Agarophyton vermiculophyllum is an alga that is attacked by facultative bacterial pathogens and can attract (Saha et Weinberger 2019) protective bacteria. In order to better understand these interactions, several bioassays were carried out to determine the interactions between bacteria and A. vermiculophyllum. Only certain bacteria (G-NORD11, G-JI1, G-NY6) were confirmed to protect against the pathogens Kordia algicida and Pseudoalteromonas arctica. A certain density of P. arctica or G-NORD11 is required to induce or affect bleaching in A. vermiculophyllum. Whether to prevent or to induce bleaching there must be a minimum of bacterial cells present and bacterial exudate alone is not enough. P. arctica cells exhibited 16 times higher absorbance than G-NORD11 when present at the cell density. G-NORD11 was the only bacterium capable to inhibit the growth of P. arctica on solid medium, but its exudate did not show such an effect. The possible role of two quorum sensing mediators was also tested, but neither for N-butyl-homoserine lactone nor for 2H4Q confirmed. Contact between P. arctica and G-NORD11 cultures in transwell plates had no effect on cell densities or presence of living or dead cells
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