Photochemical dissolution of buoyant microplastics to dissolved organic carbon: Rates and microbial impacts.

Trillions of plastic fragments are afloat at sea, yet they represent only 1-2% of the plastics entering the ocean annually. The fate of the missing plastic and its impact on marine life remains largely unknown. To address these unknowns, we irradiated post-consumer microplastics (polyethylene, PE; polypropylene, PP; and expanded polystyrene, EPS), standard PE, and plastic-fragments collected from the surface waters of the North Pacific Gyre under a solar simulator. We report that simulated sunlight can remove plastics from the sea surface. Simulated sunlight also fragmented, oxidized, and altered the color of the irradiated polymers. Dissolved organic carbon (DOC) is identified as a major byproduct of sunlight-driven plastic photodegradation. Rates of removal depended upon polymer chemistry with EPS degrading more rapidly than PP, and PE being the most photo-resistant polymer studied. The DOC released as most plastics photodegraded was readily utilized by marine bacteria. However, one sample of PE microplastics released organics or co-leachates that inhibited microbial growth. Thus, although sunlight may remove plastics from the ocean's surface, leachates formed during plastic photodegradation may have mixed impacts on ocean microbes and the food webs they support.

Carbon allocation under light and nitrogen resource gradients in two model marine phytoplankton(1).

Marine phytoplankton have conserved elemental stoichiometry, but there can be significant deviations from this Redfield ratio. Moreover, phytoplankton allocate reduced carbon (C) to different biochemical pools based on nutritional status and light availability, adding complexity to this relationship. This allocation influences physiology, ecology, and biogeochemistry. Here, we present results on the physiological and biochemical properties of two evolutionarily distinct model marine phytoplankton, a diatom (cf. Staurosira sp. Ehrenberg) and a chlorophyte (Chlorella sp. M. Beijerinck) grown under light and nitrogen resource gradients to characterize how carbon is allocated under different energy and substrate conditions. We found that nitrogen (N)-replete growth rate increased monotonically with light until it reached a threshold intensity (~200 µmol photons · m(-2)  · s(-1) ). For Chlorella sp., the nitrogen quota (pg · µm(-3) ) was greatest below this threshold, beyond which it was reduced by the effect of N-stress, while for Staurosira sp. there was no trend. Both species maintained constant maximum quantum yield of photosynthesis (mol C · mol photons(-1) ) over the range of light and N-gradients studied (although each species used different photophysiological strategies). In both species, C:chl a (g · g(-1) ) increased as a function of light and N-stress, while C:N (mol · mol(-1) ) and relative neutral lipid:C (rel. lipid · g(-1) ) were most strongly influenced by N-stress above the threshold light intensity. These results demonstrated that the interaction of substrate (N-availability) and energy gradients influenced C-allocation, and that general patterns of biochemical responses may be conserved among phytoplankton; they provided a framework for predicting phytoplankton biochemical composition in ecological, biogeochemical, or biotechnological applications.