Algicidal Bacteria in the Sea and their Impact on Algal Blooms1

Over the past two decades, many reports have revealed the existence of bacteria capable of killing phytoplankton. These algicidal bacteria sometimes increase in abundance concurrently with the decline of algal blooms, suggesting that they may affect algal bloom dynamics. Here, we synthesize the existing knowledge on algicidal bacteria interactions with marine eukaryotic microalgae. We discuss the effectiveness of the current methods to characterize the algicidal phenotype in an ecosystem context. We briefly consider the literature on the phylogenetic identification of algicidal bacteria, their interaction with their algal prey, the characterization of algicidal molecules, and the enumeration of algicidal bacteria during algal blooms. We conclude that, due to limitations of current methods, the evidence for algicidal bacteria causing algal bloom decline is circumstantial. New methods and an ecosystem approach are needed to test hypotheses on the impact of algicidal bacteria in algal bloom dynamics. This will require enlarging the scope of inquiry from its current focus on the potential utility of algicidal bacteria in the control of harmful algal blooms. We suggest conceptualizing bacterial algicidy within the general problem of bacterial regulation of algal community structure in the ocean.

Algicidal activity and gliding motility of Saprospira sp. SS98-5

A marine bacterium, Saprospira sp. SS98-5, which was isolated from Kagoshima Bay, Japan, was able to kill and lyse the cells of the diatom Chaetoceros ceratosporum. The multicellular filamentous cells of this bacterium captured the diatom cells, formed cell aggregates, and lysed them in an enriched sea water (ESS) liquid medium. Strain SS98-5 also formed plaques on double layer agar plates incorporating diatom cells. The diatom cell walls were partially degraded at the contact sites with the bacteria, the bacteria invaded from there into the diatom cells, and then the diatom cells were completely lysed. The strain possessed gliding motility and grew as spreading colonies on ESS agar plates containing lower concentrations of polypeptone (below 0.1%) while forming nonspreading colonies on ESS agar plates containing 0.5% polypeptone. Electron micrographs of ultrathin sections demonstrated that microtubule-like structures were observable only in gliding motile cells. Both the gliding motility and the microtubule-like structures were diminished by the addition of podophyllotoxin, an inhibitor of microtubule assembly, suggesting that the microtubule-like structures observed in these bacterial cells are related to their gliding motility.