1. Equilibrium between CO2 and inorganic carbon.

Fig. S1. Seawater:headspace partitioning of artificial CO2 (CO2 added to the experiment).

2. Mass balances.

Fig. S2. Mass balances for the microcosms. Mass balance calculations were based on the amount of carbon (C) in the different fractions – bottle surface, dissolved in water, CO2 and biomass. MC252 oil contains ∼ 85% C, COREXIT 9500 contains ∼ 70% C, CO2 27.3% C, and we assume that the ratio between CO2 produced and C built into biomass is 70:30.

Fig. S3. The ratio between straight chain alkanes heptadecane (n-C17) and octadecane (n-C18) and the isoprenoids pristine (pris) and phytane (phy) decreased over time, showing possible preferential degradation of alkanes.

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