Bacterial community dynamics during a harmful algal bloom of Heterosigma akashiwo

Phytoplankton bloom events result in distinct changes in the composition and availability of nutrients as well as physical conditions within aquatic ecosystems, resulting in significant effects on bacterial communities. Using a metabarcoding approach, this study investigated the effect of a harmful algal bloom (HAB) of Heterosigma akashiwo in the Sundays Estuary, South Africa, on bacterial community structures in this estuarine ecosystem over an entire bloom event. The occurrence of bacterial lineages occurring in both the oxygen-rich surface water and hypoxic bottom waters in this study reflects the respiratory flexibility and potential for both aerobic and anaerobic metabolism across a wide range of bacterial phylogenetic lineages. A close correlation between the bacterial community profiles and the physiological state of the bloom was observed, with Flavobacteria found in increased relative abundances during the H. akashiwo HAB. Flavobacteria are commonly reported in the literature associated with algal blooms, which is indicative of their central role in the degradation of algal-derived organic matter. Halieaceae were prevalent during the bloom, whilst Synechococcales, Cryomorphaceae and Sporichthyaceae were found to be positively correlated with the decay of the H. akashiwo bloom. Rhodobacteraceae correlated significantly with the H. akashiwo bloom; however, unlike the Rhodobacteraceae specific bloom-associated genera reported in literature (predominantly Roseobacter), up to 74% of the Rhodobacteraceae sequence reads in this study were assigned to the genus Litorimicrobium. The distinct bacterial community profiles recorded throughout the H. akashiwo HAB can be attributed to the influence of the bloom-forming species as well as under-representation of estuarine-occurring HABs in the literature.