The production of extracellular polymeric substances (EPS) by planktonic microbes can influence the fate of oil and chemical dispersants in the ocean through emulsification, degradation, dispersion, aggregation, and/or sedimentation. In turn, microbial community structure and function, including the production and character of EPS, is influenced by the concentration and chemical composition of oil and chemical dispersants. For example, the production of marine oil snow and its sedimentation and flocculent accumulation to the seafloor were observed on an expansive scale after the Deepwater Horizon oil spill in the Northern Gulf of Mexico in 2010, but little is known about the underlying control of these processes. Here, we review what we do know about microbially produced EPS, how oil and chemical dispersant can influence the production rate and chemical and physical properties of EPS, and ultimately the fate of oil in the water column. To improve our response to future oil spills, we need a better understanding of the biological and physiochemical controls of EPS production by microbes under a range of environmental conditions, and in this paper, we provide the key knowledge gaps that need to be filled to do so. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementExtracellular polymeric substances (EPS) are a group of chemically heterogeneous polymers released into the environment by microbes (bacteria, archaea, and phytoplankton), often in response to environmental stresses. EPS serve an important role in determining the fate and transport of oil after a spill, but relatively little is known about EPS production in relation to oil and dispersants, especially at molecular and chemical levels. Here, we summarize the scope of our current knowledge and identify major knowledge gaps. 3Limnology and Oceanography Letters 1, 2016, 3-26

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Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Marine snow was implicated in the transport of oil to the seafloor during the Deepwater Horizon oil spill, but the exact processes remain controversial. In this study, we investigated the concentrations and distributions of the 16 USEPA priority polycyclic aromatic hydrocarbons (PAHs) in marine snow aggregates collected during a mesocosm experiment. Seawater only, oil in a water accommodated fraction (WAF), and Corexit-enhanced WAF (DCEWAF) were incubated for 16 d. Both WAF and DCEWAF aggregates were enriched in heavy molecular weight PAHs but depleted in naphthalene. DCEWAF aggregates had 2.6 times more total 16 PAHs than the WAF (20.5 vs. 7.8 µg/g). Aggregates in the WAF and DCEWAF incorporated 4.4% and 19.3%, respectively of the total PAHs in the mesocosm tanks. Our results revealed that marine snow sorbed and scavenged heavy molecular weight PAHs in the water column and the application of Corexit enhanced the incorporation of PAHs into the sinking aggregates.

Diatoms play a key role in the marine carbon cycle with their high primary productivity and release of exudates such as extracellular polymeric substances (EPS) and transparent exopolymeric particles (TEP). These exudates contribute to aggregates (marine snow) that rapidly transport organic material to the seafloor, potentially capturing contaminants like petroleum components. Ocean acidification (OA) impacts marine organisms, especially those that utilize inorganic carbon for photosynthesis and EPS production. Here we investigated the response of the diatom Thalassiosira pseudonana grown to present day and future ocean conditions in the presence of a water accommodated fraction (WAF and OAWAF) of oil and a diluted chemically enhanced WAF (DCEWAF and OADCEWAF). T . pseudonana responded to WAF/DCEWAF but not OA and no multiplicative effect of the two factors (i.e., OA and oil/dispersant) was observed. T . pseudonana released more colloidal EPS (< 0.7 μm to > 3 kDa) in the presence of WAF/DCEWAF/OAWAF/OADCEWAF than in the corresponding Controls. Colloidal EPS and particulate EPS in the oil/dispersant treatments have higher protein-to-carbohydrate ratios than those in the control treatments, and thus are likely stickier and have a greater potential to form aggregates of marine oil snow. More TEP was produced in response to WAF than in Controls; OA did not influence its production. Polyaromatic hydrocarbon (PAH) concentrations and distributions were significantly impacted by the presence of dispersants but not OA. PAHs especially Phenanthrenes, Anthracenes, Chrysenes, Fluorenes, Fluoranthenes, Pyrenes, Dibenzothiophenes and 1-Methylphenanthrene show major variations in the aggregate and surrounding seawater fraction of oil and oil plus dispersant treatments. Studies like this add to the current knowledge of the combined effects of aggregation, marine snow formation, and the potential impacts of oil spills under ocean acidification scenarios.

Marine sinking particulate organic matter (POM), acting as a link between surface primary production and burial of organic matter in marine sediments, undergoes a variety of physical and biochemical alterations on its way to the deep ocean, resulting in an increase in its un-characterizable proportion with diagenesis. Further, the binding ligands in POM for iron, an essential nutrient to marine life and tightly coupled with organic matter, has rarely been studied. In the current study, we employed an approach combining sequential extraction with ultrahigh resolution mass spectrometry (ESI-FTICRMS), in order to explore and unravel the chemical characteristics of organic matter compounds relevant to marine particle flux within the mesopelagic and deep ocean, with a focus on the potential iron-carrying molecules. With increasing depth, POM increases in aliphaticity, and decreases in intensity-normalized O/C ratios, aromatics, and carboxylic-rich alicyclic molecules (CRAM)-like compounds. The potential iron-carrying molecules account for ∼14% of total identified molecules, and appear to have been incorporated into the marine particles via ion complexation, hydrophobic interaction, and/or interlayered "occlusion." The relative abundance of iron-binding organic molecules in these three operationally-defined categories changes with depth: "surficially-complexed" fraction decreases with depth, the "interlayeredoccluded" fraction increases to a comparable extent and "hydrophobic interaction" fraction occurs at all depths. Collectively, the potential iron-carrying organic molecules exhibit a set of unique molecular characteristics: a relatively lower average H/C ratio and a higher O/C ratio compared to bulk POM, a dominance of aromatics, black carbon-like compounds and CRAM-like compounds, and minor amounts of aliphatics. These molecules exhibit partial similar molecular features as precursors formed from photochemical reactions in the surface ocean, but they have been greatly modified by flux processes. Noticeably, a minor fraction of these iron-carrying molecules (<1%)

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