*3.1. Sponge-Associated Strains and Growth Conditions*

*Leptothoe kymatousa* TAU-MAC 1615 and *Le. spongobia* TAU-MAC 1115 were previously isolated from the marine sponges *Chondrilla nucula* and *Acanthella acuta*, accordingly, from a rocky sublittoral zone of the North Aegean Sea [21,24]. The strains were purified, and an axenic culture was obtained only for *Le. kymatousa* TAU-MAC 1615 due to difficulties in producing pure cultures stemming from tightly associated heterotrophic bacteria. Axenic and mono-clonal cultures were grown in MN medium [61] for 20–25 days at 20–25 ◦C, at a photo irradiance of 8–15 μmol photons m−<sup>2</sup> s−1. These strains are maintained in the Microalgae and Cyanobacteria Collection (TAU-MAC) of the Aristotle University of Thessaloniki [62].
