Long-term culture of sponge explants: conditions enhancing survival and growth, and assessment of bioactivity.

Sponges are an important source of secondary metabolites with pharmaceutical interest. This is the main reason for the increasing interest of sponge culture recent years. The optimal culture system depends on the species to be cultured: while some species easily produce sponge aggregates after dissociation (primmorphs), others show a great capacity to regenerate after fragmentation (explants). Corticium candelabrum is a Mediterranean bacteriosponge that can undergo asexual reproduction. We have taken advantage of this capability and cultured C. candelabrum explants under several experimental conditions. To find the best conditions for obtaining functional explants, we assayed a range of conditions, including seasons of collection, culture temperature, filtered versus filtered-sterile seawater, addition of antibiotics and proportion of ectosome. We monitored the changes in shape and ultrastructure during the formation of explants. After 24 h, TEM images showed the aquiferous system disarranged, in particular at the sponge periphery. From 2 to 4 weeks later, the aquiferous system regenerated, and fragments became functional sponges (explants). Explants were cultured under two regimes: in vitro and in a closed aquarium system. Antibiotics were only added to the in vitro culture to assess their effect on the symbiotic bacteria, which remained healthy despite the presence of antibiotics. Two food regimens (marine bacteria and green algae) were assayed for their ability to satisfy the metabolic requirements of explants. We monitored explant survival and growth. Explants showed a high long-term survival rate (close to 100%). Growth rates were higher in the closed aquarium system, without antibiotic addition, and fed with algae. Explants cultures were hardly contaminated because manipulation was reduced to a minimum and we used sterilized seawater. C. candelabrum produces bioactive molecules, which may play a defensive role in the sponge and may have pharmaceutical interest. The bioactivity of the explants was similar to that of wild sponges.