Actinomycetes are a clade of prokaryote that has produced some of the most economically and biotechnologically valuable secondary metabolites. Isolated strains have rendered unto us, various antibiotic agents, antitumor properties and, immuno-suppresive agents and enzymes. The number of novel strains isolated from the terrestrial environment has recently declined in conjunction with an increase in re-isolated known compounds. Due to this decline and high re-discovery rate it is essential that new Actinomycetes be discovered from lesser explored and unexploited habitats.
It has been estimated that the marine floor is more diverse in microbial life than that of a terrestrial rainforest. Whether or not this is the case, the marine floor provides a stunning array of extraordinarily different habitats ranging from high acidity, high pressures, anaerobic conditions and temperatures exceeding 100ºC; all of which have are known to have been colonized by actinomycetes. It is expected that the diversity of habitats populated by this microbe, will yield new and exciting metabolites that can be further examined for practicality. The purpose of this study is to define whether known marine actinomycetes are locally indigenous or simply swept in from nearby terrestrial habitats. The results could influence whether their conceived origins reduce or increase the novelty of their secondary metabolites.
Jensen et al collected 5 sediment samples from 15 different locations, ranging from ocean atolls, mangroves, sand spits and scrub vegetation. Each sample's dry weight was recorded and a serial dilution was conducted with sterilized seawater. The diluted sediment samples were innoculated onto two types of nutrient agar: Medium 1 for general hetrotrophic bacteria and Medium 2 designed to isolate actinomycetes. Each medium was inoculated with 2 x 10-2 and 2 x 10-4 dilutions. Separate samples were heated in favour of actinomycetes and were inoculated onto the two different agar media.
The plates were incubated for 14 and 21 days before bacterial and actinomycete counts were conducted. It is worth noting that due to the problems associated with actinomycete inoculations, some of the counts were not included. All actinomycetes were isolated and analyzed chromatographically for isomeric diaminopimelic acid configurations and whole-cell sugar analysis. This was conducted to compare results and identify each strain. The isolated actinomycetes were then grown in sterile, deionized water and total biomass was used to evaluate growth.
From the samples collected, a total of 289 actinomycete colonies were observed. From the chromatographic analysis, 91 of the samples were shown to belong to the streptomycetes and 192 were shown to be actinoplanetes. Six colonies couldn't be identified as they were too few in number. In relation to depth it was shown that stretomycetes decreased in number whereas actinoplanetes population increased with depth. Actinoplanete numbers were variable with no particular habitat being a richer source; including the oceanic atoll of which had very little terrestrial influence. In all but 5 of the 32 streptomycetes tested, deionized water reduced total growth. With the exception of 1 actinoplanete out of 31, all reduced or ceased to grow in deionized water.
Jensen et al concluded that their work was not suitable to infer the population numbers of actinomycetes in marine sediments, as the mycelin of a colony could be fragmented during the vortexing of a sample – leading to an increase of colonies during plating. However, their work did not conform to previous theories of actinomycete origin. Previous works had speculated that actinomycetes were only found in the marine environment as terrestrial spores and did not play an ecological role or metabolise under such conditions. Their salt-tolerance was also put down to them being soil microbes that need to be adaptable to varying conditions.
The team believe that actinomycetes vary in their adaptation to the sea and have differing levels of function within the marine microbial community. From the decreasing population in relation to depth and the fact that they were not present at a marine atoll, streptomycetes were suggested to be of terrestrial origin and under appropriate conditions; able to grow in a marine environment. As actinoplanetes increased in number with depth, and that their growth was inhibited by the absence of saltwater, they were deemed to be of marine origin; inferring that these microbes metabolize in the marine environment. Through morphological analysis, it was found that most of the actinoplanetes were from the genus Micromonospora. This genus has previously been studied and found at depths ranging from 0 >2000m. The team concluded that the general theory of actinomycetes not being physically adapted for the marine environment and not active members of the marine microbial community is incorrect for some members of this microbial group.
Jensen P.R, Dwight R & Fenical W. 1991. Distribution of Actinomycetes in near-shore tropical marine sediments. Applied and Enviornmental Microbiology. 57 (40) : pp.1102-1108.