Non-linear Physiology and Gene Expression Responses of Harmful Alga Heterosigma akashiwo to Ocean Acidification

Heterosigma akashiwo is a raphidophyte known for forming ichthyotoxic blooms. In order to predict the potential impacts of rising CO₂ on H. akashiwo it is necessary to understand the factors influencing growth rates over a range of CO₂ concentrations. Here we examined the physiology and gene expression response of H. akashiwo to concentrations from 200 to 1000 ppm CO₂. Growth rate data were combined from this and previous studies and fit with a CO₂ limitation-inhibition model that revealed an apparent growth optimum around 600-800 ppm CO₂. Physiological changes included a significant increase in C:N ratio at ~800 ppm CO₂ and a significant decrease in hydrogen peroxide concentration at ~1000 ppm. Whole transcriptome sequencing of H. akashiwo revealed sharp distinctions in metabolic pathway gene expression between ~600 and ~800 ppm CO₂. Hierarchical clustering by co-expression identified groups of genes with significant correlations to CO₂ and growth rate. Genes with significant differential expression with CO₂ included carbon concentrating mechanism genes such as beta-carbonic anhydrases and a bicarbonate transporter, which may underpin shifts in physiology. Genes involved in cell motility were significantly changed by both elevated CO₂ and growth rate, suggesting that future ocean conditions could modify swimming behavior in this species.