The Effects of Ocean Acidification and Eutrophication on the Growth, Lipid Composition and Toxicity of the Marine Raphidophyte Heterosigma Akashiwo.

Anthropogenic forcing, such as ocean acidification caused by rising carbon dioxide emissions, and eutrophication due to increased nutrient loadings in run-off, are causing major changes to the biogeochemistry of the oceans. As a consequence, coastal phytoplankton are susceptible to altered biogeochemical environments. This study examined the effect of a lower pH and increased levels of nutrients on the common coastal harmful alga, Heterosigma akashiwo. Growth rates, maximal cell yields, neutral lipid accumulation and toxicity of cells grown under various pH and nutrients regimes were measured. H. akashiwo growth was near maximal when grown at lower pH levels. There was a strong correlation between macronutrient concentration (nitrogen and phosphorus) and physiological responses such as cell yield, toxicity, and neutral lipid accumulation. Cells cultured on ammonium were less toxic that cells supplied with either nitrate or urea as a nitrogen source. Neutral lipid accumulation and cell toxicity varied under different environmental regimes but did not co-vary, indicating that polyunsaturated fatty acid production was not the mechanism of toxicity. Based on the ecophysiological profile, H. akashiwo will be both present and toxic in the future nutrient-rich, acidified coastal ocean waters.