Marine microzooplankton are indirectly affected by ocean acidification through direct effects on their phytoplankton prey

To date there is little evidence to suggest that marine microzooplankton are directly affected by ocean acidification (OA), and few studies have explored indirect effects of OA on microzooplankton. Microzooplankton grazing behavior is acutely sensitive to prey cell size, physiology, and nutritional state, which may all be influenced by OA in phytoplankton. Therefore, microzooplankton may be indirectly affected by OA through their prey. Due to undersaturation of CO2 for the carboxylating enzyme, RuBisCO, increasing availability of CO2 through acidification could influence phytoplankton cellular energy budgets. This could, in turn, affect algal cellular processes, physiological states, and the nutritional value for their primary consumers, the microzooplankton. In this study I tested whether there are indirect effects of ocean acidification on three microzooplankton species, representing two ecologically significant functional groups of microzooplankton, tintinnid ciliates (Favella taraikaensis and Eutintinnus sp.) and heterotrophic dinoflagellates (Oxyrrhis marina). To achieve this I first characterized the direct effects of OA on the physiology and biochemistry of their phytoplankton prey. Two phytoplankton, Emiliania huxleyi CCMP 2668 and Rhodomonas sp. CCMP 755, were cultured semi-continuously under three pCO2 treatments in media equilibrated at 400ppmv, 750ppmv, and 1000ppmv pCO2 (Ambient, Moderate, and High, respectively). After acclimation for 10 days, I quantitatively assessed cell size, C:N, growth rate, photosynthetic capacity, cellular carbohydrate and chlorophyll a concentrations. Phytoplankton cell size increased significantly under Moderate and High treatments, while there were no consistent changes in phytoplankton biochemistry with elevated pCO2. To test for indirect effects on microzooplankton, the grazers were fed a diet of pCO2-acclimated cells for durations of time ranging from minutes to hours. Epifluorescent microscopy was used to quantify ingestion at predetermined time intervals. All three microzooplankton species showed increased short-term ingestion rates, andhad a higher percentage of the population feeding on E. huxleyi grown in elevated pCO2. Multiple Linear Regression models for each grazer revealed that increased prey cell size was the sole predictive factor for the increased short-term grazing rates. I showed that OA indirectly affects microzooplankton through direct effects on the size of their phytoplankton prey. This indirect pathway for OA effects to microzooplankton has the potential for widespread impacts within marine food webs.

Kendall K., 2015. Marine microzooplankton are indirectly affected by ocean acidification through direct effects on their phytoplankton prey. MSc thesis, Western Washington University, 115 pp. Thesis.

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