22. The Effects of Ocean Acidification on Growth, Photosynthesis, and Domoic Acid Production by the Toxigenic Diatom Pseudo-nitzschia australis

Charles J. Wingert (1)*, Christopher E. Ikeda (1), Brian D. Bill (2), Vera L. Trainer (2), William P. Cochlan (1)

1 Romberg Tiburon Center for Environmental Studies, San Francisco State University, 3152 Paradise Drive, Tiburon, California, USA 94920
2 Marine Biotoxins Program, Northwest Fisheries Science Center, National Oceanic and Atmospheric Administration, 2725 Montlake Blvd. E., Seattle, WA 98112, USA

Background
Pseudo-nitzschia australis is one of the most problematic toxigenic diatoms on the west coast of North America. It is capable of producing the potent neurotoxin domoic acid (DA), responsible for amnesic shellfish poisoning events impacting both mammals and birds. Blooms of P. australis are common in the nutrient-replete coastal waters of Californian eastern boundary upwelling systems (EBUS), where increased partial pressure of CO2 (pCO2) and decreased seawater pH are common. This is the first study to investigate the potential impacts of ocean acidification (OA) on this diatom.

Methods
This laboratory study examined the effects of OA on growth, photosynthesis, and toxicity of P. australis using semi-continuous, nutrient-replete batch cultures at current (pH=8.1) and elevated (pH=7.8) CO2 concentrations using direct injection of compressed CO2/air mixture into culture flasks. Growth rates were determined by daily measurements of in vivo fluorescence and cell density. Photosynthetic productivity was assessed by incorporation of 14C isotope as a function of PPFD using photosynthetrons. Dissolved and cellular DA concentrations were determined during both the nutrient-replete exponential growth phase and the Si-deplete stationary growth phase using an indirect, competitive enzyme-linked immunosorbent assay (cELISA).

Findings
This research is ongoing; initial results demonstrate that the exponential growth rates of P. australis grown at reduced pH (7.8) are on average 11% lower than cultures grown at present-day pH (8.1). Since these experiments are being conducted over many weeks, they will also demonstrate the effects of acclimation to altered pCO2, and whether growth, photosynthetic and toxicity responses vary in short-term versus long-term cultures.

Conclusions
Preliminary results suggest P. australis grows slower at reduced pH, in contrast to published studies conduced for P. multiseries and P. fraudulenta. This laboratory study is expected to provide valuable insight into the effects of OA on this ecologically significant diatom commonly found in EBUS.