Mixotrophy in Marine Planktonic Ciliates

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This study addresses the trophic position and productivity role of chlorophyll retaining ciliates in the ocean. In order to understand the trophic position of planktonic oligotrichous ciliates, factors that determine the ciliates' dependence on both photosynthetic and phagotrophic modes of nutrition will be studied in a limited number of laboratory experiments. The major focus will be on documenting the incidence of chloroplast retention among ciliates in shelf, slope, and oceanic waters, and compare it to existing data on its incidence in neritic waters. Productivity studies based on radiolabeled carbon will be used to estimate the contribution of planktonic ciliates to primary production. Ciliates are a type of protozoan that are common in aquatic habitats. They are considered to be part of the surface water fauna and to derive their food requirements from consuming other planktonic organisms. However, previous work by Stoecker has shown that about 40% of the oligotrichous ciliates in coastal waters retain chloroplasts derived from ingested algae. Many of these ciliates therefore, derive their nutrition from both photosynthesis (from the retained algal chloroplasts), as well as from particle ingestion, and blur the usual distinction between plant and animal nutrition. This work will investigate the factors that control the relative contributions of these two feeding modes to the nutrition of the ciliate, and determine how extensive this unconventional feeding mode is in other parts of the ocean.