Extracellular organics from specific cultures of Heterosigma akashiwo (Raphidophyceae) irreversibly alter respiratory activity in mammalian cells

Raphidophyte blooms have been well documented in several coastal areas around the world. Centring raphidophyte-bloom research has been a focus evolving around issues of ichthyotoxicity, allelopathy and anti-predatory activity. However, the details of these phenomena such as the identity of the compounds and the mechanisms underlying these processes are poorly understood. One such raphidophyte, Heterosigma akashiwo (Hada) Hara et Chihara, has historically received much attention with regard to its ichthyotoxic and allelopathic properties. In this study, we collected extracellular organic compounds from cultures of nine H. akashiwo isolates and tested those exudates on two mammalian cell lines: rat osteoblastic sarcoma (UMR-106) and human embryonic kidney (HEK-293). A tetrazolium colourimetric assay was used to determine the activity of mitochondrial dehydrogenases. Exposure of the mammalian cell lines to exudates collected from cultures of H. akashiwo (strain 764) significantly increased activity in a concentration- and time-dependent manner. Exudate concentrations of as little as 0.3 mg ml⁻¹ elicited a stimulatory response in the mammalian cells. This is comparable to the range of exudate concentrations that were originally in the algal cultures (>0.1 mg ml⁻¹). Significant increases in activity were observed 12–24 h following continuous or 1 h (transient) exposure to the exudate. Production of the stimulatory bioactive exudate was not altered by nutrient-stressed H. akashiwo cultures (reduced iron, phosphate or nitrate). Collectively, these bioactive compound(s) consistently increased cellular activity 3–15-fold. Interestingly, of the nine isolates tested, four of them produced the stimulatory exudate, whereas four others did not produce the stimulatory compound(s) and isolate 560R produced a compound(s) that was inhibitory in nature. Thus, we have shown that cultures of H. akashiwo produce organic compounds that can alter the metabolic activity of mammalian cells. Future isolation and characterization of these bioactive compounds may determine them to have ecological relevance, potentially involved in the ichthyotoxic, allelopathic and/or anti-predatory behaviour of this alga.