The effect of light intensity on the colonization potential of Phaeobacter gallaeciensis on Ulva ohnoi has not yet been investigated. The project aimed to evaluate the effect of light on the colonization and maintenance of P. gallaeciensis on U. ohnoi and the interaction between Phaeobacter and Ulva in the context of IMTA-RAS. It also aimed at investigating DNA-based quantification methods to select the best one to quantify P. gallaeciensis. A small-scale Ulva culture was set up in plastic multi-well plates where Ulva disks were cultured for 3 weeks. Cultures were acclimated to low light for the first week. During the second week, Phaeobacter was introduced to Ulva cultures under three different light intensities (high, low, dark). After the colonization period, all the plates were exposed to high light during the third week. A real-time PCR method was assessed and compared to the culture-dependent method for the quantification of viable P. gallaeciensis. The results showed better colonization by Phaeobacter under dark conditions and suggested a negative effect of light on the maintenance of P. gallaeciensis on U. ohnoi. Light intensity seemed to play an important role in the interaction between Ulva and Phaeobacter as maintenance of Phaeobacter biofilm negatively correlated with Ulva growth. The real-time PCR overestimated the number of viable bacteria and needs to be optimized for future experiments.
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