THACKER, R.W.; University of Alabama, Birmingham: Field Manipulations of Sponge Symbioses: Differences Between Generalist and Host-Specific Cyanobacteria

The marine sponges Lamellodysidea herbacea and L. chlorea contain large populations of the host-specific, filamentous cyanobacterium Oscillatoria spongeliae. Other marine sponges, including Xestospongia exigua, contain generalist, unicellular cyanobacteria, Synechococcus spp. Symbiotic cyanobacteria may benefit their host sponges by fixing carbon or nitrogen that is subsequently acquired by the host sponge. However, these symbionts have also been reported to overgrow and kill their host sponges. The impact of cyanobacterial photosynthesis on sponge metabolism can be manipulated by shading sponge-cyanobacteria associations. To determine if sponges containing different cyanobacterial symbionts respond differently to reduced light availability, we shaded L. chlorea and X. exigua simultaneously in a randomized block design. After two weeks, shaded L. chlorea lost 92% more mass than unshaded L. chlorea. Shaded X. exigua gained 28% more mass than unshaded X. exigua. These results suggest that Oscillatoria symbionts benefit their host sponges in a mutualistic association, with the host sponge gaining more nutrition from photosynthesis than by feeding from the water column. Synechococcus symbionts may be commensals that exploit the resources provided by their sponge hosts without significantly affecting sponge growth.