Differential antibacterial activities of fusiform and oval morphotypes of Phaeodactylum tricornutum (Bacillariophyceae)

The diatom, Phaeodactylum tricornutum is a common inhabitant of inshore waters and can exist in different morphotypes that are thought to be adapted for survival in different habitats. Despite this diatom being widely used for physiological and genetic studies of microalgae, little is known about biochemical or physiological differences between the cell morphotypes. The present study was aimed at comparing differences in the antibacterial properties of the fusiform and oval morphotypes, the dominant cell types found in laboratory cultures of most strains of P. tricornutum. In cultures differing in proportions of fusiform and oval cells, there is a significant and positive correlation between the proportion of cells in the fusiform morphotype and the antibacterial activity of cell extracts. Extracts prepared from cultures enriched for fusiform cells ( 76%) show greater antibacterial activity against the Gram-positive bacterium, Staphylococcus aureus, than those prepared from pure (100%) oval cultures. Thus fusiform cells contain greater antibacterial activity per cell compared to the ovals. Gas–liquid chromatographic analyses of the extracts reveal that those from enriched fusiform populations contain significantly greater levels of the free fatty acids, eicosapentaenoic acid (EPA), hexadecatrienoic acid (HTA) and palmitoleic acid (PA) than the pure oval cell cultures. These free fatty acids from P. tricornutum have been previously shown by us to have potent antibacterial activity against S. aureus. Free fatty acids, released from damaged microalgal cells, defend the microalgal population against grazing predators but, here, we suggest that these free fatty acids could also act against pathogenic bacteria in the vicinity of the algae. As cell extracts from the fusiform cells contain greater quantities of these fatty acids, fusiform cells may have greater potential than the ovals for this type of protection.