Prokaryotic Heterotrophic Production Research Articles

Distinct bacterial succession and functional response to alginate in the South, Equatorial, and North Pacific Ocean.

The availability of alginate, an abundant macroalgal polysaccharide, induces compositional and functional responses among marine microbes, but these dynamics have not been characterized across the Pacific Ocean. We investigated alginate-induced compositional and functional shifts (e.g., heterotrophic production, glucose turnover, hydrolytic enzyme activities) of microbial communities in the South Subtropical, Equatorial, and Polar Frontal North Pacific in mesocosms. We observed that shifts in response to alginate were site-specific. In the South Subtropical Pacific, prokaryotic cell counts, glucose turnover, and peptidase activities changed the most with alginate addition, along with the enrichment of the widest range of particle-associated taxa (161 amplicon sequence variants; ASVs) belonging to Alteromonadaceae, Rhodobacteraceae, Phormidiaceae, and Pseudoalteromonadaceae. Some of these taxa were detected at other sites but only enriched in the South Pacific. In the Equatorial Pacific, glucose turnover and heterotrophic prokaryotic production increased most rapidly; a single Alteromonas taxon dominated (60% of the community) but remained low (<2%) elsewhere. In the North Pacific, the particle-associated community response to alginate was gradual, with a more limited range of alginate-enriched taxa (82 ASVs). Thus, alginate-related ecological and biogeochemical shifts depend on a combination of factors that include the ability to utilize alginate, environmental conditions, and microbial interactions.

What happens to biomass burning-emitted particles in the ocean? A laboratory experimental approach based on their tracers

Wildfires, controlled burns, and biofuel combustion (biomass burning or BB) are major contributors to particulate matter in the atmosphere and thus have an impact on climate, human health, and ecosystems. Once emitted, the particulate matter derived from BB can be taken up by the oceans. However, the fate and impact of BB in the marine biological carbon pump, and carbon cycle are largely unknown. This work presents the first attempt to investigate the bioavailability of two BB tracers, levoglucosan and galactosan, in seawater inoculated with marine prokaryotes. Levoglucosan and galactosan were incubated with a marine bacterial inoculum and monitored for six weeks under controlled laboratory conditions. Along with the anhydrosugar concentrations, multiple chemical and biological parameters were monitored over time. The results indicate that levoglucosan and galactosan can be assimilated by marine prokaryotes as their concentrations decreased by 97 ± 4 % and 36 ± 21 % (n = 3) of their initial values. However, this decrease occurred only after a 9 and 15 days from the beginning of the experiment, respectively. The decrease in the levoglucosan and galactosan concentrations was accompanied by an increase in both heterotrophic prokaryotic production, and abundance. These results demonstrate that these anhydrosugars have the potential to be assimilated by heterotrophic prokaryotes and thus contribute to the microbial food web functioning. Under our experimental conditions, levoglucosan exhibited a bacterial growth efficiency of 17 ± 5 % (n = 3), suggesting that most of the levoglucosan is mineralized into CO2. Prokaryotic diversity analyses revealed the predominance of a few bacterial genera from the Roseobacter clade that were selected after the addition of the anhydrosugars. The presence of this widespread marine bacterial clade reflects its ability to process semilabile compounds (here levoglucosan and galactosan) originating from BB and contribute to the dissolved organic matter pool in surface seawaters.

Microbial Parameters as Predictors of Heterotrophic Prokaryotic Production in the Ross Sea Epipelagic Waters (Antarctica) during the Austral Summer

A regression-based approach was used to test the suitability of a range of parameters, including total prokaryotic cell abundance and biomass, as lipopolysaccharide (LPS) content, and exoenzymatic activities (leucine aminopeptidase, LAP, beta-glucosidase, ß-G, and alkaline phosphatase, AP) as predictors of heterotrophic prokaryotic production (HPP) in the Ross Sea epipelagic waters. A close association between HPP and protein hydrolysis mediated by enzymatic activity (LAP), and to a lower significance level with the other variables, was recorded. Three multiple regression equations were developed from two microbial datasets collected during middle austral summer periods. All showed a good predictive ability for HPP, and this was further validated through a comparison between the predicted and the observed HPP values. The obtained regression equations proved to represent a promising example of empirical models for further predictive studies in the Ross Sea where—through the incorporation of additional microbiological and environmental parameters—the developed models could find a practical application to cover the entire austral summer period.

Evidence of coupled autotrophy and heterotrophy on plastic biofilms and its influence on surrounding seawater

We measured phytoplankton primary production and heterotrophic bacterial activities on microplastics and seawater in the Northwestern Mediterranean Sea during two 3-month spring periods over 2 consecutive years. Microorganisms growing on a 5 mm diameter low density polyethylene films (LDPE; 200 μm thick) faced two contrasting conditions depending on the year. Spring 2018 was characterized by consistent nutrient inputs and bloom development. In spring 2019, nutrient inputs and bloom were low. For the first time, we observed a clear coupling between primary production and heterotrophic prokaryote production on microplastics during both years, but with different intensity between years that reflected the crucial role of the trophic environmental conditions (nutrient supply) in shaping microbial activities on plastics. We found that high primary production on plastics could support the whole (net autotrophy) or the majority of the bacterial carbon demand needed for heterotrophic activities, supplemented by other carbon sources if surrounding waters are highly productive. We propose that microbial activity on plastics influences the microbial community in the surrounding seawater, especially when the environmental conditions are less favorable. An illustrative image of the role of plastics in the environment could be that of an inverter in an electrical circuit that mitigates both positive and negative variations. Our results highlight the potential role of the plastisphere in shaping biogeochemical cycles in the context of increasing amounts of plastic particles in the marine environment.

Availability of vitamin B12 and its lower ligand intermediate α-ribazole impact prokaryotic and protist communities in oceanic systems

Genome analyses predict that the cofactor cobalamin (vitamin B12, called B12 herein) is produced by only one-third of all prokaryotes but almost all encode at least one B12-dependent enzyme, in most cases methionine synthase. This implies that the majority of prokaryotes relies on exogenous B12 supply and interacts with producers. B12 consists of a corrin ring centred around a cobalt ion and the lower ligand 5’6-dimethylbenzimidazole (DMB). It has never been tested whether availability of this pivotal cofactor, DMB or its intermediate α-ribazole affect growth and composition of prokaryotic microbial communities. Here we show that in the subtropical, equatorial and polar frontal Pacific Ocean supply of B12 and α-ribazole enhances heterotrophic prokaryotic production and alters the composition of prokaryotic and heterotrophic protist communities. In the polar frontal Pacific, the SAR11 clade and Oceanospirillales increased their relative abundances upon B12 supply. In the subtropical Pacific, Oceanospirillales increased their relative abundance upon B12 supply as well but also downregulated the transcription of the btuB gene, encoding the outer membrane permease for B12. Surprisingly, Prochlorococcus, known to produce pseudo-B12 and not B12, exhibited significant upregulation of genes encoding key proteins of photosystem I + II, carbon fixation and nitrate reduction upon B12 supply in the subtropical Pacific. These findings show that availability of B12 and α-ribazole affect growth and composition of prokaryotic and protist communities in oceanic systems thus revealing far-reaching consequences of methionine biosynthesis and other B12-dependent enzymatic reactions on a community level.

Patterns of microbial abundance and heterotrophic activity along nitrogen and salinity gradients in coastal wetlands

Coastal wetlands are valuable aquatic ecosystems with high biological productivity, which provide services such as a reduction in nitrogen loading into coastal waters and storage of organic carbon acting as carbon dioxide sinks. The predicted rise of sea level or freshwater extractions, particularly in the arid Mediterranean biome, will salinize many coastal wetlands. However, there is considerable uncertainty about how salinization will affect microbial communities and biogeochemical processes. We determined the abundance of total prokaryotes, cyanobacteria, and viruses and quantified the heterotrophic production of prokaryotes sensitive- (predominantly Bacteria) and resistant- (predominantly Archaea) to erythromycin in 112 ponds from nine coastal wetlands. We explored the main drivers of prokaryotic abundance and heterotrophic production using generalized linear models (GLMs). The best GLM, including all the wetlands, indicated that the concentration of total dissolved nitrogen (TDN) positively affected the total abundance of prokaryotes and the heterotrophic erythromycin-resistant (ery-R) production. In contrast, heterotrophic erythromycin-sensitive (ery-S) production was negatively related to TDN. This negative relationship appeared to be mediated by salinity and virus abundance. Heterotrophic ery-S production declined as salinity and virus abundance increased. Consequently, we observed a switch from heterotrophic ery-S production towards ery-R production as salinity and virus abundance increased. Our results imply that microbial activity will change from heterotrophic bacterial-dominated processes to archaeal-dominated processes with anthropogenic nitrogen and salinization increases. However, more studies are required to link the mineralization rates of dissolved nitrogen and organic carbon with specific archaeal taxa to enable more accurate predictions on future scenarios in coastal wetlands.

Microbial Growth and Organic Matter Cycling in the Pacific Ocean Along a Latitudinal Transect Between Subarctic and Subantarctic Waters

The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a, particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in these gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a, POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions.

Empirical leucine-to-carbon conversion factors in north-eastern Atlantic waters (50\u20132000\xa0m) shaped by bacterial community composition and optical signature of DOM

Microbial heterotrophic activity is a major process regulating the flux of dissolved organic matter (DOM) in the ocean, while the characteristics of this DOM strongly influence its microbial utilization and fate in the ocean. In order to broaden the vertical resolution of leucine-to-carbon conversion factors (CFs), needed for converting substrate incorporation into biomass production by heterotrophic bacteria, 20 dilution experiments were performed in the North Atlantic Ocean. We found a depth-stratification in empirical CFs values from epipelagic to bathypelagic waters (4.00 ± 1.09 to 0.10 ± 0.00 kg C mol Leu−1). Our results demonstrated that the customarily used theoretical CF of 1.55 kg C mol Leu−1 in oceanic samples can lead to an underestimation of prokaryotic heterotrophic production in epi- and mesopelagic waters, while it can overestimate it in the bathypelagic ocean. Pearson correlations showed that CFs were related not only to hydrographic variables such as temperature, but also to specific phylogenetic groups and DOM quality and quantity indices. Furthermore, a multiple linear regression model predicting CFs from relatively simple hydrographic and optical spectroscopic measurements was attempted. Taken together, our results suggest that differences in CFs throughout the water column are significantly connected to DOM, and also reflect differences linked to specific prokaryotic groups.

Hydrothermal plumes as hotspots for deep-ocean heterotrophic microbial biomass production

Carbon budgets of hydrothermal plumes result from the balance between carbon sinks through plume chemoautotrophic processes and carbon release via microbial respiration. However, the lack of comprehensive analysis of the metabolic processes and biomass production rates hinders an accurate estimate of their contribution to the deep ocean carbon cycle. Here, we use a biogeochemical model to estimate the autotrophic and heterotrophic production rates of microbial communities in hydrothermal plumes and validate it with in situ data. We show how substrate limitation might prevent net chemolithoautotrophic production in hydrothermal plumes. Elevated prokaryotic heterotrophic production rates (up to 0.9 gCm−2y−1) compared to the surrounding seawater could lead to 0.05 GtCy−1 of C-biomass produced through chemoorganotrophy within hydrothermal plumes, similar to the Particulate Organic Carbon (POC) export fluxes reported in the deep ocean. We conclude that hydrothermal plumes must be accounted for as significant deep sources of POC in ocean carbon budgets.

Particulate biogenic barium tracer of mesopelagic carbon remineralization in the Mediterranean Sea (PEACETIME project)

Abstract. We report on the sub-basin variability in particulate organic carbon (POC) remineralization in the western and central Mediterranean Sea in late spring during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean sea) cruise. POC remineralization rates were estimated using the excess biogenic particulate barium (Baxs) inventories in the mesopelagic layers (100–1000 m depth) and compared with prokaryotic heterotrophic production (PHP). Baxs-based mesopelagic remineralization rates (MRs) ranged from 25±2 to 306±70 mgCm-2d-1. MRs were larger in the Algero-Provençal (ALG) Basin than in the Tyrrhenian (TYR) and Ionian (ION) basins. Our Baxs inventories and integrated PHP data also indicated that significant mesopelagic remineralization occurred down to 1000 m depth in the ALG Basin in contrast to the ION and TYR basins, where remineralization was mainly located above 500 m depth. We propose that the higher and deeper MRs in the ALG Basin were sustained by an additional particle export event driven by deep convection. The TYR Basin (in contrast to the ALG and ION basins) presented the impact of a previous dust event, as reflected by our particulate Al water column concentrations. The ION and TYR basins showed small-scale heterogeneity in remineralization processes, reflected by our Baxs inventories and integrated PHP data at the Tyrr long-duration station. This heterogeneity was linked to the mosaic of blooming and non-blooming patches reported in this area during the cruise. In contrast to the western Mediterranean Sea (ALG Basin), the central Mediterranean Sea (ION and TYR basins) showed lower remineralization rates restricted to the upper mesopelagic layer during the late spring PEACETIME cruise.

COVID-19 lockdown moderately increased oligotrophy at a marine coastal site

COVID-19 has led to global population lockdowns that have had indirect effects on terrestrial and marine fauna, yet little is known on their effects on marine planktonic communities. We analysed the effect of the spring 2020 lockdown in a marine coastal area in Blanes Bay, NW Mediterranean. We compared a set of 23 oceanographic, microbial and biogeochemical variables sampled right after the strict lockdown in Spain, with data from the previous 15 years after correcting for long-term trends. Our analysis shows a series of changes in the microbial communities which may have been induced by the combination of the decreased nitrogen atmospheric load, the lower wastewater flux and the reduced fishing activity in the area, among other factors. In particular, we detected a slight decrease beyond the long-term trend in chlorophyll a, in the abundance of several microbial groups (phototrophic nanoflagellates and total prokaryotes) and in prokaryotic activity (heterotrophic prokaryotic production and β-glucosidase activity) which, as a whole, resulted in a moderate increase of oligotrophy in Blanes Bay after the lockdown.

Influence of atmospheric deposition on biogeochemical cycles in an oligotrophic ocean system

Abstract. The surface mixed layer (ML) in the Mediterranean Sea is a well-stratified domain characterized by low macronutrients and low chlorophyll content for almost 6 months of the year. In this study we characterize the biogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in situ measurements of atmospheric deposition, nutrients in seawater, hydrological conditions, primary production, heterotrophic prokaryotic production, N2 fixation and leucine aminopeptidase activity. Dry deposition was continuously measured across the central and western open Mediterranean Sea, and two wet deposition events were sampled, one in the Ionian Sea and one in the Algerian Basin. Along the transect, N budgets were computed to compare the sources and sinks of N in the mixed layer. In situ leucine aminopeptidase activity made up 14 % to 66 % of the heterotrophic prokaryotic N demand, and the N2 fixation rate represented 1 % to 4.5 % of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of nitrate and ammonium in aerosols, was higher than the N2 fixation rates in the ML (on average 4.8-fold). The dry atmospheric input of inorganic N represented a highly variable proportion of biological N demand in the ML among the stations, 10 %–82 % for heterotrophic prokaryotes and 1 %–30 % for phytoplankton. As some sites were visited on several days, the evolution of biogeochemical properties in the ML and within the nutrient-depleted layers could be followed. At the Algerian Basin site, the biogeochemical consequences of a wet dust deposition event were monitored through high-frequency sampling. Notably, just after the rain, nitrate was higher in the ML than in the nutrient-depleted layer below. Estimates of nutrient transfer from the ML into the nutrient-depleted layer could explain up to a third of the nitrate loss from the ML. Phytoplankton did not benefit directly from the atmospheric inputs into the ML, probably due to high competition with heterotrophic prokaryotes, also limited by N and phosphorus (P) availability at the time of this study. Primary producers decreased their production after the rain but recovered their initial state of activity after a 2 d lag in the vicinity of the deep chlorophyll maximum layer.

Linking Microbial Functioning and Trophic Pathways to Ecological Status in a Coastal Mediterranean Ecosystem

Coastal marine ecosystems host complex microbial communities whose composition and metabolism are influenced by continental inputs and mesoscale properties of seawater masses. The identifying traits of the phytoplankton and bacteria such as biomass, size, shape and their metabolism related to organic matter production and degradation, recognized as indicators of the functioning of an ecosystem, were observed in the Gulf of Manfredonia (South Adriatic Sea, Italy) in late spring. This Gulf area is characterized by terrestrial inputs and mesoscale circulation influence such as coastal waters flowing southward from the North Adriatic and offshore waters interested by the Ionian Sea. Water samples were grouped in clusters (Coastal, Intermediate, Offshore and Deep Systems) according to the water column properties. Phytoplankton community biomass and composition, autotrophic and total prokaryotic abundances and microbial metabolism such as enzyme activity rates and prokaryotic heterotrophic production were analyzed to elucidate the trophic pathways with the objective to infer on the ecosystem status. As expected, size-fractionated phytoplankton biomass and production showed greater concentration in coastal waters with prevalence of the largest fractions (micro- and nano-) supported by the diatoms. Conversely, lower biomass and production were measured in all off-shore waters, mainly sustained by smallest fractions (nano-sized phytoflagellates and picophytoplankton). Total and autotrophic prokaryotic abundance decreased from coastal to offshore stations, inversely with respect to cell volume. Prokaryotic heterotrophic production was just below 50% compared to that of phytoplankton in all waters, evidencing an active biomass synthesis. High alkaline phosphatase and leucine aminopeptidase in coastal and offshore waters suggested the quick regeneration of Phosphorus and protein decomposition, respectively. Different levels of phytoplankton-bacteria association might provide a tool to define the ecological status of the studied system in the observed period; an approach to ecosystem assessment exportable to other coastal systems is proposed.

On the barium–oxygen consumption relationship in the Mediterranean Sea: implications for mesopelagic marine snow remineralization

Abstract. In the ocean, remineralization rate associated with sinking particles is a crucial variable. Since the 1990s, particulate biogenic barium (Baxs) has been used as an indicator of carbon remineralization by applying a transfer function relating Baxs to O2 consumption (Dehairs's transfer function, Southern Ocean-based). Here, we tested its validity in the Mediterranean Sea (ANTARES/EMSO-LO) for the first time by investigating connections between Baxs, prokaryotic heterotrophic production (PHP) and oxygen consumption (JO2-Opt; optodes measurement). We show that (1) higher Baxs (409 pM; 100–500 m) occurs in situations where integrated PHP (PHP100/500=0.90) is located deeper, (2) higher Baxs occurs with increasing JO2-Opt, and (3) there is similar magnitude between JO2-Opt (3.14 mmol m−2 d−1; 175–450 m) and JO2-Ba (4.59 mmol m−2 d−1; transfer function). Overall, Baxs, PHP and JO2 relationships follow trends observed earlier in the Southern Ocean. We conclude that such a transfer function could apply in the Mediterranean Sea.

Deep maxima of phytoplankton biomass, primary production and bacterial production in the Mediterranean Sea

Abstract. The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant to our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both the biomass and primary production but not in the growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the ratio of carbon to chlorophyll a (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mg C m−3 at the surface to 10–15 mg C m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The ratio of fucoxanthin to total chlorophyll a increased markedly with depth, suggesting an increased contribution of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fuelled by new nutrients but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the peaks of deep primary production associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

Mesopelagic microbial carbon production correlates with diversity across different marine particle fractions

The vertical flux of marine snow particles significantly reduces atmospheric carbon dioxide concentration. In the mesopelagic zone, a large proportion of the organic carbon carried by sinking particles dissipates thereby escaping long term sequestration. Particle associated prokaryotes are largely responsible for such organic carbon loss. However, links between this important ecosystem flux and ecological processes such as community development of prokaryotes on different particle fractions (sinking vs. non-sinking) are yet virtually unknown. This prevents accurate predictions of mesopelagic organic carbon loss in response to changing ocean dynamics. Using combined measurements of prokaryotic heterotrophic production rates and species richness in the North Atlantic, we reveal that carbon loss rates and associated microbial richness are drastically different with particle fractions. Our results demonstrate a strong negative correlation between prokaryotic carbon losses and species richness. Such a trend may be related to prokaryotes detaching from fast-sinking particles constantly enriching non-sinking associated communities in the mesopelagic zone. Existing global scale data suggest this negative correlation is a widespread feature of mesopelagic microbes.

Assessing Viral Abundance and Community Composition in Four Contrasting Regions of the Southern Ocean.

We explored how changes of viral abundance and community composition among four contrasting regions in the Southern Ocean relied on physicochemical and microbiological traits. During January–February 2015, we visited areas north and south of the South Orkney Islands (NSO and SSO) characterized by low temperature and salinity and high inorganic nutrient concentration, north of South Georgia Island (NSG) and west of Anvers Island (WA), which have relatively higher temperatures and lower inorganic nutrient concentrations. Surface viral abundance (VA) was highest in NSG (21.50 ± 10.70 × 106 viruses mL−1) and lowest in SSO (2.96 ± 1.48 × 106 viruses mL−1). VA was positively correlated with temperature, prokaryote abundance and prokaryotic heterotrophic production, chlorophyll a, diatoms, haptophytes, fluorescent organic matter, and isoprene concentration, and was negatively correlated with inorganic nutrients (NO3−, SiO42−, PO43−), and dimethyl sulfide (DMS) concentrations. Viral communities determined by randomly amplified polymorphic DNA–polymerase chain reaction (RAPD-PCR) were grouped according to the sampling location, being more similar within them than among regions. The first two axes of a canonical correspondence analysis, including physicochemical (temperature, salinity, inorganic nutrients—NO3−, SiO42−, and dimethyl sulfoniopropionate -DMSP- and isoprene concentrations) and microbiological (chlorophyll a, haptophytes and diatom, and prokaryote abundance and prokaryotic heterotrophic production) factors accounted for 62.9% of the variance. The first axis, temperature-related, accounted for 33.8%; the second one, salinity-related, accounted for 29.1%. Thus, different environmental situations likely select different hosts for viruses, leading to distinct viral communities.

Effects of climate changes on the microbial activities and prokaryotic abundances in the euphotic layer of the Central Mediterranean Sea

Prokaryotic microorganisms are at the base of marine food web and are sensitive to climate changes. In the Mediterranean Sea, an important link between surface water warming and microbial activities is hypothesized to exist on a seasonal and inter-annual scale, with high amounts of organic matter utilized by prokaryotes and recycled in warm periods. To test this hypothesis, from 1998 to 2013, the prokaryotic abundance and microbial activities (enzymatic activities, prokaryotic heterotrophic production, and respiration), together with trophic parameters (chlorophyll, particulate organic carbon, and nitrogen), were measured in the euphotic layer of the Ionian Sea (Central Mediterranean). A statistical simultaneous analysis of the environmental and prokaryotic data was performed with the aim of depicting the evolution of microbial activities and their implications in the biogeochemical cycles. Results showed that despite the large spatial and temporal variability, temperature was the main factor affecting the variability of microbial abundance and metabolism with significant differences on both seasonal and inter-annual scales. Particularly interesting is the increase of production in summer and of the respiration in winter. In spite of low levels of chlorophyll and organic carbon, an increase of prokaryotic heterotrophic production, coupled with that of alkaline phosphatase, was found. As a consequence, the increase of prokaryotic biomass was bottom-up sustained. Most of prokaryotic activities and respiration rates were significantly correlated to organic carbon pools; the process of organic matter decomposition was frequently enhanced at higher temperature, and both prokaryotic production and growth efficiency increased in summer. In view of climate changes, the rise in temperature was associated to the increase of heterotrophic activities and this trend can have a predictive value to describe the microbial processes in the Central Mediterranean Sea.

Regulation of Low and High Nucleic Acid Fluorescent Heterotrophic Prokaryote Subpopulations and Links to Viral-Induced Mortality Within Natural Prokaryote-Virus Communities.

Flow cytometric analysis of marine prokaryotes routinely reveals two distinct clusters of heterotrophic cells referred to as high nucleic acid fluorescent (HNA) and low nucleic acid fluorescent (LNA) populations. Evidence suggests that these may represent physiologically and ecologically distinct prokaryote populations. According to the "kill the winner" hypothesis, viral lysis reduces the efficiency of the microbial loop by decreasing the biomass and activity of the most abundant and active members of a population (i.e., competition specialist). Thus, viral-induced mortality may vary according to the physiology of HNA and LNA cells, with implications for the marine carbon cycle. Here, the abundance and production of heterotrophic prokaryotic populations were assessed in the North Atlantic during two phases of the annual plankton cycle and related to bottom-up (i.e., organic carbon variability) and top-down processes (i.e., viral abundance and lytic production). Our results demonstrate that the relative abundance of HNA and LNA heterotrophic cells and heterotrophic prokaryote production vary according to organic carbon variability in the water column, which can be strongly influenced by the physical eddy field (i.e., type of eddy: cyclonic, anticyclonic, or no eddy). In addition, the abundance and lytic production of virus subpopulations were correlated with the cellular production and abundance of heterotrophic HNA and LNA prokaryote communities. Our data suggest group- and activity-specific linkages between hosts and viruses (i.e., HNA-V1 and LNA-V2). Specifically, V1 had a greater contribution to total viral production (i.e., 2.6-fold higher than V2 viruses), similar to their putative host. Finally, we explore potential implications of group- and activity-specific linkages between host and virus groups on the flux of carbon through the microbial food web.

Microbial metabolic rates in the Ross Sea: the ABIOCLEAR Project

The Ross Sea is one of the most productive areas of the Southern Ocean and includes several functionally different marine ecosystems. With the aim of identifying signs and patterns of microbial response to current climate change, seawater microbial populations were sampled at different depths, from surface to the bottom, at two Ross Sea mooring areas southeast of Victoria Land in Antarctica. This oceanographic experiment, the XX Italian Antarctic Expedition, 2004-05, was carried out in the framework of the ABIOCLEAR project as part of LTER-Italy. Here, microbial biogeochemical rates of respiration, carbon dioxide production, total community heterotrophic energy production, prokaryotic heterotrophic activity, production (by3H-leucine uptake) and prokaryotic biomass (by image analysis) were determined throughout the water column. As ancillary parameters, chlorophylla, adenosine-triphosphate concentrations, temperature and salinity were measured and reported. Microbial metabolism was highly variable amongst stations and depths. In epi- and mesopelagic zones, respiratory rates varied between 52.4–437.0 and 6.3–271.5 nanol O2l-1h-1; prokaryotic heterotrophic production varied between 0.46–29.5 and 0.3–6.11 nanog C l-1h-1; and prokaryotic biomass varied between 0.8–24.5 and 1.1–9.0 µg C l-1, respectively. The average heterotrophic energy production ranged between 570 and 103 mJ l-1h-1in upper and deeper layers, respectively. In the epipelagic layer, the Prokaryotic Carbon Demand and Prokaryotic Growth Efficiency averaged 9 times higher and 2 times lower, respectively, than in the mesopelagic one. The distribution of plankton metabolism and organic matter degradation was mainly related to the different hydrological and trophic conditions. In comparison with previous research, the Ross Sea results, here, evidenced a relatively impoverished oligotrophic microbial community, throughout the water column.