The effects of elevated CO₂ on the growth and toxicity of field populations and cultures of the saxitoxin-producing dinoflagellate, <i>Alexandrium fundyense</i>

Abstract

The effects of coastal acidification on the growth and toxicity of the saxitoxin-producing dinoflagellate <i>Alexandrium fundyense</i> were examined in culture and ecosystem studies. In culture experiments, <i>Alexandrium</i> strains isolated from Northport Bay NY, USA, and the Bay of Fundy, Canada, grew significantly faster (16 -190%; <i>p</i><0.05) when exposed to elevated levels of pCO₂ (~ 800- 1900μatm) compared to lower levels (~390μatm). Exposure to higher levels of pCO2 also resulted in significant increases (71 - 81%) in total cellular toxicity (fg STX eq. cell^-1) in the Northport Bay strain, while no changes in toxicity were detected in the Bay of Fundy strain. The positive relationship between pCO₂ enhancement and elevated growth was reproducible using natural populations from Northport; <i>Alexandrium</i> densities were significantly and consistently enhanced when natural populations were incubated at 1500 μatm pCO₂, a value at the upper range of those recorded in Northport Bay, 390 - 1500 µatm. During natural <i>Alexandrium</i> blooms in Northport Bay, pCO₂ concentrations increased over the course of a bloom to more than 1700μatm and were highest in regions with the greatest <i>Alexandrium</i> abundances, suggesting <i>Alexandrium</i> may be further exacerbating acidification or be especially adapted to these extreme, acidified conditions. The co-occurrence of <i>Alexandrium</i> blooms and elevated pCO₂ represents a previously unrecognized, compounding environmental threat to coastal ecosystems. The ability of elevated pCO₂ to enhance the growth and toxicity of <i>Alexandrium</i> indicates that acidification promoted by eutrophication or climate change can intensify these, and perhaps other, harmful algal blooms.