Resilience of a Cosmopolitan Diatom to Induced Ocean Acidification Based on C, N, Si and Chlorophyll-a Stoichiometric Ratios, Nutrient Uptake Rates, and Morphology

Diatoms contribute to approximately 20% of global primary production and are therefore a driving force in the biological carbon and silica pump in the oceans. They also form the base of efficient food webs and can produce compounds harmful to other organisms. Despite their biogeochemical and ecological importance, little is known about the effects of ocean acidification on the physiology and morphology of marine diatoms. We performed manipulation experiments by exposing unialgal cultures of Thalassiosira rotula to current and elevated atmospheric CO₂ concentrations of 410 ppm and 1000 ppm, respectively. Cultures were maintained at exponential growth under optimal light and nutrient conditions, and allowed to acclimate to the experimental treatment over ten generations. These experiments investigated the effects of the decrease in pH at high pCO₂ on growth rate, biomass, and ratios of particulate carbon, nitrogen, silica and chlorophyll-a. Uptake rates of C, N, and Si were estimated from the disappearance of dissolved nutrient concentrations in the media, and measured directly from the incorporation of stable (¹³C and ¹⁵N) and radioactive (³²Si) isotopes. Morphological measurements were made to determine surface area-to-volume ratios, while optical and SEM microscopy were used to assess the integrity of the silica frustule. Results from this project will shed new light on the effects of ocean acidification on diatom silicification and other growth parameters in a cosmopolitan centric diatom.