P2.109 Saturday, Jan. 5 Reduced Irradiance Alters Cyanobacterial Symbiont Abundance and Growth Rate of three Tropical Sponges MATTERSON, K.M.*; THACKER, R.T.; FREEMAN, C.J.; University of Alabama at Birmingham; University of Alabama at Birmingham; University of Alabama at Birmingham Kenanm@uab.edu
Tropical sponges can host cyanobacterial symbionts that supply essential, photosynthetically fixed nutrients. In such species, symbioses with the sponge-specific cyanobacterium Synechococcus spongiarium are of particular interest, especially since this taxon is composed of at least 12 genetically distinct clades that may differ in their benefit to a host sponge. To determine if this genetic diversity translates to variation in host benefit, we coupled manipulative shading experiments with light-dark bottle incubations to determine the effects of reduced irradiance on photosymbiont populations and host sponges. We carried out these experiments using three common Caribbean sponges that each host at least one unique clade of S. spongiarium: Verongula rigida, Neopetrosia proxima and Ircinia felix. Photosymbiont abundance and growth rates varied substantially across species. I. felix exhibited a 20% reduction in growth under shaded conditions, while V. rigida varied little between control and shaded treatments. Interestingly, N. proxima had positive growth rates under shade, but these growth rates were significantly less than those of control treatments, suggesting a method of compensation for reduced light. Respiration and productivity (P:R) values from light-dark bottle incubations of sponges used in these shading experiments also varied across species. We observed no significant differences in P:R ratios in V. rigida and N. promixa between shaded and control treatments. However, I. felix exhibited significantly higher oxygen production in controls. Combined, these results support the hypothesis that symbioses between sponge hosts and cyanobacterial symbionts are highly variably across host species, spanning a range of interactions from commensalisms to facultative and obligate mutualisms.