Lab experiments were performed to investigate algicidal activity in the 10 species of Kordia marine bacteria available in culture. Co-cultures of each Kordia strain with 54 distinct phytoplankton strains representing a large range of taxa (diatoms, dinoflagellates, chlorophytes, raphidophytes, haptophytes, and cyanobacteria) showed that all Kordia species are algicidal against at least 3 phytoplankton strains. Interestingly, the number and identity of sensitive phytoplankton strains differed among Kordia species and revealed that 6 of the 10 species had a broad spectrum of sensitive phytoplankton strains (between 18 and 31) while the 4 remaining Kordia affected a smaller range of phytoplankton strains (between 3 and 12). At least one strain from each phytoplankton taxa tested was sensitive to at least one Kordia species. These results show that overall, the Kordia genus is capable of negatively impacting a wide spectrum of phytoplankton taxa.
Further experiments were performed to decipher the mechanism of the algicidal attack. Co-cultures of a sensitive diatom strain with each of the 10 Kordia species showed conserved effects of the different Kordia on the diatom. These experiments showed a quick inhibition of the diatom growth followed by rapid depletion of the diatom chlorophyll content. Changes in diatom chlorophyll content were accompanied by decreased photosynthetic efficiency. These effects were also observed in the presence of Kordia culture filtrates instead of the Kordia themselves, demonstrating that the attack is mediated by extracellular release of one or several algicidal molecule(s). Interestingly, membrane integrity markers showed that no changes in the integrity of the diatom membrane were observed in these experiments, suggesting that, instead of cell lysis, a yet elusive mechanism is involved in chlorophyll a depletion. Finally, experiments where different Kordia preculture conditions were tested showed that no activation of the bacteria by phytoplankton cues were necessary to trigger the attack, but that Kordia growth rather than abundances was necessary to observe an efficient bacterial attack.
Overall, the results of this project suggest that, despite large genetic differences between the 10 Kordia species, a common algicidal mechanism involving the production of extracellular compounds that inhibit phytoplankton growth and photosynthesis and trigger chlorophyll depletion in otherwise intact phytoplankton cells is conserved in this marine bacterial genus. The ongoing analysis of Kordia genome expression during phytoplankton attack will further pinpoint the biosynthetic pathways involved in Kordia algicidal activity. Through these conserved algicidal features, the Kordia group can attack a large range of phytoplankton taxa, making this ubiquitous marine bacterial genus an excellent model to study the conditions and ecological consequences of bacterial algicidal activity in the ocean in subsequent research projects.
