Marine Actinomycetes: A Review

The marine environment is a prolific resource for the isolation of less exploited microorganisms, and as a matter of fact in the sea, untapped habitats exist with unique characteristics. This is partly caused by the lack of effort spent in exploring marine actinomycetes, whereas terrestrial actinomycetes have been, until recently, a successful source of novel bioactive metabolites. Furthermore, scepticism regarding the existence of indigenous population of marine actinomycetes arises from the fact that the terrestrial bacteria produce resistant spores that are known to be transported from land into sea, where they remain available but dormant for many years. Thus, it has been frequently assumed that actinomycetes isolated from marine samples are merely of terrestrial origin. It may be better to seek evidence instead that marine derived actinomycetes are metabolically active or capable of growing in the marine environment. Other useful characteristics include the display of specific marine adaptations, the formation of associations with plants or invertebrates and the demonstration of a role in nutrient cycling, all of which provide an indication that these bacteria are not merely present as dormant spores. When coupled with phylogenetic novelty, evidence for any of the above traits provides a strong case that the specific population under study can be considered as marine. As marine organisms live in a significantly different environment from those of the terrestrial organisms, it is reasonable to expect that their metabolites will differ considerably. Although actinomycetes can be readily isolated from marine sediments using appropriate selective isolation techniques, it is only through the use of molecular phylogenetics, a clear picture of the diversity, distributions and taxonomic uniqueness of certain populations can be obtained. Once recognized as unique, this population can then be selected as the focal point for intensive natural product studies. Clearly, actinomycetes have the potential to continue to play a major role in microbial drug discovery, and the recent observations on significant population of marine-adapted actinomycetes occur in ocean sediments, could certainly emphasize the potential importance of these strains in future drug discovery efforts.