Spatial distribution of vertical carbon fluxes on the Agulhas Bank and its possible implication for the benthic nepheloid layer

Vertical particle fluxes of particulate organic carbon (POC), chlorophyll a (Chl a) and biogenic silica (bSi) were measured on the productive shelf of southern Africa, the Agulhas Bank (AB), in March 2019. Sinking particulate material in the form of aggregates is hypothesized to form the benthic nepheloid layer (BNL) which is a turbid layer found near the seabed. This layer is known to affect the spawning success of squid as it is linked to high turbidity which reduces visibility during mating. To determine the distribution of fluxes and particle composition in the AB, we collected water samples below the surface mixed layer (‘export’) and near the seabed (‘bottom’) using a Marine Snow Catcher. POC export fluxes were significantly higher inshore than offshore (mean ± SD: 944.6 ± 302.0 & 461.1 ± 162.1 mg POC m−2 d−1, respectively). There was no significant difference in the cross-shelf distribution of Chl a and bSi export fluxes, however the inshore fluxes of Chl a and bSi were higher than offshore, suggesting a link between export fluxes and sinking organic matter derived from the more productive inshore surface waters. All bottom fluxes were significantly higher inshore, suggesting the contribution of sinking organic particles and resuspended bottom sediments to inshore fluxes. POC export efficiency (ratio of exported POC flux relative to net primary production (NPP)) was higher on the AB (range: 0.58–9.56) compared to the global shelf seas ratio of 0.18 and not related to NPP, suggesting an export of standing stock of carbon biomass, likely produced before the cruise. Transfer efficiency (i.e., the amount of exported flux that reaches the bottom) was also high (max: 0.99, 1.0 and 33.04 for POC, Chl a and bSi, respectively) but did not show a clear spatial pattern. We observed a significant positive correlation between bottom turbidity (a proxy for BNL presence) and export POC flux, suggesting the possibility that sinking organic matter is contributing to BNL formation on the AB

Benthic protists and fungi of Mediterranean deep hypsersaline anoxic basin redoxcline sediments

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Frontiers in Microbiology 5 (2014): 605, doi:10.3389/fmicb.2014.00605.Some of the most extreme marine habitats known are the Mediterranean deep hypersaline anoxic basins (DHABs; water depth ∼3500 m). Brines of DHABs are nearly saturated with salt, leading many to suspect they are uninhabitable for eukaryotes. While diverse bacterial and protistan communities are reported from some DHAB water-column haloclines and brines, the existence and activity of benthic DHAB protists have rarely been explored. Here, we report findings regarding protists and fungi recovered from sediments of three DHAB (Discovery, Urania, L’ Atalante) haloclines, and compare these to communities from sediments underlying normoxic waters of typical Mediterranean salinity. Halocline sediments, where the redoxcline impinges the seafloor, were studied from all three DHABs. Microscopic cell counts suggested that halocline sediments supported denser protist populations than those in adjacent control sediments. Pyrosequencing analysis based on ribosomal RNA detected eukaryotic ribotypes in the halocline sediments from each of the three DHABs, most of which were fungi. Sequences affiliated with Ustilaginomycotina Basidiomycota were the most abundant eukaryotic signatures detected. Benthic communities in these DHABs appeared to differ, as expected, due to differing brine chemistries. Microscopy indicated that only a low proportion of protists appeared to bear associated putative symbionts. In a considerable number of cases, when prokaryotes were associated with a protist, DAPI staining did not reveal presence of any nuclei, suggesting that at least some protists were carcasses inhabited by prokaryotic scavengers.K. Kormas was partially supported by the University of Thessaly through a sabbatical in 2013. Supported by NSF grants OCE-0849578 to Virginia P. Edgcomb and Joan M. Bernhard and OCE-1061391 to Joan M. Bernhard and Virginia P. Edgcomb

Picoplankton and nanoplankton composition on and around a seamount, affected by an eddy dipole south of Madagascar

Picoplankton and nanoplankton community abundances were sampled on and around a seamount located on the Madagascar Ridge, in November 2016. Prochlorococcus, Synechococcus, picoeukaryote and nanoplankton community abundances both on the seamount and in the immediate vicinity (approximately 100 km) surrounding it were analysed using flow cytometry. The original aim of the study was to estimate whether or not the seamount had an effect on the picoplankton and nanoplankton community distribution. However, coupled altimetry and in situ ship data indicated that a dipole eddy was also sampled during this time, which had a substantial effect on the results. Results reveal that picoeukaryotes contributed the most to carbon biomass within the anticyclone, showing higher abundances in the stations situated within the southern slopes of the seamount which also fell within the anticyclone. The southern slope of the seamount contained higher Synechococcus, Prochlorococcus and picoeukaryote abundances, whereas nanoplankton dominated in the cyclonic eddy. Species composition and abundance was significantly different on versus off the seamount, driven by nanoplankton biomass. These results indicate significantly high abundance above the slopes of the seamount, driven by picoeukaryotes, which seem to be amplified by the dipole presence. The resulting variability in microbial abundance both on the southern slopes of the seamount as well as within the dipole has far reaching implications with regard to larger organism proliferation and carbon export during such events in this region

Protist Community Grazing on Prokaryotic Prey in Deep Ocean Water Masses

Oceanic protist grazing at mesopelagic and bathypelagic depths, and their subsequent effects on trophic links between eukaryotes and prokaryotes, are not well constrained. Recent studies show evidence of higher than expected grazing activity by protists down to mesopelagic depths. This study provides the first exploration of protist grazing in the bathypelagic North Atlantic Deep Water (NADW). Grazing was measured throughout the water column at three stations in the South Atlantic using fluorescently-labeled prey analogues. Grazing in the deep Antarctic Intermediate water (AAIW) and NADW at all three stations removed 3.79% +/- 1.72% to 31.14% +/- 8.24% of the standing prokaryote stock. These results imply that protist grazing may be a significant source of labile organic carbon at certain meso- and bathypelagic depths

Temperature (°C), salinity (PSU) and dissolved oxygen (mL· L^-1) depth profiles for stations 2, 7 and 23.

<p>Sampling depths are marked with an asterisk.</p

Temperatures, light levels and sampling times for grazing experiments at stations 2, 7 and 23.

<p>FLP = Fluorescently labeled prey (or prokaryotes).</p><p>Temperatures, light levels and sampling times for grazing experiments at stations 2, 7 and 23.</p

Fig 3A: Eukaryote concentrations (#Euks ml^-1) at 0 hours obtained through DAPI stained counts at stations 2, 7 and 23.

<p>Fig 3B: Prokaryote concentrations (# bacteria·mL^-1) obtained through DAPI stained counts at stations 2, 7 and 23. Error bars represent the standard deviation of the mean (n = 2).</p

Fig 4A: Community grazing rates of prokaryotes represented as the number of prokaryotes grazed per day.

<p>Fig 4B: Community grazing rates of prokaryotes as a percentage of prokaryote standing stock grazed. Error bars represent the standard deviation of the mean (n = 2).</p