Marine sponges harbor many symbionts, which constitute a large amount of the sponge's dry weight. The reef sponges Agelas sp. produce a brominated pyrrole alkaloid, Ageladine A, which accumulates in acidic cellular compartments and has previously been used as a pH sensitive, fluorescent dye. Ageladine A may therefore accumulate in the acidic thylakoid lumen of photosynthetic cells. The ecophysiological role of Ageladine A in the sponge biome, however, is still unresolved. Here we show that Ageladine A possibly acts as an additional light harvesting molecule for photosynthesis in Synechococcus bacillaris, a cyanobacteria similar to the sponge's symbionts. A joint modeling and experimental approach revealed that Ageladine A facilitates photosynthesis of Synechococcus, when the cells are exposed to UV light of 370-380nm wavelength. While Ageladine A absorbs in the UV range, it fluoresces blue, matching the blue absorbance of chlorophyll a. In the presence of Ageladine A, photosynthetic production of O2 increased 2.54- and 3.1-fold, in the experiments and the model, respectively. This study presents a type of symbiosis, in which light is the major currency of species interaction. To verify the applicability of these results to other close relatives of sponge symbionts, two additional strains of Synechococcus sp., and two diatom species, are being tested. The current experiments differ from the initial ones, however, since UV-A radiation of a wider spectrum is used. In addition to these experiments, the applicability of the principle to a wide variety of marine and freshwater algae will be assessed.