Substantial microbial community shifts in response to an exceptional harmful algal bloom in coastal Southern California

Phytoplankton blooms create organic matter that stimulates entire marine ecosystems, including other components of the microbial community. How the ecosystem responds varies depending on the intensity, duration, and composition of the bloom. When the bloom has a direct or indirect negative impact on the ecosystem, it is termed a harmful algal bloom (HAB). HAB frequency is expected to increase in response to changing oceanic conditions and coastal nutrient supply. Characterizing the response of the bacterial and archaeal communities to HABs will improve our understanding of the ecological impacts of these phenomena. We utilized time series of chlorophyll a, phaeophytin, dissolved oxygen, flow cytometry cell counts, and microbial community structure (assessed via 16S rRNA gene sequences) maintained by several observing programs to investigate how the microbial community was affected by an exceptional bloom of Lingulodinium polyedra in coastal Southern California. These multi-year datasets allowed us to compare the microbial community response to past events, such as a smaller L. polyedra bloom the previous year. We demonstrated that the bacterial and archaeal response to the 2020 bloom was unique taxonomically, with many novel heterotrophs, and higher trophic state variance. The measured heterotrophic response to the bloom resulted in massive oxygen drawdown and may have impacted the length of the bloom and contributed to a secondary diatom bloom following the main HAB event. Taken together, these data illustrate how the massive 2020 L. polyedra bloom created unique ecological conditions for coastal Southern California.