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.
Highlights
(Table 1a), chromophoric dissolved organic matter (CDOM) a(355) was different in the surface microlayer (SML) of dark samples with respect to the ULW (Wilcoxon test, p = 0.02) while no significant difference was observed between SML and ULW in the samples exposed to light
To keep the system as simple as possible, and due to methodological constraints, limited parameters were measured in a non-destructive manner; CDOM and oxygen were chosen as representatives of organic matter presence and processing
Surface slicks are often associated with a high bacterial biomass comparable to biofilms on solid surfaces [13], and the SML microbial community could potentially produce significant amounts of CO2 at the ocean–air interface [28]
Summary
The sea surface microlayer (SML) is the uppermost oceanic boundary; it is an organic surface biofilm [1] up to 1 mm thick, [2,3,4]. It is observable on sunny low-wind days with calm waters when the surface of the sea appears as a smooth perfectly reflecting film. Studies on the biological features of the SML first appeared in the 1960s in the Sargasso Sea, when Trichodesmium blooms formed visible surface slicks covering areas up to 25 km [7]
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