Many investigations have shown that sponges yield a large number of highly diversified marine natural products with novel structures and thus they are excellent biological systems for chemical defenses, inasmuch as they have a sessile lifestyle and soft, unprotected body tissues. Their abundance and the persistence of the chemical substances that are released can act as antifoulants against larval settlement, and prevent overgrowth by neighboring organisms. My research project was focused on: (1) isolating bioactive compounds from marine sponge Acanthella cavernosa and its associated microbes in order to investigate how these compounds protect the sponge against biofouling; (2) optimize the culture conditions for bioactive compound production from sponge -associated microbes....[ Read more ]

Many investigations have shown that sponges yield a large number of highly diversified marine natural products with novel structures and thus they are excellent biological systems for chemical defenses, inasmuch as they have a sessile lifestyle and soft, unprotected body tissues. Their abundance and the persistence of the chemical substances that are released can act as antifoulants against larval settlement, and prevent overgrowth by neighboring organisms. My research project was focused on: (1) isolating bioactive compounds from marine sponge Acanthella cavernosa and its associated microbes in order to investigate how these compounds protect the sponge against biofouling; (2) optimize the culture conditions for bioactive compound production from sponge -associated microbes.

Kalihinol A isolated from the sponge tissue showed a strong inhibition effect on fouling bacteria as well as the polychaete Hydroides elegans and the barnacle Balanus amphitrite with an EC₅₀ of 0.5 and 0.36 μg ml^-1, respectively. On the other hand, kalihinol A inhibited larval settlement through the modification of the bacterial community on the Phytagel^® surface. The result indicated that A. cavernosa could prevent larval settlement of H. elegans on its surface through the release of the secondary metabolites to inhibit larval settlement or through the modification of the bacterial community on their surfaces.

Succinic acid with both antibacterial and anti-larval settlement activity was isolated from the sponge A. cavernosa and its surface associated fungus, Fusarium sp. using a bioassay-guided isolation and purification procedure. The results indicated that succinic acid substantially altered the cell density and species diversity of the bacterial communities in the biofilms on hydrogel surface in comparison to the control hydrogel discs. This study provides the first piece of evidence that compound(s) originating from both sponge tissues and the sponge surface-associated fungus can act as an effective antifouling agent, strongly arguing that sponge surface-associated microbes can indeed contribute the sponge's chemical defense against fouling organisms.

Two antifouling compounds, 3-methyl-N-(2-phenylethyl) butanamide and cyclo(Pro-Phe), were isolated from another A. cavernosa-associated fungus Letendraea helminthicola. These antifouling compounds were found to be maximized at 35-45 ppt with a favorable temperature and pH at 18-25°C and 3.5-4.5, respectively. Glucose, xylose, yeast and peptone were found to be the most satisfactory carbon and nitrogen sources to manipulate the production of antifouling compounds. These optimum conditions can facilitate antifouling compound production in a large scale.

Besides, a marine sponge associated bacterium, Pseudoalteromonas luteoviolacea, produced violacein when growing attached to agar, but not in suspended culture, and this compound showed potent antibacterial activity. After growing the bacterium at different agitation speeds (0, 50, 100, 150, and 200 rpm), the highest amount of violacein was produced under stagnant conditions. A quorum sensing molecule, N-(3-Oxo-octanoyl)-L-homoserine lactone (OOHL), was detected under stagnant conditions at a concentration 5-fold higher than that in an agitated state (200 rpm). Overall, elevated agitation reduced the bacterial aggregated size and the amount of the quorum sensing molecule (OOHL), which in turn may affect the production of violacein by P. luteoviolacea.