Bacteria-Oil Microaggregates Are an Important Mechanism for Hydrocarbon Degradation in the Marine Water Column

Vast quantities of oil-associated marine snow (MOS) formed in the water column as part of the natural biological response to the Deepwater Horizon drilling accident. Despite the scale of the event, uncertainty remains about the mechanisms controlling MOS formation and its impact on the environment.

Machine Learning for the Study of Plankton and Marine Snow from Images

Quantitative imaging instruments produce a large number of images of plankton and marine snow, acquired in a controlled manner, from which the visual characteristics of individual objects and their in situ concentrations can be computed. To exploit this wealth of information, machine learning is necessary to automate tasks such as taxonomic classification. Through a review of the literature, we highlight the progress of those machine classifiers and what they can and still cannot be trusted for. Several examples showcase how the combination of quantitative imaging with machine learning has brought insights on pelagic ecology. They also highlight what is still missing and how images could be exploited further through trait-based approaches. In the future, we suggest deeper interactions with the computer sciences community, the adoption of data standards, and the more systematic sharing of databases to build a global community of pelagic image providers and users. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

From Nano-Gels to Marine Snow: A Synthesis of Gel Formation Processes and Modeling Efforts Involved with Particle Flux in the Ocean

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 morphology illuminates the evolution of phytoplankton blooms and determines their subsequent vertical export

The organic carbon produced in the ocean’s surface by phytoplankton is either passed through the food web or exported to the ocean interior as marine snow. The rate and efficiency of such vertical export strongly depend on the size, structure and shape of individual particles, but apart from size, other morphological properties are still not quantitatively monitored. With the growing number of in situ imaging technologies, there is now a great possibility to analyze the morphology of individual marine snow. Thus, automated methods for their classification are urgently needed. Consequently, here we present a simple, objective categorization method of marine snow into a few ecologically meaningful functional morphotypes using field data from successive phases of the Arctic phytoplankton bloom. The proposed approach is a promising tool for future studies aiming to integrate the diversity, composition and morphology of marine snow into our understanding of the biological carbon pump.

Massive export of diazotrophs across the tropical south Pacific Ocean

Diazotrophs are widespread microorganisms that regulate marine productivity in 60% of our oceans by alleviating nitrogen limitation. Yet, their contribution to organic carbon and nitrogen export fluxes has never been quantified, making an assessment of their impact on the biological carbon pump impossible. Here, we examine species-specific fates of several groups of globally-distributed unicellular (UCYN) and filamentous diazotrophs in the mesopelagic ocean. We used an innovative approach consisting of the combined deployment of surface-tethered drifting sediment traps, Marine Snow Catcher, and Bottle-net, in which we performed nifH sequencing and quantitative PCR on major diazotroph groups across the subtropical South Pacific Ocean. nifH sequencing data from sediment traps deployed at 170 m, 270 m and 1000 m provide clear evidence that cyanobacterial and non-cyanobacterial diazotrophs are systematically present in sinking particles down to 1000 m, with export fluxes being the highest for the UCYN-A1 symbiosis, followed by UCYN-B or Trichodesmium (depending on station and depth), Gamma A and UCYN-C. Specific export turnover rates (a metric similar to the export efficiency adapted to organisms) point to a more efficient export of UCYN groups relative to the filamentous Trichodesmium. This is further confirmed by Marine Snow catcher data showing that the proportion of sinking cells was significantly higher for UCYN compared to Trichodesmium. Phycoerythrin-containing UCYN-B and UCYN-C-like cells were indeed recurrently found embedded in large (> 50 micrometers) seemingly organic aggregates, or organized into clusters of tens to hundreds of cells linked by an extracellular matrix, facilitating the export. Overall, diazotrophs accounted for 6-13% (170 m) to 45-100% (1000 m) of the total particulate nitrogen export fluxes in our study. We thus conclude that diazotrophs are important contributors to carbon sequestration in the subtropical South Pacific Ocean and need to be considered in future studies to improve the accuracy of current regional and global estimates of export.

Potential Role of Marine Snow in the Fate of Spilled Oil in Cook Inlet, Alaska

ABSTRACT The objective of the research is to inform response decision-making and understanding of the potential association of spilled oil with marine snow in Cook Inlet, Alaska. While extensive research has been conducted on minerals aggregating with spilled oil, larger organic aggregates, such as marine snow, have only recently been studied as a transport mechanism. This knowledge gap in understanding the fate of oil was highlighted as part of the Gulf of Mexico Research Initiative (GoMRI) following the Deepwater Horizon (DWH) spill. It was determined that significant percentages of spilled oil reached the seafloor as a result of association with marine snow during both DWH and the Ixtoc 1 blowouts. As development of oil resources continues in Alaska and the Arctic, marine snow is a significant oil exposure pathway that must be considered during oil spill response. In parallel with a corresponding sedimentation study, input from local, federal and industry experts was used to develop laboratory scale oil exposure experiments to evaluate the potential for oil-marine snow aggregate formation in Cook Inlet. Roller-bottle experiments were conducted from May to July 2019 to assess the interactions between a 5 μm sheen of Alaska North Slope crude oil and Cook Inlet surface water. Aggregate formation was documented and sinking flocs were observed and analyzed with fluorescence microscopy to estimate oil content. The total oil volumes estimated in aggregates were between 0.01 to 0.4 μl. Estimates of total oil volume associated with the aggregates ranged from 0.6 to 9.3 % ± 1.4% of the total oil volume (80 μl) that was added to the bottles. The incorporation of spilled oil in surface forming aggregates will contribute to understanding fate and response implications in Cook Inlet and other northern regions at risk of spilled oil entering the benthic food web via association with sinking marine snow.

Crude oil and particulate fluxes including marine oil snow sedimentation and flocculant accumulation: Deepwater Horizon oil spill study

The Deepwater Horizon oil spill is the largest in US history in terms of oil released and the amount of dispersants applied. It is also the first spill in which the incorporation of oil and/or dispersant into marine snow was directly observable. Marine snow formation, incorporation of oil (MOS – marine oil snow) and subsequent settling to the seafloor, has been termed MOSSFA: Marine Oil Snow Sedimentation and Flocculent Accumulation. This pathway accounts for a significant fraction of the total oil returning back to the sea floor. GOMRI funded studies have determined that important drivers of MOSSFA include, but are not limited to, an elevated and extended Mississippi River discharge, which enhanced phytoplankton production and suspended particle concentrations, zooplankton grazing, and enhanced mucus formation (operationally defined as EPS, TEP, marine snow). Efforts thus far to understand the mechanisms driving these processes are being used to aid in the development of response strategies. These include modeling efforts towards predicting plume dynamics. Although much has been learned during the GOMRI program (reviewed herein and elsewhere), there are still important unknowns that need to be addressed. Understanding of the conditions under which significant MOSSFA events occur, the consequences to the biology, the sinking velocity and distribution of the MOSSFA as well as its ultimate fate are amongst the most important consideration for future studies. Also important is the modification of the oil and dispersant within the MOS and its transport as part of MOSSFA. Ongoing studies are needed to further develop our understanding of these complex and interrelated phenomena.