Benthic Prokaryotes Research Articles

Role of polymetallic-nodule dependent fauna on carbon cycling in the eastern Clarion-Clipperton Fracture Zone (Pacific)

The abyssal seafloor in the Clarion-Clipperton Fracture Zone (CCZ) in the central Pacific is covered with large densities of polymetallic nodules, i.e., metal concretions containing iron, manganese, nickel, cobalt, and copper. Nodules are of economic importance for these metals, but they also host a variety of deep-sea fauna. In a recent study it was estimated that the removal of these nodules would lead to a loss of up to 18% of all taxa in the CCZ. Here, I assess the impact of removing these nodule-dependent taxa on carbon cycling at two sites (B4S03, B6S02) of the Belgian exploration license area in the eastern CCZ. For this purpose, I developed two highly resolved carbon-based food web models with 71 (B6S02) to 75 (B4S03) food-web compartments consisting of different detritus pools, bacteria, metazoan meiobenthos, macrobenthic isopods, polychaetes and other macrobenthos, megabenthic cnidarians, crustaceans, poriferans, holothurians and other invertebrate megabenthos, and fish. These compartments were connected with 303 (B6S02) to 336 (B4S03) links which were reduced by 5–9% when nodule-dependent faunal compartments were removed. The models estimated the “total system throughput” T.. i.e., the sum of all carbon flows in the food webs, in intact food webs as 1.18 mmol C m-2 d-1 and 1.20 mmol C m-2 d-1 at B4S03 and B6S02, respectively, whereby 69.8% (B6S02) to 71.2% (B4S03) of T.. flowed through the microbial loop. A removal of the nodule-dependent fauna did not affect this microbial loop but reduced the scavenger loop by 56.5% (B6S02) to 71.6% (B4S03). Overall, nodule-dependent fauna is responsible for only a small fraction of total carbon cycling at the eastern CCZ. Therefore, when the effect of prospective deep-seabed mining on carbon cycling is investigated, its impact on benthic prokaryotes and the microbial loop should be addressed specifically.

Synecological response of spring benthic prokaryotes and macroinvertebrates to Paleozoic roof pendant‐derived calcium

AbstractDespite the ecological significance of desert springs, little is known about relationships between spring hydrogeochemistry and ecology, particularly over multiple trophic levels. Here, we surveyed microbial communities (bacteria and archaea) and benthic macroinvertebrate (BMI) communities in springs that recharge through granitoid bedrock in Owens Valley, CA, to determine whether subtle geochemical differences imparted by metamorphosed roof pendant weathering in the source area affect spring ecosystems. Relative to other springs, roof‐pendant‐recharged springs were characterized by elevated (1) Ca2+/Na+, Ca2+/Mg2+, and divalent/monovalent cation ratios, (2) relative abundance of taxa inferred to be benthic aerobes and prosthecate/stalked bacteria, and (3) abundance and diversity of shredder and collector gatherer BMIs. These BMI feeding groups graze on biofilm communities and stimulate bacterial degradation of particulate organic matter, consistent with extensive bacteria–BMI connections in co‐occurrence networks in these springs. Springs not impacted by roof pendants were instead enriched with taxa inferred to be anaerobes, chemolithotrophs, and low‐diversity BMI communities with poor bacteria–BMI network connectivity. We speculate that excess Ca2+ derived from roof pendant weathering plays a key role in biofilm formation on coarse granite substrates, with subsequent trophic interactions between benthic microbial biofilms and diverse BMI shredders/collector gatherers.

The ProkaBioDen database, a global database of benthic prokaryotic biomasses and densities in the marine realm

Benthic prokaryotes include Bacteria and Archaea and dominate densities of marine benthos. They play major roles in element cycles and heterotrophic, chemoautotrophic, and phototrophic carbon production. To understand how anthropogenic disturbances and climate change might affect these processes, better estimates of prokaryotic biomasses and densities are required. Hence, I developed the ProkaBioDen database, the largest open-access database of benthic prokaryotic biomasses and densities in marine surface sediments. In total, the database comprises 1,089 georeferenced benthic prokaryotic biomass and 1,875 density records extracted from 85 and 112 studies, respectively. I identified all references applying the procedures for systematic reviews and meta analyses and report prokaryotic biomasses as g C cm−3 sediment, g C g−1 sediment, and g C m−2. Density records are presented as cell cm−3 sediment, cell g−1 sediment/ sulfide/ vent precipitate, and cell m−2. This database should serve as reference to close sampling gaps in the future.

In situ experimental evidences for responses of abyssal benthic biota to shifts in phytodetritus compositions linked to global climate change

Abyssal plains cover more than half of Earth's surface, and the main food source in these ecosystems is phytodetritus, mainly originating from primary producers in the euphotic zone of the ocean. Global climate change is influencing phytoplankton abundance, productivity, and distribution. Increasing importance of picoplankton over diatom as primary producers in surface oceans (especially projected for higher latitudes) is projected and hence altering the quantity of organic carbon supplied to the abyssal seafloor as phytodetritus, consequences of which remain largely unknown. Here, we investigated the in situ responses of abyssal biota from viruses to megafauna to different types of phytoplankton input (diatoms or cyanobacteria which were labeled with stable isotopes) at equatorial (oligotrophic) and temperate (eutrophic) benthic sites in the Pacific Ocean (1°N at 4277 m water depth and 39°N at 5260 m water depth, respectively). Our results show that meiofauna and macrofauna generally preferred diatoms as a food source and played a relatively larger role in the consumption of phytodetritus at higher latitudes (39°N). Contrarily, prokaryotes and viruses showed similar or even stronger responses to cyanobacterial than to diatom supply. Moreover, the response of prokaryotes and viruses was very rapid (within 1–2 days) at both 1°N and 39°N, with quickest responses reported in the case of cyanobacterial supply at higher latitudes. Overall, our results suggest that benthic deep‐sea eukaryotes will be negatively affected by the predicted decrease in diatoms in surface oceans, especially at higher latitudes, where benthic prokaryotes and viruses will otherwise likely increase their quantitative role and organic carbon cycling rates. In turn, such changes can contribute to decrease carbon transfer from phytodetritus to higher trophic levels, with strong potential to affect oceanic food webs, their biodiversity and consequently carbon sequestration capacity at the global scale.

Patterns in Benthic Microbial Community Structure Across Environmental Gradients in the Beaufort Sea Shelf and Slope.

The paradigm of tight pelagic-benthic coupling in the Arctic suggests that current and future fluctuations in sea ice, primary production, and riverine input resulting from global climate change will have major impacts on benthic ecosystems. To understand how these changes will affect benthic ecosystem function, we must characterize diversity, spatial distribution, and community composition for all faunal components. Bacteria and archaea link the biotic and abiotic realms, playing important roles in organic matter (OM) decomposition, biogeochemical cycling, and contaminant degradation, yet sediment microbial communities have rarely been examined in the North American Arctic. Shifts in microbial community structure and composition occur with shifts in OM inputs and contaminant exposure, with implications for shifts in ecological function. Furthermore, the characterization of benthic microbial communities provides a foundation from which to build focused experimental research. We assessed diversity and community structure of benthic prokaryotes in the upper 1 cm of sediments in the southern Beaufort Sea (United States and Canada), and investigated environmental correlates of prokaryotic community structure over a broad spatial scale (spanning 1,229 km) at depths ranging from 17 to 1,200 m. Based on hierarchical clustering, we identified four prokaryotic assemblages from the 85 samples analyzed. Two were largely delineated by the markedly different environmental conditions in shallow shelf vs. upper continental slope sediments. A third assemblage was mainly comprised of operational taxonomic units (OTUs) shared between the shallow shelf and upper slope assemblages. The fourth assemblage corresponded to sediments receiving heavier OM loading, likely resulting in a shallower anoxic layer. These sites may also harbor microbial mats and/or methane seeps. Substructure within these assemblages generally reflected turnover along a longitudinal gradient, which may be related to the quantity and composition of OM deposited to the seafloor; bathymetry and the Mackenzie River were the two major factors influencing prokaryote distribution on this scale. In a broader geographical context, differences in prokaryotic community structure between the Beaufort Sea and Norwegian Arctic suggest that benthic microbes may reflect regional differences in the hydrography, biogeochemistry, and bathymetry of Arctic shelf systems.

Recovery of subtropical coastal intertidal system prokaryotes from a destruction event and the role of extracellular polymeric substances in the presence of endocrine disrupting chemicals

Intertidal sediments constitute the micro-environment for the co-existence of endocrine disrupting chemicals (EDCs) and biofilms consisting of the microbial community and extracellular polymeric substances (EPS). However, the interactions and the resulting eco-function of this community are complex and poorly characterized, especially after a destruction event. This study evaluates the re-construction of biofilms in terms of the abundance of prokaryotic cells and related EPS characterization in two destroyed sedimentary matrices from subtropical environments simulated by sterilization in the presence of EDCs and investigates the role of EPS. The results show that benthic prokaryotes recover from the deposition of active prokaryotes in natural seawater and form biofilms after sterilization. Sterilization triggers the release of polysaccharides and protein from lysed native microbial cells and bound EPS in sedimentary organic matter, thus increasing their concentrations. The increased portion of EPS also acts as a persistent stress on re-colonizing prokaryotes and leads to the overproduction of sedimentary EPS. Due to the protective role mediated by EPS, the effect of EDCs on biofilm composition in sterilized sediment is not significant. The sedimentary matrix is the most important determinant of the composition of the biofilm and the occurrence of EDCs. At the end of an 84-day experiment, the abundance of prokaryotic cells and the concentrations of polysaccharides and protein in mangrove sediment are 1.6–1.8 times higher than those in sandflat sediment, regardless of EDCs. Sandflat sediment exhibits higher concentrations of nonylphenol and bisphenol A but a lower concentration of 17α-ethinylestradiol than mangrove sediment. This study enhances our understanding of the role of sedimentary biofilms and the fate of EDCs in intertidal systems and highlights the benefit of a destructive event in enhancing ecosystem function, particularly tolerance to EDC adversity due to EPS production.

Impacts of Anthropogenic Pollutants on Benthic Prokaryotic Communities in Mediterranean Touristic Ports.

Ports and marinas are central nodes in transport network and play a strategic role in coastal development. They receive pollution from land-based sources, marine traffic and port infrastructures on one side and constitute a potential pollution source for the adjacent coastal areas on the other. The aim of the present study was to evaluate the effects of organic and inorganic co-contamination on the prokaryotic communities in sediments from three Mediterranean ports. The structure and composition of the bacterial and archaeal communities were assessed by targeted metagenomic analysis of the 16S rRNA gene, and the links of prokaryotic communities with environmental and pollution variables were investigated. The harbors presented pronounced site-specificity in the environmental properties and pollution status. Consistently, the structure of archaeal and bacterial communities in surface sediments exhibited a strong spatial variation among the three investigated ports. On the contrary, a wide overlap in composition of prokaryotic assemblages among sites was found, but local variation in the community composition and loss of prokaryotic diversity was highlighted in a heavily impacted port sector near a shipyard. We provided evidences that organic matter, metals and PAHs as well as temperature and salinity play a strong role in structuring benthic bacterial communities significantly contributing to the understanding of their responses to anthropogenic perturbations in marine coastal areas. Among metals, copper was recognized as strongly associated with the observed changes in bacterial assemblages. Overall, this study provides the first assessment of the effects exerted by multiple organic and inorganic contaminations on benthic prokaryotes in ports over a large spatial scale and designates bacterial community as a candidate tool for the monitoring of the sediment quality status in harbors.

Chemical contamination can promote turnover diversity of benthic prokaryotic assemblages: The case study of the Bagnoli-Coroglio bay (southern Tyrrhenian Sea)

Chemical contamination of marine ecosystems represents a major concern for the detrimental consequences at different levels of biological organization. However, the impact of chronic contamination on the diversity and assemblage composition of benthic prokaryotes is still largely unknown, and this limits our understanding of the potential implications on ecosystem functioning. The Bagnoli-Coroglio bay (Gulf of Naples, Tyrrhenian Sea) is a typical example of coastal area heavily contaminated by metals and hydrocarbons, released for decades by industrial activities, which ceased at the beginning of nineties. In the present study we analyzed the abundance, diversity and assemblage composition of benthic prokaryotic assemblages at increasing distance from the historical source of contamination in relation to the heavy hydrocarbons (C > 12), polycyclic aromatic hydrocarbons (PAHs) and heavy metal concentrations in the sediments. Prokaryotic abundance in the sediments differed among sites, and was mostly driven by environmental factors rather than by contamination levels. Conversely, the richness of prokaryotic taxa was relatively high in all samples, was driven by contamination levels and decreased significantly with increasing contamination (15–38%). Moreover, our results indicate large variations in the composition of the benthic prokaryotic assemblages among sites, mostly explained by the different levels and types of chemical contaminants found in the sediments. Overall, our findings suggest that chemical contaminants, even after decades from the end of their release, can profoundly influence the richness and turnover diversity of the benthic prokaryotic assemblages, in turn promoting a high diversification of the benthic bacterial and archaeal assemblages by selecting those lineages more adapted to specific mixtures of different contaminants. Our results open new perspectives for understanding of the long-term effects of chemical contamination on the benthic prokaryotic assemblages and the ecological processes they mediate.

Prokaryotic community composition and distribution in coastal sediments following a Po river flood event (northern Adriatic Sea, Italy)

The influence of the freshwater runoff on marine prokaryotes has been largelly studied in the pelagic environment, little is known about the effects on the prokaryotes in coastal sediments. We investigated the distribution of benthic prokaryotes in the northern Adriatic Sea, when an extreme Po River flood event determined high heterogeneity along the estuarine salinity gradient (from mainly freshwater to marine). The microbial diversity was assessed by 16S rRNA gene Illumina sequencing, the quantification of the dominant taxa of Bacteria and Archaea by microscopy techniques (CARD-FISH). Our results showed dominance of Bacteria (eight-fold higher) over Archaea, and highlighted Proteobacteria as the most represented phylum, followed by Bacteroidetes. Prokaryotic abundance, quantified by DAPI staining, was mainly affected by sediment characteristics (Total Organic C and Total N contents, and silt fraction). Abundance reached the highest values in sites directly affected by the plume (range 7.3 × 108 - 2.8 × 109 cell g−1). A considerable difference in prokaryotes composition was observed along the salinity gradient, with an increasing presence of freshwater taxa at stations more influenced by the river discharge. CARD-FISH analyses corroborated these findings, showing higher abundance of beta-Proteobacteria and Planctomycetes at stations closest to the river mouths. Nevertheless, a core microbiome (about 10% of the total OTUs) was present across all the study areas, indicating the ability of this assemblage to survive in rather diverse environmental conditions, differently impacted by the river plume. Our results show that the river plume can affect the diversity and distribution patterns of benthic prokaryotes far into the pro-delta, although further investigations are needed in order to understand how the mixing of bacterial communities of different origin might reflect on ecosystem functioning.

Viral Infections Boost Prokaryotic Biomass Production and Organic C Cycling in Hadal Trench Sediments.

Hadal trenches are among the most remote and least explored ecosystems on Earth and can support high benthic microbial standing stocks and activities. However, information on the role of viruses in such ecosystems and their interactions with prokaryotic hosts is very limited. Here, we investigated activities of benthic viruses and prokaryotes and their interactions in three hadal trenches (Japan, Izu-Ogasawara and Mariana trenches) and in their nearby abyssal sites. Our findings reveal that these hadal trenches, compared with the surrounding abyssal sites, support higher abundances and biomasses of prokaryotes. In addition, the high prokaryotic biomasses of hadal trenches could favor high rates of viral infection and cell lysis, especially in the Japan Trench. Hadal viruses can release large amounts of highly labile and promptly available organic material by inducing cell lysis, which could contribute to sustain benthic prokaryotes and decrease their dependency on the enzymatic digestion of the more refractory fraction of sediment organic matter. Our results suggest that this process can contribute to explain the discrepancy between high prokaryote biomass and apparent low efficiency in the utilization of the sedimentary organic matter in the hadal ecosystems. Concluding, hadal trenches may be characterized by a highly dynamic viral component, which can boost prokaryotic biomass production, thereby profoundly influencing the functioning of these remote and extreme ecosystems.

Microbial processing of sedimentary organic matter at a shallow LTER site in the northern Adriatic Sea: an 8-year case study

Benthic prokaryotes are the key-players in C-cycling at the sediment-seawater interface, one of the largest biologically active interfaces on Earth. Here, microbial-mediated processes, such as the degradation of organic matter and the incorporation of mobilized C into microbial biomass, depend on several factors such as environmental temperature and substrate availability, especially in shallow sediments at mid-high latitudes where seasonal fluctuations of these variables occur. In the present study, four degradative activities (β-glucosidase, lipase, chitinase and aminopeptidase), Heterotrophic C Production (HCP), Total Organic C (TOC), Total Nitrogen (TN) and Biopolymeric C (BPC) were investigated seasonally from April 2010 to April 2018 in the surface sediments of a shallow Long-Term Ecological Research (LTER) station of the northern Adriatic Sea. Significant temperature-dependences were described by Arrhenius-type equations for HCP and each of the degradative activities tested with the exception of aminopeptidase. The relatively low apparent Activation Energies suggested that these microbial-mediated processes were enhanced by the availability of palatable substrates over the study period. Nevertheless, a clear and tight dependence from such substrates was detected only for aminopeptidase, the most pronounced degradative activity observed. TN was identified by the stepwise multiple regression analysis as the environmental variable that mainly drove this exoenzymatic activity. Enhanced aminopeptidase rates mirrored peaks of TN that seemed, in turn, linked to the seasonal proliferation of benthic microalgae. By supplying prokaryotes with promptly available substrates, these autotrophs, represented mainly by diatoms, seemed to play an important role in the C-cycling regulation at the studied LTER station.

High diversity of benthic bacterial and archaeal assemblages in deep-Mediterranean canyons and adjacent slopes

Submarine canyons and slopes increase the topographic heterogeneity of continental margins and enhance mass and energy transfer from the shelf to the deeper basins, profoundly influencing the biodiversity and functioning of deep-sea ecosystems. However, information on the distribution and diversity of microbial assemblages in such habitats is extremely scant. In the present study, for the first time, we investigated abundances of benthic prokaryotes and their diversity (i.e., bacterial and archaeal) in two of the largest canyons of the Mediterranean Sea (Bisagno and Polcevera Canyons, in the Ligurian Margin) and their adjacent open slope along a water-depth gradient from 200 to 2000 m. We found that prokaryotic abundance and the richness of Operational Taxonomic Units (OTUs) varied within a narrow range despite the high variability of thermohaline and trophic conditions in the habitats investigated. Conversely, the taxonomic composition (in terms of OTUs) was highly variable among the different habitats and depths investigated. The largest fraction of OTUs (representing >80% of the total OTUs identified) were unique of each sampling site, indicating a high prokaryotic β-diversity among the investigated systems. Thermo-haline and trophic conditions and water depth only partially influenced the composition of the prokaryotic assemblages suggesting that other factors, potentially related to physical forcing typically occurring in margin ecosystems, could promote a high diversification of benthic bacterial and archaeal assemblages. This study provides new insights into the benthic prokaryotic diversity of Ligurian canyons and suggests that microbial assemblages thriving in continental margins can largely contribute to the whole diversity of Mediterranean deep-sea ecosystems.

Rapid response of benthic deep-sea microbes (viruses and prokaryotes) to an intense dense shelf water cascading event in a submarine canyon of the NW Mediterranean Sea

A major dense shelf water cascading (DSWC) event occurred in 2005 downward the Cap de Creus Canyon (Gulf of Lion, NW Mediterranean Sea), which caused a significant change in environmental parameters and biological components. Here we describe the effects of this DSWC event on benthic microbes and on virus-prokaryote interactions, and we explore their implications on the functioning of the canyon’s ecosystem. We collected sediment samples at increasing depths inside the canyon and in the adjacent deep continental margin over a period of five years, i.e. during and after the DSWC event, which led to the deposition of high amounts of fresh and labile organic matter that stimulated C production by benthic prokaryotes and increased their abundance and biomass. The enhanced prokaryotic metabolism, still evident 6 months after the DSWC event, was associated with high viral replication rates and prokaryotic mortality, which released 3.4–6.3 gC m−2 over such a 6 months period. Such values are up to 3-times higher than the yearly C-flux to the seafloor reported in this area in years without DSWC. We conclude that DSWC can significantly enhance benthic prokaryotic metabolism and C cycling through viral-induced prokaryotic mortality.

Impact of pyrene and cadmium co-contamination on prokaryotic community in coastal sediment microcosms

Acute ecological impacts of co-contamination of polycyclic aromatic hydrocarbons (PAHs) and heavy metals on diversity and composition of coastal benthic prokaryotes were unclear. We took pyrene (Pyr) and cadmium (Cd) as the representatives and mimicked an eight-week exposure of moderate and high levels of Pyr, Cd and their mixtures. 16S rRNA amplicon sequencing was used to investigate interaction of the contaminants in temporal succession of prokaryotes. Generally, concentrations of Pyr and HCl-extractable Cd in the sediments were stable over time. Effects and interaction of Pyr and Cd on prokaryotic α-diversity were temporally- and dose-dependent with a decreasing trend in richness and Shannon index under various contamination regimes, particularly in the single-Cd contaminated groups at the early stage. Temporal variability and Pyr-induced pattern in prokaryotic composition were observed. However, Pyr and Cd showed a persistent interaction in prokaryotic composition after 7 days, altering successional trajectories of communities. The communities under Pyr contamination regardless of Cd could be at a developing stage for an active PAH-degrading community with appearance of a pioneer Cycloclasticus phylotype, persistently showing a strong correlation with Pyr level. The associations of phylotypes and Cd level were short-lived and weak, corresponding to the overall resistance of prokaryotic composition to Cd. In the high-throughput sequencing era, using microcosm experiment, we renewed the knowledge about how prokaryotes vary in terms of α-diversity, composition and specific taxa in response to co-contamination of model contaminants at a temporal scale.

Regional variations in the diversity and predicted metabolic potential of benthic prokaryotes in coastal northern Zhejiang, East China Sea.

Knowledge about the drivers of benthic prokaryotic diversity and metabolic potential in interconnected coastal sediments at regional scales is limited. We collected surface sediments across six zones covering ~200 km in coastal northern Zhejiang, East China Sea and combined 16 S rRNA gene sequencing, community-level metabolic prediction, and sediment physicochemical measurements to investigate variations in prokaryotic diversity and metabolic gene composition with geographic distance and under local environmental conditions. Geographic distance was the most influential factor in prokaryotic β-diversity compared with major environmental drivers, including temperature, sediment texture, acid-volatile sulfide, and water depth, but a large unexplained variation in community composition suggested the potential effects of unmeasured abiotic/biotic factors and stochastic processes. Moreover, prokaryotic assemblages showed a biogeographic provincialism across the zones. The predicted metabolic gene composition similarly shifted as taxonomic composition did. Acid-volatile sulfide was strongly correlated with variation in metabolic gene composition. The enrichments in the relative abundance of sulfate-reducing bacteria and genes relevant with dissimilatory sulfate reduction were observed and predicted, respectively, in the Yushan area. These results provide insights into the relative importance of geographic distance and environmental condition in driving benthic prokaryotic diversity in coastal areas and predict specific biogeochemically-relevant genes for future studies.

Quantification of Viral and Prokaryotic Production Rates in Benthic Ecosystems: A Methods Comparison.

Viruses profoundly influence benthic marine ecosystems by infecting and subsequently killing their prokaryotic hosts, thereby impacting the cycling of carbon and nutrients. Previously conducted studies, based on different methodologies, have provided widely differing estimates of the relevance of viruses on benthic prokaryotes. There has been no attempt so far to compare these independent approaches, including contextual comparisons among different approaches for sample manipulation (i.e., dilution or not of the sediments during incubations), between methods based on epifluorescence microscopy (EFM) or radiotracers, and between the use of different radiotracers. Therefore, it has been difficult to identify the most suitable methodologies and protocols to be used as standard approaches for the quantification of viral infections of prokaryotes. Here, we compared for the first time different methods for determining viral and prokaryotic production rates in marine sediments collected at two benthic sites, differing in depth and environmental conditions. We used a highly replicated experimental design, testing the potential biases associated to the incubation of sediments as diluted or undiluted. In parallel, we also compared EFM counts with the 3H-thymidine incubations for the determination of viral production rates, and the use of 3H-thymidine versus 3H-leucine radiotracers for the determination of prokaryotic production. We show here that, independent from sediment dilution, EFM-based values of viral production ranged from 1.4 to 4.6 × 107 viruses g-1 h-1, and were similar but overall less variable compared to those obtained by the 3H-thymidine method (0.3 to 9.0 × 107 viruses g-1h-1). In addition, the prokaryotic production rates were not affected by sediment dilution, and the use of different radiotracers provided very consistent estimates (10.3–35.1 and 9.3–34.6 ngC g-1h-1 using the 3H-thymidine or 3H-leucine method, respectively). These results indicated that viral lysis was responsible for the abatement of 55–81% of the prokaryotic heterotrophic production, corroborating previous findings of the major role of viruses in benthic deep-sea ecosystems. Moreover, our methodological comparison for the analysis of viral production in marine sediments suggests that microscopy-based approaches are simpler and more cost-effective than those based on radiotracers. These approaches also reduce time to results and overcome issues related to generation of radioactive waste.

Impact of the biocide Irgarol on meiofauna and prokaryotes from the sediments of the Bizerte lagoon-an experimental study.

The biocide Irgarol 1051 has been reported to have negative effects on a large number of living components including non-target organisms, but information on its impact on the marine meiofauna and benthic prokaryotes is completely lacking. Here, we report the results of long-term experimental studies in which we determined the effects of increasing Irgarol concentrations (from 11.5 to 315ngg(-1) sediment dry weight) on meiofauna and benthic prokaryotes. We found that this biocide had a significant impact on meiofauna abundance, even at the lowest concentrations, causing a drastic decline in the abundance of nematodes (the dominant meiofaunal taxon) and an increase of the relative importance of oligochaetes. Even if no direct effects of Irgarol were found on prokaryotic abundance and biomass, the molecular fingerprinting analyses (automated ribosomal intergenic spacer analysis) showed that the prokaryotic diversity was significantly altered by the biocide. The results of the present study indicate that Irgarol 1051 in marine sediments has a significant impact on the smallest eukaryotic and microbial components also at very low concentrations (ca 12ngg(-1)).

Integrated approach for the assessment of the benthic ecosystem functioning at a coastal site in the northern Adriatic Sea

The assessment of an ecosystem functioning is very complex and requires an integrated approach based on the combined and synoptic estimates of biological processes and the study of communities at different trophic levels. Here we present the main results gained over a 2 year-period aimed to investigate the benthic ecosystem functioning at a virtually pristine coastal site. Our suggested approach follows consecutive and interconnected steps: from the chemical characterisation of the sediments to the evaluation of the main autotrophic and heterotrophic metabolic processes, passing through the study of microphytobenthos and meiofauna. Our results indicate that in summer the system experienced pulsed inputs of fresh organic matter mostly due to the proliferation of benthic diatoms. This freshly produced material increased the sediment nutritional value supporting higher meiofaunal densities and stimulating benthic prokaryotes in mobilising C and incorporating it in new biomass. In winter, the lower availability of fresh organic matter, mainly due to the lower diatom abundances, contributed to a reduced prokaryotic C reworking and overall to a major confinement of C flow within the benthic microbial loop. The system shifted from net autotrophy to net heterotrophy as a response to temperature and light variations at the bottom. The latter factors directly influence the microalgal photosynthetic efficiency and are therefore responsible for the observed uncoupling between microphytobenthic abundances and primary production rates.

Benthic ecosystem functioning in the severely contaminated Mar Piccolo of Taranto (Ionian Sea, Italy): focus on heterotrophic pathways.

The benthic ecosystem functioning is a rarely applied holistic approach that integrates the main chemical and biological features of the benthic domain with the key processes responsible for the flux of energy and C through the system. For the first time, such conceptual model, with an emphasis on the heterotrophic pathways, has been applied to the sediments at four stations within one of the most polluted coastal areas in Italy: the Mar Piccolo of Taranto. The functioning of the benthic ecosystem was different according to the investigated site. Nearby the military arsenal, i.e., the main source of organic contaminants and heavy metals, the system seemed inhibited at all the investigated structural and functional levels. Slow microbial processes of C reworking together with very limited densities of benthic fauna suggested a modest transfer of C both into a solid microbial loop and to the higher trophic levels. On the other hand, the ingression of marine water through the "Navigabile" channel seemed to stimulate the organic matter degradation and, consequently, the proliferation of meiofauna and macrofauna. In the innermost part of the basin, the system functioning, to some extent, is less impacted by contaminants and more influenced by mussel farms. The organic matter produced by these bivalves fueled faster C reworking by benthic prokaryotes and enhanced the proliferation of filter feeders.

Impact of silver nanoparticles on benthic prokaryotes in heavy metal-contaminated estuarine sediments in a tropical environment

Little knowledge is available about the potential impact of commercial silver nanoparticles (Ag-NPs) on estuarine microbial communities. The Hugli river estuary, India, is susceptible to heavy metals pollution through boat traffic, and there is the potential for Ag-NP exposure via effluent discharged from ongoing municipal and industrial activities located in close proximity. This study investigated the effects of commercial Ag-NPs on native microbial communities in estuarine sediments collected from five stations, using terminal restriction fragment length polymorphism (T-RFLP) technique. An increase in the number of bacteria in consortium in sediments was observed following exposure to Ag-NPs. In general microbial communities may be resistant in estuarine systems to the antimicrobial effects of commercial Ag-NPs, but key microorganisms, such as Pelobacter propionicus, disappeared following exposure to Ag-NPs. In conclusion, the T-RFLP analysis indicated that Ag-NPs have the potential to shape estuarine sediment bacterial community structure.