Dissolution of Antarctic diatoms at low temperatures

The bSiO2 ooze of the Southern Ocean (S.O) has long provided a source of discussion over how and why such thick accumulations exist underlying a region of relatively low diatom productivity. The low temperatures and high nutrient conditions of many regions of the S.O are understood to be optimal for Fragilariopsis kerguelensis, a slow growing diatom with a high silicate (Si), yet low iron requirement, thus making it a dominant species in the surface ocean of this region. The high level of silification and robust characteristics of F.kerguelensis has been hypothesised as being a main factor contributing to its persistence in the sediments of the S.O. However, specific dissolution characteristics of this species have not previously been elucidated, nor have the effects that temperature and aggregation might have in determining the diatom composition of deep ocean sediments. Laboratory experiments tested the hypothesis that the rate of bSiO2 dissolution of aggregated F.kerguelensis is lower than that of the less silicified Chaetoceros debilis. The effects of temperature and physiological stage of the cells on the dissolution rate of freshly aggregated cells was also investigated. Four experiments were undertaken; one with F.kerguelensis at 5º C, one with senescent C.debilis at 5º C, one with senescent C.debilis at 15º C, and one with exponentially growing C.debilis at 5º C. Aggregates were formed in rolling tanks and Si dissolution monitored for 4 months. bSiO2 dissolution was significantly lower for F.kerguelensis as compared to C.debilis at 5º C. Dissolution of C.debilis aggregates formed using exponentially growing cells started with a lag period of 1 week in comparison to those formed using senescent cells, and dissolution increased markedly with temperature

Marine snow formation in the aftermath of the Deepwater Horizon oil spill in the Gulf of Mexico

The large marine snow formation event observed in oil-contaminated surface waters of the Gulf of Mexico (GoM) after the Deepwater Horizon accident possibly played a key role in the fate of the surface oil. We characterized the unusually large and mucus-rich marine snow that formed and conducted roller table experiments to investigate their formation mechanisms. Once marine snow lost its buoyancy, its sinking velocity, porosity and excess density were then similar to those of diatom or miscellaneous aggregates. The hydrated density of the component particles of the marine snow from the GoM was remarkably variable, suggesting a wide variety of component types. Our experiments suggest that the marine snow appearing at the surface after the oil spill was formed through the interaction of three mechanisms: (1) production of mucous webs through the activities of bacterial oil-degraders associated with the floating oil layer; (2) production of oily particulate matter through interactions of oil components with suspended matter and their coagulation; and (3) coagulation of phytoplankton with oil droplets incorporated into aggregates. Marine snow formed in some, but not all, experiments with water from the subsurface plume of dissolved hydrocarbons, emphasizing the complexity of the conditions leading to the formation of marine snow in oil-contaminated seawater at depth

The Ecosystem Baseline for Particle Flux in the Northern Gulf of Mexico

Response management and damage assessment during and after environmental disasters such as the Deepwater Horizon (DWH) oil spill require an ecological baseline and a solid understanding of the main drivers of the ecosystem. During the DWH event, a large fraction of the spilled oil was transported to depth via sinking marine snow, a routing of spilled oil unexpected to emergency response planners. Because baseline knowledge of particle export in the Northern Gulf of Mexico and how it varies spatially and temporally was limited, we conducted a detailed assessment of the potential drivers of deep (~1400 m depth) particle fluxes during 2012–2016 using sediment traps at three contrasting sites in the Northern Gulf of Mexico: near the DWH site, at an active natural oil seep site, and at a site considered typical for background conditions. The DWH site, located ~70 km from the Mississippi River Delta, showed flux patterns that were strongly linked to the Mississippi nitrogen discharge and an annual subsequent surface bloom. Fluxes carried clear signals of combustion products, which likely originated from pyrogenic sources that were transported offshore via the Mississippi plume. The seep and reference sites were more strongly influenced by the open Gulf of Mexico, did not show a clear seasonal flux pattern, and their overall sedimentation rates were lower than those at the DWH site. At the seep site, based on polycyclic aromatic hydrocarbon data, we observed indications of three different pathways for “natural” oiled-snow sedimentation: scavenging by sinking particles at depth, weathering at the surface before incorporation into sinking particles, and entry into the food web and subsequent sinking in form of detritus. Overall, sedimentation rates at the three sites were markedly different in quality and quantity owing to varying degrees of riverine and oceanic influences, including natural seepage and contamination by combustion products

Editorial: Microbial exopolymers: Sources, chemico-physiological properties, and ecosystem effects in the marine environment

A large proportion of the total carbon in theWorld Ocean is in the formof dissolved organic matter (DOM), which is comparable in mass to the carbon in atmospheric CO2 (Hansell and Carlson, 1998). A major source of this material derives from the synthesis and release of exopolymers, or extracellular polymeric substances (EPS), mainly by bacteria and eukaryotic phytoplankton (Verdugo, 1994; Aluwihare et al., 1997). An initial understanding on the secretion of EPS by microorganisms, and their potential stabilizing effects for microbial cells, emerged during the last century with the first report by ZoBell and Allen (1935). We now know that most bacteria, and other microorganisms, occur associated with biofilms, either attached to surfaces or as suspended-aggregates in the water column. Exopolymer secretions thus serve important functions in marine environments, where they may be involved in microbial adhesion to solid surfaces and biofilm formation (Thavasi and Banat, 2014). They have also been shown to be involved in emulsification of hydrocarbon oils to enhance biodegradation (Gutierrez et al., 2013), mediating the fate and mobility of heavymetals and tracemetal nutrients (Bhaskar and Bhosle, 2005; Gutierrez et al., 2008, 2012), or interacting with dissolved and/or particulate organic matter (Long and Azam, 2001). This wide spectrum of functional activity is reflected not merely in the complex chemistry of these biopolymers, but also in the diversity of bacterial and phytoplankton genera found producing them

Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens

Biological dinitrogen (N-2) fixation is performed solely by specialized bacteria and archaea termed diazotrophs, introducing new reactive nitrogen into aquatic environments. Conventionally, phototrophic cyanobacteria are considered the major diazotrophs in aquatic environments. However, accumulating evidence indicates that diverse non-cyanobacterial diazotrophs (NCDs) inhabit a wide range of aquatic ecosystems, including temperate and polar latitudes, coastal environments and the deep ocean. NCDs are thus suspected to impact global nitrogen cycling decisively, yet their ecological and quantitative importance remain unknown. Here we review recent molecular and biogeochemical evidence demonstrating that pelagic NCDs inhabit and thrive especially on aggregates in diverse aquatic ecosystems. Aggregates are characterized by reduced-oxygen microzones, high C:N ratio (above Redfield) and high availability of labile carbon as compared to the ambient water. We argue that planktonic aggregates are important loci for energetically-expensive N-2 fixation by NCDs and propose a conceptual framework for aggregate-associated N-2 fixation. Future studies on aggregate-associated diazotrophy, using novel methodological approaches, are encouraged to address the ecological relevance of NCDs for nitrogen cycling in aquatic environments

Polysaccharide hydrolysis in the presence of oil and dispersants: Insights into potential degradation pathways of exopolymeric substances (EPS) from oil-degrading bacteria

Oceanic oil-degrading bacteria produce copious amounts of exopolymeric substances (EPS) that facilitate their access to oil. The fate of EPS in the water column is in part determined by activities of heterotrophic microbes capable of utilizing EPS compounds as carbon and energy sources. To evaluate the potential of natural microbial communities to degrade EPS produced during oil degradation, we measured potential hydrolysis rates of six structurally distinct polysaccharides in two roller bottle experiments, using water from a natural oil seep in the northern Gulf of Mexico. The suite of polysaccharides used to measure the initial step in carbon degradation is indicative of polymers within microbial EPS. The treatments included (i) unamended surface or deep waters (whole water), and water amended with (ii) a water-accommodated fraction of oil (WAF), (iii) oil dispersant Corexit 9500, and (iv) WAF chemically-enhanced with Corexit (CEWAF). The oil and Corexit treatments were employed to simulate conditions during the Deepwater Horizon oil spill. Polysaccharide hydrolysis rates in the surface-water treatments were lowest in the WAF treatment, despite elevated levels of EPS in the form of transparent exopolymer particles (TEP). In contrast, the three deep-water treatments (WAF, Corexit, CEWAF) showed enhanced hydrolysis rates and TEP levels (WAF) compared to the whole water. We also observed variations in the spectrum of polysaccharide-hydrolyzing enzyme activities among the treatments. These substrate specificities were likely driven by activities of oil-degrading bacteria, shaping the pool of EPS and TEP as well as degradation products of hydrocarbons and Corexit compounds. A model calculation of potential turnover rates of organic carbon within the TEP pool suggests extended residence times of TEP in oil-contaminated waters, making them prone to serve as the sticky matrix for oily aggregates known as marine oil snow

Effects of rising temperature on pelagic biogeochemistry in mesocosm systems: a comparative analysis of the AQUASHIFT Kiel experiments

A comparative analysis of data, obtained during four indoor-mesocosm experiments with natural spring plankton communities from the Baltic Sea, was conducted to investigate whether biogeochemical cycling is affected by an increase in water temperature of up to 6 °C above present-day conditions. In all experiments, warming stimulated in particular heterotrophic bacterial processes and had an accelerating effect on the temporal development of phytoplankton blooms. This was also mirrored in the build-up and partitioning of organic matter between particulate and dissolved phases. Thus, warming increased both the magnitude and rate of dissolved organic carbon (DOC) build-up, whereas the accumulation of particulate organic carbon (POC) and phosphorus (POP) decreased with rising temperature. In concert, the observed temperature-mediated changes in biogeochemical components suggest strong shifts in the functioning of marine pelagic food webs and the ocean’s biological carbon pump, hence providing potential feedback mechanisms to Earth’s climate system