Semi-quantitative assay for polyketide prymnesins isolated from Prymnesium parvum (Haptophyta) cultures

•A new, chemifluorometric assay can semi-quantify purified polyketide prymnesins (PPs) from the alga Prymnesium parvum.•8–10 × more PPs were purified from culture media than from pelleted cells.•On a per-cell basis, external PP levels peaked in lag and post-log phases of growth, when cell numbers are lower.•Both lower salinity and phosphate limitation promoted higher PP levels in culture media.•PPs are not secreted by Golgi-mediated processes, but are probably excreted passively or by cell lysis.

A fluorometric assay was developed to semi-quantify co-purified polyketide prymnesins-1 and -2 (PPs) from Prymnesium parvum cultures. Evaluations performed throughout the growth cycle of 5 practical salinity unit (PSU) cultures detected relatively 8–10 × more PPs in the culture medium (exotoxins) than in cells (endotoxins). The [exotoxin] remained stable and relatively low until post-log growth, when they increased significantly. However, on a per-cell basis, [exotoxin] declined throughout log phase and subsequently increased dramatically during late- and post-log phases. The [endotoxin] remained stable until late- and post-log phases, when it achieved its highest level before declining sharply. Shaking cultures of strains from Texas, South Carolina and the United Kingdom displayed dramatically different [exotoxin] during post-log decline. Cultures adapted to 30 PSU had significantly lower [exotoxin] over the course of cultivation than those grown at 5 PSU. Phosphate limitation enhanced [exotoxin] on a per-cell basis, especially in late- and post-log cultures. Media containing streptomycin exhibited a ∼20% increase in [exotoxin] in post-log cultures vs. control treatments, but it had only negligible effects on endotoxin levels. Brefeldin A had minimal effects on [exotoxin], suggesting that the presence of PPs in the medium may be largely derived from cell lysis or some other passive means.

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