Mixotrophy in the dineflagellate Prorocentrum minimum

Abstract

Prorocentrum minimum (formerly also known as P. mariae-lebouriae) is a common bloomforming, photosynthetic dineflagellate in Chesapeake Bay, USA. It is also capable of ingesting other cells. In Chesapeake Bay, P. minimum usually co-occurs with cryptophytes. Ingested cryptophyte material is observable in the dineflagellate under an epifluorescent microscope as orange-fluorescent inclusions (OFI). During April and May, the frequency of OFI was ≤ 10% in both surface and pycnocline assemblages. In summer, up to 50% of the P. minimum contained OFI. The frequency of OFI was positively correlated with cryptophyte abundance, but OFI were not frequent in all populations of P. minimum when cryptophyte densities were high. On-deck experimental incubations were done to determine the conditions that influence feeding. Light level and inorganic nutrient availability over the previous 24 h affect feeding. Incidence of feeding is lower when populations are maintained in the dark for 24 h than on a natural light:dark cycle. Addition of nitrate and phosphate together can inhibit feeding ingestion has a diel pattern, with frequency of OFI highest in the afternoon and evening and lowest in the morning. Feeding is influenced by a complex of factors, but the spatial-temporal pattern of ingestion and the experiments both suggest that feeding is primarily a mechanism for obtaining limiting inorganic nutrients rather than a mechanism for supplementing carbon nutrition during light limitation. Ingestion of other protists, including competitors for light and nutrients, may be an important strategy which allows bloom-forming dinoflagellates to dominate plankton assemblages for extended periods and during changing nutrient regimes.