Plastic Accumulation in the Sea Surface Microlayer: An Experiment-Based Perspective for Future Studies

Abstract

Plastic particles are ubiquitous in the marine environment. Given their low density, they have the tendency to float on the sea surface, with possible impacts on the sea surface microlayer (SML). The SML is an enriched biofilm of marine organic matter, that plays a key role in biochemical and photochemical processes, as well as controlling gas exchange between the ocean and the atmosphere. Recent studies indicate that plastics can interfere with the microbial cycling of carbon. However, studies on microplastic accumulation in the SML are limited, and their effects on organic matter cycling in the surface ocean are poorly understood. To explore potential dynamics in this key ocean compartment, we ran a controlled experiment with standard microplastics in the surface and bulk water of a marine monoculture. Bacterial abundance, chromophoric dissolved organic matter (CDOM), and oxygen concentrations were measured. The results indicate an accumulation of CDOM in the SML and immediate underlying water when microplastic particles are present, as well as an enhanced oxygen consumption. If extrapolated to a typical marine environment, this indicates that alterations in the quality and reactivity of the organic components of the SML could be expected. This preliminary study shows the need for a more integrated effort to our understanding the impact of microplastics on SML functioning and marine biological processes.

Microplastics increase the marine production of particulate forms of organic matter

“…Interestingly, in MP treatments only, the increase in DOC correlated to an increased abundance of heterotrophic bacteria (Spearman rho=0.57, p=0.002, n=29) and to a decrease in phaeopigments (Spearman rho=−0.42, p=0.037, n=25). This indicates a higher bacterial metabolic activity and transformation of phytoplankton-derived organic substrates (Galgani et al 2018, Galgani and Loiselle 2019), with a subsequent reintegration of inorganic nutrients, mainly PO 4 3− , into the system. This is further demonstrated by the different dynamics of PO 4 3− concentrations in the treatment mesocosms, being significantly higher in MP treatments throughout the experiment (Two-Way ANOVA F(1,40)=20.83, p<0.0001) (Kirchman 1994).…”

“…While it is a complex process with multiple pathways, these results indicate a possible mechanism. As growth substrates for marine plankton, microplastics may increase the biological production of DOM (Galgani et al 2018, Galgani andLoiselle 2019) and its aggregation into marine gel particles around suspended particulates. This is facilitated by the phytoplanktonbacteria interplay and would favor the downward export of natural or artificial materials through marine snow (Galloway et al 2017).…”

How will marine plastic pollution affect bacterial primary producers?

“…In addition to providing a surface to colonize, plastics can also leach a variety of compounds. Plastics may provide some dissolved organic carbon sources for some bacteria to metabolise 6,7 but they can also leach organic compounds and metals that negatively impact growth of a variety of microorganisms, including marine larvae 8,9 and algae 10 . Most plastic items are manufactured using a variety of additives, such as UV stabilisers, plasticizers, metals, dyes and flame retardants 11 .…”

Validation of a method to quantify microfibres present in aquatic surface microlayers

“…Surface tension may act to retain light, low density particles (such as microplastics) in the SML, as they already float at the surface due to their low density. The SML has also been described as 'sticky' due to its organic matter content 7 , which may contribute to the enriched abundance of microplastics (and other hydrophobic substances) observed in the SML as compared to underlying water [7][8][9][10] . High abundances of microplastics have been recorded in the SML, with an average value of 152,688 (± 92,384) particles m −3 recorded in the coastal seas of South Korea 8 .…”

“…The SML forms a vital habitat for a variety of species, including commercially important species (e.g. cod larvae) 10 , which could be vulnerable to microplastic ingestion. Risks may also be increased by the interaction of microplastics with other anthropogenic pollutants which are recorded to have an enriched abundance in the SML, including persistent organic pollutants, chlorinated hydrocarbons, and heavy metals 7,11 .…”