Understanding the Role of Phycotoxins in the Biological Carbon Pump of the Southern California Bight

Anomalously warm conditions associated with climate change are causing increases in harmful algal bloom events in the California Current. Biogeochemical consequences are not fully understood; however, preliminary studies concluded that abundant phycotoxin domoic acid has allelopathic effects on other microbes, providing the diatoms responsible for its production with a competitive advantage and potentially altering the flow of carbon through the microbial loop. Phycotoxins likely influence carbon flow in the ecosystem by changing predator-prey interactions, bioaccumulating in and interrupting a significant population of higher organisms, and impacting subsurface heterotrophic communities associated with marine snow. Using a novel UPLC/MS method to quantify marine phycotoxins, the seasonal succession of HABs in the Southern California Bight was observed during a 6-month time series that sampled 6 depths from 0 to 890 m, February to July 2019. A trade-off between domoic acid and okadaic acid was observed. To understand the biogeochemical impact of these phycotoxins, a series of amendment experiments were performed exposing representative microbial isolates to varying concentrations of phycotoxins observed in the field. The growth of non-toxic diatoms was inhibited at domoic acid concentrations of 4000 ng/L and similar results were observed for okadaic acid experiments. Heterotrophic bacteria, cyanobacteria, and dinoflagellates were more tolerant to these phycotoxins. Transcriptomics will be used to determine phycotoxin driven changes in microbial gene expression associated with nutrient uptake, stress response, and organic matter degradation. Future iterations of this work will compare field observations with laboratory findings to form more conclusive hypotheses. However, these results do suggest HABs that we can expect to become more frequent with a changing climate will shift the microbial community structure and thus carbon flow through coastal ecosystems.