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 100ยบC;
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.
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