The balance of autotrophy and heterotrophy during mixotrophic growth of Karlodinium micrum (Dinophyceae)

We studied autotrophic and heterotrophic C metabolism during mixotrophic growth of Karlodinium micrum (leadbeter et Dodge) Larsen (Dinophyceae) on prey Storeatula major (Cryptophyceae). Our goal was to determine the balance of autotrophy and heterotrophy that supports mixotrophic growth in K micrum. Assimilation of inorganic C-14 and C-14-labeled prey was used to separate the quantify and quality, (i.e., lipid, polysaccharide and protein) of C obtained by autotrophy and heterotrophy, respectively. Growth rates (mu) of mixotrophic K micrum were 0.52-0.75 div. (.) day(-1), equal to or greater than the maximum autotrophic growth rate (0.55 div. (.) day(-)) of K. micrum. Autotrophy represented 27-69% of gross C uptake during mixotrophic growth. Cellular photosynthetic performance (PPcell pg C cell(-1) (.) day(-1)) was 24-52 % lower during mixotrophic growth than during autotrophic growth of K micrum. Mixotrophic K micrum assimilated 16% less photosynthate as protein compared to autotrophic K. micrum, while protein was the major net assimilation product (52 %) from ingested prey C. Growth efficiency (% GE) of mixotrophic cultures, based on both autotrophic and heterotrophic C sources, averaged 36 +/- 2.9%, slightly lower than the 40-50% GE typical of purey autotrophic K. micrum, but higher C ginns associated with heterotrophic feeding more than compensated for the decrease in %GE in mixotrophic K. micrum. We conclude that mixotrophic growth of K micrum is dominated by heterotrophic metabolism, although photosynthesis continues at a lowered rate. This is consistent with a shift toward secondary production in plankton assemblages dominated by mixotrophically growing K. micrum.