Acrylate in Phaeocystis colonies does not affect the surrounding bacteria

Acrylate accumulates to concentrations of 1.3–6.5 mM in the mucus of Phaeocystis colonies and may have an effect on the surrounding bacterial community, either as an inhibitor or as a carbon source. Both in the field and in the laboratory, effects of acrylate on bacterial growth and on its consumption were investigated. During a Phaeocystis bloom, acrylate-consuming bacteria were found to be present (1% of total number counted by microscopy) and a 5-fold increase of the number of these bacteria was observed after the Phaeocystis bloom (4.9% of the total number counted by microscopy). Acrylate consumption rates were higher in filtered (≤20 μm) seawater samples than in unfiltered samples, indicating that particles larger than 20 μm, mostly Phaeocystis colonies, caused a reduction in the rate of acrylate consumption. This was not found when axenic Phaeocystis was added to an acrylate-consuming bacterium (strain AC-2) that had been isolated from the highest MPN dilution from field samples. Furthermore, we could not find a decrease in growth rates of the total bacterial community or of isolated strains in the presence of high acrylate concentrations (≤10 mM). In co-cultures of Phaeocystis and strain AC-2 we observed that the production of acrylate was not affected by the bacterium and that the consumption of acrylate by strain AC-2 was not affected by the presence of Phaeocystis. Acrylate concentrations in the mucous layer of the Phaeocystis colonies in those co-cultures were high (6.7–7.7 mM) and comparable with acrylate concentrations in the mucous layer of axenic Phaeocystis colonies. Acrylate seems to be sorbed to the mucus matrix of the colony and diffusion of acrylate out of this mucus matrix appears to be slow. Upon disruption of the colony skin acrylate was immediately solubilised from the mucus matrix.