas their total biomass decreased from 280±46 to 50±14 pg C ml(-1). The parameters that better explained the variance of HP abundance were depth and prokaryote abundance, and to lesser extent oxygen concentration. The generally good correlation with prokaryotic abundance suggested active grazing of HP on prokaryotes. On a finer scale, the prokaryote:HP abundance ratio varied at a regional scale, and sites with the highest ratios exhibited a larger contribution of fungi molecular signal. Our study is a step forward towards determining the relationship between HP and their environment, unveiling their importance as players in the dark ocean's microbial food web.