Meeting Abstract
Sponges are bacterivores, removing picoplankton from the water column with high efficiency, and thereby affecting water column properties. Bacterial supply in deep-water is limited, yet kilometre-long reefs of glass sponges occur on the shelf of Pacific Canada. Meter-large Geodia barretti also form beds of thousands of individuals in fjords and on the continental shelf of the North Atlantic. Geodia (Demospongiae, Astrophorida, Geodiidae) is a ‘high microbial abundance’ sponge whose tissues are packed with bacterial symbionts. We asked what supports such large populations in deep, bacteria-poor fjords and shelves, and whether Geodia filters bacteria with high or poor efficiency? In waters with high dissolved organic carbon sponges can live heavily off DOC via symbionts. We used direct measures (inhaled-vs-exhaled water) to study feeding and excretion in a population of Geodia inhabiting fjords in southern Norway. We used microscopy to describe the structure of, and to model the resistance through, the canals and collar filter. We also carried out experiments to determine whether 1µm fluorescent beads can bypass the choanocyte filter. Ambient TOC (bacteria and DOC) were low, and although Geodia filtered bacteria with 91% efficiency, no particles were found to bypass the filter. Geodia removed nitrite and released nitrate, but not ammonium. Electron microscopy showed the filter has a ‘gasket’ that envelops all collars, much like the secondary reticulum of the Hexactinellida. The tightness of the collar filter suggests high resistance to flow and a relatively high cost to feeding in this sponge. The role of symbiotic bacteria in reducing the cost is suspected.
