Physiological responses and sexual compatibility within a massive toxic bloom of Alexandrium minutum

In summer 2018, a massive bloom of Alexandrium minutum, a causative agent of paralytic shellfish poisoning, occurred in the Ría de Vigo (NW Spain) reaching a maximum density of 745,470 cells L^-1. This study assessed the bloom's physiological state, by analyzing its cell abundance, cell size, toxin content and DNA content, as well as how those cell parameters evolved in time. While toxicity decreased over time, cell size remained constant. DNA content analysis revealed a scarcity of sexual stages and a decline in mitotic growth rate between the two dates. Nine months later, 39 sediment samples were analyzed for resting cyst deposition, showing highly variable distribution. The highest cyst concentrations were at the bloom initiation site and the farthest sampling point. Generally low cyst deposition values aligned with the low number of sexual stages detected in the bloom samples analyzed. To estimate the intrinsic potential for sexuality of the bloom, i.e., that shown under conditions specifically designed to induce cyst formation 43 strains from different bloom locations were isolated and mixed in all possible combinations of pairs (945 crosses). Sexual reproduction occurred in 86.46 % of the crosses, but with low compatibility indices (CI = 0.10-0.88, AV = 1.50-2.55, RC = 0.14-2.12). The low intrinsic potential for sexuality of the bloom, was coincident with all previous sexual aspects analyzed in the bloom subsamples, an aspect which may relate to a bloom with low genetic variability. This study provides insights into the dynamics of sexual reproduction and physiological changes in harmful dinoflagellate blooms, contributing to understanding bloom persistence and future risks.