Costs, benefits and characteristics of mixotrophy in marine oligotrichs

1. Oligotrich ciliates are an important part of most marine plankton commun ities. Mixotrophic (chloroplast-sequestering) oligotrichs, a common compone nt of marine oligotrich communities, obtain fixed carbon from both photosyn thesis as well as the ingestion of particulate food. Mixotrophy, in general , is often considered an adaptation permitting exploitation of food-poor en vironments. We examined the hypothesis that, among oligotrichs, mixotrophs may be at a disadvantage relative to heterotrophs in food-rich conditions i n a nutrient-enrichment experiment. We compared growth responses of mixotro phic and heterotrophic oligotrichs in natural communities from the N.W. Med iterranean Sea in microcosms with daily nutrient additions resulting in inc reases in nanoflagellates and Synechococcus populations. The results indica ted that both mixotrophic and heterotrophic oligotrichs respond to prey inc reases with rapid growth (mu = 1.2 d(-1)). 2. To examine the hypothesis that the proportion of mixotrophic to heterotr ophic oligotrichs changes with the trophic status of a system, increasing w ith oligotrophy, we examined data from a variety of marine systems. Across systems ranging in chlorophyll concentration from about 0.1 to 40 mug L-1, oligotrich cell concentrations are correlated with chlorophyll concentratio ns, and mixotrophs are a consistent component of oligotrich communities, av eraging about 30% of oligotrich cell numbers. 3. We discuss the costs, benefits and possible uses of mixotrophy in marine oligotrichs and suggest that mixotrophy in marine oligotrichs is not close ly linked to the exploitation of food-poor environments, but probably serve s a variety of purposes.