Description: Transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP) are gel-like particles, ubiquitous in the ocean, that affect important biogeochemical processes including organic carbon cycling by planktonic food webs. Despite much research on both groups of particles (especially TEP) over many years, whether they exist as distinctly stainable fractions of the same particles or as independent particles, each with different driving factors, remains unclear. To address this question, we examined the temporal dynamics of TEP and CSP over 2 complete seasonal cycles at 2 coastal sites in the Northwestern Mediterranean Sea, the Blanes Bay Microbial Observatory (BBMO) and the L’Estartit Oceanographic Station (EOS), as well as their spatial distribution along a coast-to-offshore transect. Biological, chemical, and physical variables were measured in parallel. Surface concentrations (mean ± standard deviation [SD]) of TEP were 36.7 ± 21.5 µg Xanthan Gum (XG) eq L–1 at BBMO and 36.6 ± 28.3 µg XG eq L–1 at EOS; for CSP, they were 11.9 ± 6.1 µg BSA eq L–1 at BBMO and 13.0 ± 5.9 µg BSA eq L–1 at EOS. Seasonal variability was more evident at EOS, where surface TEP and CSP concentrations peaked in summer and spring, respectively, and less predictable at the shore-most station, BBMO. Vertical distributions between surface and 80 m, monitored at EOS, showed highest TEP concentrations within the surface mixed layer during the stratification period, whereas CSP concentrations were highest before the onset of summer stratification. Phytoplankton were the main drivers of TEP and CSP distributions, although nutrient limitation and saturating irradiance also appeared to play important roles. The dynamics and distribution of TEP and CSP were uncoupled both in the coastal sites and along the transect, suggesting that they are different types of particles produced and consumed differently in response to environmental variability.

Description: Pollution of the marine environment is an emerging threat. Nowadays, engineered nanoparticles (〈100 nm) such as zinc, copper and silver are widely used as antimicrobial agents, therefore often present in daily-life products. Consequently, the demand and production of nanoparticles are expected to increase. Here, we specifically focus on silver nanoparticles (AgNP). Once released into the environment, AgNPs pose an obvious ecotoxicological risk, potentially affecting ecosystem structure and functioning. For instance, phytoplankton-derived exudates, rich in acidic polysaccharides and amino acids, can abiotically aggregate into microgels such as transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP). Hence, microgels can bridge dissolved and particulate size fractions and facilitate aggregate formation with organic and mineral particles. Both physical and chemical properties make TEP and CSP attractive nutrient hotspots for heterotrophic bacterioplankton. Bacteria, in turn, utilize extracellular enzymes to access these carbon and nitrogen pools. However, knowledge about the mechanisms by which AgNPs might interact with and affect the biogeochemical cycling of TEP and CSP is still insufficient. Therefore, we conducted a mesocosm experiment in the Eastern Mediterranean Sea and investigated the effects of environmentally relevant concentrations of silver ions (Ag+) and AgNP on the properties of TEP and CSP (i.e., area and abundance) along with enzymatic activity measurements. Our results showed that cyanobacteria were likely the primary source of CSP in the ultra-oligotrophic Mediterranean Sea. Also, CSP contributed more to the microgel pool than TEP, as indicated by a strong relationship between CSP and heterotrophic microbial dynamics. While silver (i.e., Ag+ or AgNP) had overall only marginal effects, both species affected the relationships between cell-specific LAPase activity and CSP and cell-specific APase activity and phosphate levels. Thus, Ag+ and AgNP have the potential to regulate microgel dynamics. However, future studies are needed to derive a robust understanding of the effects of silver pollution on the coupling of microgel formation and degradation and the follow-on effect on biogeochemical cycles.

Description: Phytoplankton forms the base of aquatic food webs and element cycling in diverse aquatic systems. The fate of phytoplankton-derived organic matter, however, often remains unresolved as it is controlled by complex, interlinked remineralization and sedimentation processes. We here investigate a rarely considered control mechanism on sinking organic matter fluxes: fungal parasites infecting phytoplankton. We demonstrate that bacterial colonization is promoted 3.5-fold on fungal-infected phytoplankton cells in comparison to non-infected cells in a cultured model pathosystem (diatom Synedra, fungal microparasite Zygophlyctis, and co-growing bacteria), and even ≥17-fold in field-sampled populations (Planktothrix, Synedra, and Fragilaria). Additional data obtained using the Synedra–Zygophlyctis model system reveals that fungal infections reduce the formation of aggregates. Moreover, carbon respiration is 2-fold higher and settling velocities are 11–48% lower for similar-sized fungal-infected vs. non-infected aggregates. Our data imply that parasites can effectively control the fate of phytoplankton-derived organic matter on a single-cell to single-aggregate scale, potentially enhancing remineralization and reducing sedimentation in freshwater and coastal systems.

Description: In the ocean, downward flux of particles produced in sunlit surface waters is the major component of the biological carbon pump, which sequesters atmospheric carbon dioxide and fuels deep-sea ecosystems. The efficiency of downward carbon transfer is expected to be particularly high in tropical upwelling systems where hypoxia occurring beneath the productive surface waters is thought to hamper particle consumption. However, observations of both particle feeders and carbon export in low-oxygen waters are scarce. Here, we provide evidence that hypoxia-tolerant zooplankton feed on sinking particles in the extensive Oxygen Minimum Zone (OMZ) off Peru. Using several arrays of drifting sediment traps and in situ imaging, we show geochemical and morphological transformations of sinking particles and substantial control of carbon export by zooplankton. Our findings challenge the assumption of a consistently efficient biological carbon pump in OMZs and further demonstrate the need to consider mesopelagic organisms when studying oceanic carbon sequestration.

Description: Deoxygenation is tied to organic carbon (Corg) supply and utilization in marine systems. Under oxygen-depletion, bacteria maintain respiration using alternative electron acceptors such as nitrate. Since anaerobic respiration's energy yield is lower, Corg remineralization may be reduced and its residence time increased. We investigated the influence of oxygen and alternative electron acceptors' availability on Corg cycling by heterotrophic bacteria during a continuous culture experiment with Shewanella baltica, a facultative anaerobic γ-Proteobacteria in the Baltic Sea. We tested six different oxygen levels, from suboxic (〈5 µmol L-1 ) to fully oxic conditions, using media (salinity=14 g L-1 ) supplied with high (HighN) or low (LowN) inorganic nitrogen concentrations relative to glucose as labile Corg source. Our results show that suboxia limited DOC (glucose) uptake and cell growth only under LowN, while higher availability of alternative electron acceptors seemingly compensated oxygen limitation under HighN. N-loss was observed under suboxia in both nitrogen treatments. Under HighN, N-loss was highest and a C:N loss ratio of ~2.0 indicated that Corg was remineralized via denitrification. Under LowN, the C:N loss ratio under suboxia was higher (~5.5), suggesting dominance of other anaerobic respiration pathways, such as dissimilatory nitrate reduction to ammonium (DNRA). Bacterial growth efficiency was independent of oxygen concentration but higher under LowN (34±3.0%) than HighN (26±1.6%). Oxygen concentration also affected dissolved organic matter (DOM) cycling. Under oxic conditions, the release of dissolved combined carbohydrates was enhanced, and the amino acid-based degradation index (DI) pointed to more diagenetically altered DOM. Our results suggest bacterial Corg uptake in low-oxygen systems dominated by S. baltica can be limited by oxygen but compensated by high nitrate availability. Hence, suboxia diminishes Corg remineralisation only when alternative electron acceptors are lacking. Under high nitrate:Corg supply, denitrification leads to a higher N:C loss ratio, potentially counteracting eutrophication in the long run. Low nitrate:Corg supply may favour other anaerobic respiration pathways like DNRA, which sustains labile nitrogen in the system, potentially intensifying the cycle of eutrophication. Going forward, it will be crucial to establish the validity of our findings for S. baltica in natural systems with diverse organic substrates and microbial consortia.