Bryozoan metabolites: an ecological perspective

Historically, the study of natural products has focused on the biosynthetic pathways of terrestrial plants and micro-organisms, particularly the Actinomycetes. Terrestrial habitats have been screened extensively in order to find producers of natural products with novel activities. The diversity of soil micro-organisms has been exploited for many years and most natural products of economic value, such as antibiotics, have been derived from this source. The number of novel natural products has decreased significantly in recent years largely because of the high rediscovery frequency encountered in screening programmes where standard microbial cultivation techniques are being used. This is of particular concern because the rapid increase in microbial resistance to antibiotics has serious implications for the prevention and treatment of disease. It is imperative, therefore, that the search for novel active compounds incorporates unexplored habitats and an extensive range of species from varied environments. These issues have led to the search for novel natural products being widened to encompass the flora and fauna of marine habitats. This Highlight will focus on marine fauna belonging to the phylum Bryozoa. The majority of research into marine metabolites has centred on algae and sponges, although there is now increasing emphasis on investigating other phyla from this environment. Since 1986 the annual Royal Society of Chemistry Marine Natural Products reviews have described a large number of marine metabolites from the published literature of the preceding year. In the 2004 annual review the highest numbers of novel compounds were isolated from sponges, followed by coelenterates and then micro-organisms. In contrast, the lowest number of novel compounds was isolated from the phylum Bryozoa. Although over 8000 species are known, and having featured in the Marine Natural Products review every year, only 1% of natural products characterized so far are from Bryozoa. In this Highlight we will focus on research into the chemical ecology of marine bryozoans, an area that appears to be significantly under-investigated considering the ubiquitous nature and large number of species of bryozoans living in the marine environment. The importance of the ecological context has been somewhat neglected with much of the research having centred mainly on the isolation and structural characterisation of bryozoan natural products (over 70 studies). There have only been two reviews specifically covering bryozoan metabolites since 1978, the most recent of these being in 1990. Significant progress has been made in the area since then, with particular emphasis being placed on research into the functionality of bryozoan secondary metabolites.