Diarrhetic shellfish toxins production appears to be driven by photosynthesis and phosphate–revealed by different light-adapted strains of Prorocentrum lima complex and P. caipirginum

Introduction Diarrhetic shellfish toxins (DST) harm shellfish aquaculture and endanger human health, and include well-known marine dinoflagellate-produced toxins such as okadaic acid (OA) and analogues, such as dinophysistoxin-1 (DTX-1). Toxin-producing species have different toxin profiles and contents, with unclear interactions, with the toxins hypothesized to be produced under stress conditions. Methods Five morphotypes of Prorocentrum lima complex, a well-studied DST-producing species with remarkable phylogenetic variability, were chosen and exposed to three distinct light conditions (photosynthetic active radiation, PAR; PAR+UVA; PAR+UVA and UVB) for 18 or 24 days. Results and discussion The studied morphotypes were classified as either light-sensitive (LS) or light-tolerant (LT) types based on their light responses and varying abilities to produce DTX-1 across three orders of magnitude (0.001-1 pg cell -1 d -1 , abbreviated as LL, ML, and HL in order of rank). All toxin production rates ( R tox ) initially increased and then decreased, with the first peaks varying between days 3 to 12. The results of earlier peaks in LL and ML and a linear correlation between R tox values for OA and DTX-1 in HL indicated that the two compounds may be competitive. When light conditions initially altered, R tox was either stagnant or negative under all light treatments in the LS cohort, and with UV addition in LT, which subsequently delayed the early peaks. The R tox data for DTX-1 all demonstrated earlier and higher peaks with UVA addition in LL and ML. Likewise, their growth was facilitated following the addition of UVA light. Therefore, R tox of pre-peaks is relevant to photosynthetic status and photoprotection ability. As decreasing R tox data points corresponded closely with phosphate depletion, the phosphate consumption rate was calculated and shown to be linearly or exponentially associated with all downward R tox . This study proposes a supply-demand link between photosynthetic products and phosphate with DST biosynthesis, inferring a likely competitive interaction between OA and DTX-1 production.