Influence of bioturbation by the polychaete Nereis diversicolor on the structure of bacterial communities in oil contaminated coastal sediments

Patterns of change in the structure of bacterial communities monitored by ribosomal intergenic spacer analysis (RISA) in oil contaminated sediments inhabited or not by the marine polychaete Nereis diversicolor were studied during 45 days under laboratory conditions. Results supported by principal component analysis showed a marked response of the bacterial communities to the oil contamination and to the presence of N. diversicolor. Phylogenetic affiliation of specific RISA bands showed that, in the contaminated sediments, the presence of the marine polychaetes favoured the development of bacteria which may play an active role in natural bioremediation processes of oil polluted environments

Biovolume and biomass estimates of key diatoms in the Southern Ocean

Linear measurements of 31 Southern Ocean diatom species or genus groupings are presented and used in the derivation of species-specific biovolume, biomass and surface to volume ratios. Species measurements are in tune with summarised values in the literature and range from 2.6 to 2296 μm in length and 8.5 to 140 μm in diameter. Average cell volume ranged from 63 to 1.43 × 10⁵ μm³ and biomass from 10 to 3686 pg C cell⁻¹, the values being smallest in pennate shaped species and greatest in cylindrical centrics. The 31 species can be divided into 7 small, 12 intermediate and 12 large biomass contributors based on their cell volume and/or their surface to volume ratio. Carbon biomass estimates from biovolume equations are discussed and suggest that future work including large Southern Ocean species is warranted. Species-specific biomass values reported here should serve as a baseline for future analyses of major diatom carbon contributors in the generally high nutrient, low chlorophyll Southern Ocean.14 page(s

Impact of iron on silicon utilization by diatoms in the Southern Ocean: A case study of Si/N cycle decoupling in a naturally iron-enriched area

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Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N-2 fixation activity and diversity in the western tropical South Pacific Ocean

Biological dinitrogen (N-2) fixation provides the major source of new nitrogen (N) to the open ocean, contributing more than atmospheric deposition and riverine inputs to the N supply. Yet the fate of the diazotrophderived N (DDN) in the planktonic food web is poorly understood. The main goals of this study were (i) to quantify how much of DDN is released to the dissolved pool during N-2 fixation and how much is transferred to bacteria, phytoplankton and zooplankton, and (ii) to compare the DDN release and transfer efficiencies under contrasting N-2 fixation activity and diversity in the oligotrophic waters of the western tropical South Pacific (WTSP) Ocean. We used nanometre-scale secondary ion mass spectrometry (nanoSIMS) coupled with N-15(2) isotopic labelling and flow cytometry cell sorting to track the DDN transfer to plankton, in regions where the diazotroph community was dominated by either Trichodesmium or by UCYN-B. After 48 h, 2 similar to 20-40% of the N-2 fixed during the experiment was released to the dissolved pool when Trichodesmium dominated, while the DDN release was not quantifiable when UCYNB dominated; similar to 7-15% of the total fixed N (net N-2 fixation + release) was transferred to non-diazotrophic plankton within 48 h, with higher transfer efficiencies (15 +/- 3 %) when UCYN-B dominated as compared to when Trichodesmium dominated (9 +/- 3 %). The pico-cyanobacteria Synechococcus and Prochlorococcus were the primary beneficiaries of the DDN transferred (similar to 65-70 %), followed by heterotrophic bacteria (similar to 23-34 %). The DDN transfer in bacteria was higher (34 +/- 7 %) in the UCYN-B-dominating experiment compared to the Trichodesmium-dominating experiments (24 +/- 5 %). Regarding higher trophic levels, the DDN transfer to the dominant zooplankton species was less efficient when the diazotroph community was dominated by Trichodesmium (similar to 5-9% of the DDN transfer) than when it was dominated by UCYN-B (similar to 28 +/- 13% of the DDN transfer). To our knowledge, this study provides the first quantification of DDN release and transfer to phytoplankton, bacteria and zooplankton communities in open ocean waters. It reveals that despite UCYN-B fix N-2 at lower rates compared to Trichodesmium in the WTSP, the DDN from UCYN-B is much more available and efficiently transferred to the planktonic food web than the DDN originating from Trichodesmium

FlowCAM as a tool for studying small (80-1000 mu m) metazooplankton communities

FlowCAM was assessed as a tool for studying small (80-1000 mu m) metazooplankton communities by comparing the abundances estimated using FlowCAM with those estimated using a stereomicroscope for eight groups of organisms. With the exception of poecilostomatoid copepods, estimates of the number of organisms in samples using FlowCAM were similar to those using the stereomicroscope. These results suggest that FlowCAM is an effective tool for enumerating small metazooplankton

Composition of the sinking particle flux in a hot spot of dinitrogen fixation revealed through polyacrylamide gel traps

International audienceDiazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N 2 fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). Since N isotope budgets show that export is mainly supported by diazotrophy at these stations, these results suggest that the diazotroph-derived N has been efficiently transferred to the foodweb up to zooplankton and fecal pellets before being exported, pleading for an indirect export of diazotrophy. However, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, suggesting that diazotrophs are also directly exported, with a potential contribution to overall POC fluxes increasing with depth. Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N 2 fixation

Using a new fluorescent probe of silicification to measure species-specific activities of diatoms under varying environmental conditions

info:eu-repo/semantics/publishe