Phototrophic nutrition and symbiont diversity of two Caribbean sponge-cyanobacteria symbioses

Coral reef sponges frequently harbor symbiotic cyanobacteria that can potentially benefit their hosts by providing supplemental nutrition. Although nutritional mutualisms have been documented for Indo-Pacific sponges, previous investigations suggest that Caribbean sponges obtain no nutritional benefits from their symbionts. We conducted field-based shading experiments to determine the effects of reduced irradiance on symbiont abundance and host growth in the Caribbean coral reef sponges Aplysina fulva and Neopetrosia subtriangularis, both of which host the unicellular cyanobacterium Synechococcus spongiarum. For both host species, 6 wk of shading significantly reduced chlorophyll a concentrations to less than half of those in control sponges. Shaded A. fulva individuals exhibited less than half the growth of control sponges. For N. subtriangularis, no significant differences in growth rates were observed between shaded and control sponges. Measurements of gross photosynthesis to respiration (P:R) ratios predicted that both host sponges were heterotrophic at low irradiances (< 250 mu mol m(-2) s(-1)) and phototrophic at high irradiances (> 500 mu mol m(-2) s(-1)), with measurements obtained at high irradiances likely to be more indicative of natural conditions. Phylogenetic analyses of cyanobacterial internal transcribed spacer (ITS) ribosomal RNA gene sequences revealed 3 genetically distinct clades of S. spongiarum (designated A, B and C). Clade A symbionts were isolated exclusively from A. fulva, Clade B symbionts from both host sponge species and Clade C symbionts exclusively from N. subtriangularis. These findings demonstrate that Caribbean sponges associate with diverse clades of cyanobacterial symbionts that may confer variable nutritional benefits; in some cases, these symbionts appear to substantially enhance host growth rates.