Induced Bacteriovory in a Declining Culture of the Mixotrophic Dinoflagellate Prorocentrum minimum (Pavillard) Schiller

Bacteriovory was reported previously in the dinoflagellate, Prorocentrum minimum, but it was unclear if this is constitutive or induced under certain conditions. We tested the hypothesis that phosphate deficiency, or cessation of autotrophic growth for other reasons, would induce bacteriovory in a culture of P. minimum that is harmful to shellfish. Phosphate-starved cells did not ingest fluorescently labeled bacteria and died. In stationary-phase, full-enrichment cultures, more than half of viable P. minimum cells showed declines in chlorophyll that was coincident with incorporation of fluorescently labeled bacteria. Declining populations of P. minimum increase in toxicity to suspension-feeding shellfish; this suggests a possible association between bacteriovory and toxicity. 1. Introduction Prorocentrum minimum (Pavillard) Schiller is a dinoflagellate with world wide distribution in coastal waters; blooms can have ecosystem-disrupting consequences [1]. Sporadic expression of toxicity to mammals [2] and to suspension-feeding molluscs [3–6] has been reported. Although toxicity in this species appears to be strain dependent [2], recently we demonstrated conclusively that senescent cultures of one strain are more bioactive against bivalves than actively growing cultures [7]. The chemical identities of toxic agents produced by this species are unknown, although the term “venerrupin” has been associated with it [1]. Prorocentrum minimum has been described as a mixotroph [8]. The species grows photosynthetically on inorganic nutrients with light as the sole energy source but has been shown to assimilate dissolved, organic nitrogen and carbon compounds [9]. Furthermore, phycoerythrin fluorescence within P. minimum cells growing together with cryptophytes, and incorporation of coincubated, fluorescently-labeled bacteria [10] indicate the potential for phagotrophy in this species. Previous observations in our laboratory of lowered chlorophyll content in declining cultures of Prorocentrum minimum (unpublished microscope and flow-cytometer observations) suggested loss of photosynthetic capacity, yet cultures remain viable for months after nearly all pigments have disappeared. Previous studies have shown that P. minimum can release a bacteriostatic compound when cultured under phosphate-depleted conditions [11], which we confirmed with the JA-98-01 strain of P. minimum used in the present study (data not shown). In an initial experiment, we found no evidence that fluorescently labeled bacteria were incorporated within cells of actively growing cultures of the