Reduction in CO2 uptake rates of red tide dinoflagellates due to mixotrophy

We investigated a possible reduction in CO2 uptake rate by phototrophic red tide dinoflagellates arising from mixotrophy. We measured the daily ingestion rates of Prorocentrum minimum by Prorocentrum micans over 5 days in 10 L experimentalbottles, and the uptake rates of total dissolved inorganic carbon (CT) by a mixture of P. micans and P. minimum(mixotrophic growth), and for the predator P. micans (phototrophic growth; control) and prey P. minimum (phototrophicgrowth; control) alone. To account for the effect of pH on the phototrophic growth rates of P. micans and P. minimum,measurements of CT and pH in the predator and prey control bottles were continued until the pH reached the same level(pH 9.5) as that in the experimental bottles on the final day of incubation. The measured total CT uptake rate by the mixtureof P. micans and P. minimum changed from 123 to 161 μmol CT kg-1 d-1 over the course of the experiment, and waslower than the CT uptake rates shown by P. micans and P. minimum in the predator and prey control bottles, respectively,which changed from 132 to 176 μmol CT kg-1 d-1 over the course of the experiment. The reduction in total CT uptake ratearising from the mixotrophy of P. micans was 7-31% of the daily CT uptake rate seen during photosynthesis. The resultssuggest that red tide dinoflagellates take up less CT during mixotrophy.1131Ysciescopuskc