[Department of Ocean Science] PhD Thesis Defense Seminar: Responses of Marine Protozoa to Ocean Alkalinity Enhancement in the Changing Ocean

Abstract:

Ocean alkalinity enhancement (OAE) is proposed to be a promising approach for atmospheric CO2 removal and climate change mitigation, yet its ecological impacts on marine protozoa remain poorly understood. This thesis investigates the physiological and transcriptomic responses of key marine heterotrophic protists, including nanoflagellates and ciliates, to OAE across a series of experiments, aiming to provide scientific evidence to evaluate the biological risks associated with its large-scale deployment in a changing ocean.

Acute exposure and acclimation experiments on heterotrophic nanoflagellates (Cafeteria burkhardae and Paraphysomonas longispina) and ciliates (Uronema marinum and Pseudocohnilembus persalinus) revealed significant physiological stress under acute OAE conditions, including reduced growth rates and enhanced ingestion. Following acclimation, species-specific tolerances emerged, with some species demonstrating resilience at low OAE levels while others suffered from chronic oxidative stress, metabolic disruption, and reduced growth efficiency. Underlying transcriptomic mechanisms further revealed divergent acclimation strategies, including inhibition of replication and metabolic pathways in sensitive species and upregulation of stress-response and translation-related pathways in more tolerant ones, reflecting fundamentally different cellular responses to alkalinization stress.

Since OAE should usually be conducted in subtropical and tropical oceans where seawater is a carbon source, understanding its biological consequences under concurrent thermal stress is critical. Integrating OAE with a broad temperature range across five heterotrophic protozoan species revealed that OAE significantly shifts thermal performance curves upward and toward higher temperatures, increasing both optimal temperatures and maximal rates of growth and ingestion while reducing oxidative stress under warming conditions. These shifts indicate that OAE may enhance protozoan thermal resilience, extending their functional thermal niche under future climate scenarios. Collectively, these findings demonstrate that OAE can elicit complex, species-specific biological responses that may restructure marine protist communities, alter microbial food web dynamics, and influence carbon and nutrient cycling. The findings highlight the importance of incorporating microbial responses into ecological risk assessments and biogeochemical models to predict the outcomes of OAE deployment.