MIXOTROPHY IN THE DINOFLAGELLATE PROROCENTRUM-MINIMUM

Prorocentrum minimum (formerly also known as P. mariae-lebouriae) is a common bloom-forming, photosynthetic dinoflagellate in Chesapeake Bay , USA. II is also capable of ingesting other cells. In Chesapeake Bay, P, minimum usually co-occurs with cryptophytes. Ingested cryptophyte material is observable in the dinoflagellate under an epifluorescent m icroscope as orange-fluorescent inclusions (OFI). During April and May , the frequency of OFI was less than or equal to 10% in both surface a nd pycnocline assemblages. In summer, up to 50% of the P, minimum cont ained OFI. The frequency of OFI was positively correlated with cryptop hyte abundance, but OFI were not frequent in all populations of P. min imum when cryptophyte densities were high. On-deck experimental incuba tions were done to determine the conditions that influence feeding. Li ght level and inorganic nutrient availability over the previous 24 h a ffect feeding. Incidence of feeding is lower when populations are main tained in the dark for 24 h than on a natural Light:dark cycle. Additi on of nitrate and phosphate together can inhibit feeding. Ingestion ha s a diel pattern, with frequency of OFI highest in the afternoon and e vening and lowest in the morning. Feeding is influenced by a complex o f factors, but the spatial-temporal pattern of ingestion and the exper iments both suggest that feeding is primarily a mechanism for obtainin g limiting inorganic nutrients rather than a mechanism for supplementi ng carbon nutrition during light limitation. Ingestion of other protis ts, including competitors for light and nutrients, may be an important strategy which allows bloom-forming dinoflagellates to dominate plank ton assemblages for extended periods and during changing nutrient regi mes.