Aquaculture of sponges for production of bioactive metabolites.

摘要

Sponges have proved to be the best source among marine organisms of biologically active metabolites for use as drugs or biomedical tools. If successful in clinical trials, bioactive metabolites will be needed in vast quantities, but most sponges contain only trace amounts of them. Of the supply methods currently being examined, aquaculture is considered to be the most cost-effective or perhaps the only method to guarantee sufficient supplies of some sponge metabolites. Two factors restricting the commercial development of sponge aquaculture are a poor understanding of how the environment affects the growth, survival and metabolite biosynthesis of sponges, and the lack of a farming structure that can supply sufficient quantities of bioactive metabolites. This study examined these factors focusing on two species, Latrunculia brevis and Polymastia croceus, both of which contain metabolites with biomedical application. For three years the population dynamics, growth and bioactivity (measure of metabolite biosynthesis) of wild L. brevis and P. croceus were examined to further our knowledge about sponge ecology and also to provide information to help develop good methods and procedures to farm sponges. For both species, survival of adult sponges was high in all seasons, while juvenile sponges had poor survival. Recruitment of L. brevis occurred in all seasons indicating that it is reproductively active throughout the year. P. croceus recruited mostly in autumn, this observation supports previous work that found the sponge to be reproductively active in summer and early autumn only. For both species, growth rates varied greatly between individuals and were unaffected by sponge size within the range examined. Sponges generally grew during winter and spring as the water temperature rose and shrank during summer and autumn as the water temperature fell. This growth pattern may relate to seasonal variation in food abundance, and for P. croceus it may result also from seasonal differences in reproductive investment. After 2 years, L. brevis and P. croce us had on average, halved and doubled in size, respectively. This indicates that wild sponges generally grow slowly and can also shrink in size. L. brevis showed a seasonal pattern of bioactivity, being most active in spring possibly to prevent the surface overgrowth of fouling organisms. P. croceus had no seasonal pattern of bioactivity, but individuals were either very active or inactive. These patterns of bioactivity may indicate an optimal defence strategy whereby sponges increase metabolite synthesis when they are most required. The bioactive metabolites in both species probably aid in competitive interactions and prevent predation and biofouling. The major environmental factors that are likely to influence the growth, survival and metabolite biosynthesis of farmed sponges and thus directly affect the success of a farming operation are season, exposure or water movement and depth. The effect of these environmental factors were examined in a series of short-term transplant experiments. Both survival and growth of L. brevis were greatest in winter when the water temperature was lowest, which probably reduces stress during transplanting. For P. croceus, survival was similar in winter, spring and autumn, while growth was greatest in spring probably because of greater food availability. Therefore, the response to the farming season can vary greatly between sponge species. During the summer transplant the toxic alga Gymnodinium brevisulcatum bloomed, killing most farmed explants. This highlights the danger to sponge aquaculture of adverse stochastic events that cannot be planned for or controlled. Growth of L. brevis and P. croceus generally increased as exposure increased thus showing clearly that although sponges are active suspension feeders, they rely greatly on the passive flow of water to provide food. The depth range (5-15m) examined in this study had no overall effect on the growth or survival of either L. brevis or P. croceus. Explants of both species farmed in similar environmental conditions varied greatly in growth. For both species, farming promoted greater synthesis of bioactive metabolites, which may indicate an optimal defence strategy. A reciprocal transplant experiment between northern and central New Zealand indicated that P. croceus could be transplanted great distances and survive. However, growth of relocated explants is reduced until they adjust to their new environment. A series of experiments was done to develop farming structures suitable for sponge aquaculture for metabolite production. Four general methods were examined: farming explants inside mesh structures, attached to substrate, with rope threaded through them and rope wrapped around them. Each was further divided into several specific methods examining the effects of various mesh sizes and rope materials. Most methods were found to be unsuitable because the farmed explants did not attach to the substrate but instead moved away from it and dislodged themselves. The two methods that showed the most potential for farming sponges, in terms of good growth, survival and metabolite biosynthesis, were threaded PVA rope and individual mesh bags with large holes and thin strand. These were developed into "rope" and "mesh" arrays. For nine months, L. brevis and P. croceus were farmed in rope and mesh arrays and harvested at different times. Harvesting involved the removal of new tissue growth leaving the explant "core" behind to regrow. The water temperature at the time of harvesting greatly affected the survival of L. brevis but not P. croceus. This supports the results of the short-term transplant experiments. Growth after harvesting was similar between harvested and non-harvested explants, indicating that healing of cut tissue and reorganisation of the canal system is not a drain on resources. This experiment showed that sponges can have very high growth rates. For example, explants of L. brevis and P. croceus in one treatment had grown by an average of 950% and 740% of their initial volume, respectively, in six months. Both rope and mesh arrays were found to be good farming structures, but differing patterns of growth and survival indicated that the two arrays are most suited for a particular type of sponge depending on its tissue structure. Rope arrays should be used to farm firm sponges such as P. croceus that can survive the threading process, while mesh arrays are best for farming soft, fleshy sponges like L. brevis that can grow quickly through the mesh strands. In some treatments, overall tissue yields were double the initial transplanted weight. As before, farmed sponges were generally more bioactive than wild sponges. An experiment to examine whether harvesting wild sponge populations is a suitable alternative method of supplying bioactive metabolites found that individuals of L. brevis and P. croceus could survive after removal of ≥90% of their biomass. Tissue regrowth was rapid and it was estimated to take between 1-4 years for individuals to grow back to their pre-harvested size. Although this suggests that harvesting wild populations of L. brevis and P. croceus can be an alternative method of metabolite supply, it is limited because of the relative scarcity of the sponges in the natural environment. This study examined the effect of different environments on the growth, survival and metabolite biosynthesis of sponges and developed methods and structures suitable for farming sponges. The high tissue yields from some treatments and the elevated bioactivity of farmed explants suggests that sponge aquaculture is a viable commercial method of supplying bioactive metabolites.