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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.

Polar petroleum components enter marine environments through oil spills and natural seepages each year. Lately, they are receiving increased attention due to their potential toxicity to marine organisms and persistence in the environment. We conducted a laboratory experiment and employed state-of-the-art Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to characterize the polar petroleum components within two operationally-defined seawater fractions: the water-soluble fraction (WSF), which includes only water-soluble molecules, and the water-accommodated fraction (WAF), which includes WSF and microscopic oil droplets. Our results show that compounds with higher heteroatom (N, S, O) to carbon ratios (NSO:C) than the parent oil were selectively partitioned into seawater in both fractions, reflecting the influence of polarity on aqueous solubility. WAF and WSF were compositionally distinct, with unique distributions of compounds across a range of hydrophobicity. These compositional differences will likely result in disparate impacts on environmental health and organismal toxicity, and thus highlight the need to distinguish between these often-interchangeable terminologies in toxicology studies. We use an empirical model to estimate hydrophobicity character for individual molecules within these complex mixtures and provide an estimate of the potential environmental impacts of different crude oil components.