|
|
|
|
Current issues of Marine Microbiology |
K.
Kathiresan
Centre of Advanced Study in Marine Biology,
Annamalai University,
Parangipettai: 608 502,Tamil Nadu, India
&
A. Duraisamy
Ozone Cell,
Ministry of Environment & Forests,
Govt. of India, New Delhi |
|
Marine
microbes are taxonomically diverse, genetically special
and largely unexplored species. It is estimated that
there are 1029 prokaryotes in the world's oceans, and
there should be about ten times more viruses. The number
of prokaryotic microbes is obviously abundant in the
ocean, as compared to eukaryotic microbes; however,
biomass in both the types is probably the same. With
more than a billion individual microbes in a liter of
seawater, the microbes constitute 98% of oceanic biomass!
They have dominating roles in oceanic processes, such
as making oceanic food, break down the food materials,
recycling of nutrients and climate change. |
|
Developments in marine
microbiology:Marine
microbiology is an interesting field of biology that
began in the late 19th century. Ocean expeditions such
as the Challenger expedition held between l855 and 1890
laid a foundation for subsequent microbiological studies
of the ocean. In l950s, the Galathea expedition explored
microbial aspects of the deep sea and marine psychrophilic
bacteria. In l960s, marine microbes were described for
their distribution. Between l975 to l980, the roles
of marine microbes in ecology, productivity, food chain
and biofilm production were discovered. During l990-2005,
marine microbes were studied for biotechnological aspects
such as genomic identification and microbial drugs.
Scanning electron microscope along with fluorescent
probes are helpful to understand the activities of bacteria.
Future development of marine microbiology depends on
new methodologies and instruments yet to be built-up. |
|
The
remarkable event in the history of marine microbiology
occurred in the year l977 on discovery of microbes in
hydrothermal vents at the sea bottom. The microbes living
therein are reported to be chemoautotrophic bacteria
utilizing H2S and endosymbiotically associated with
a giant worm namely Riftia which grows at a rate of
33 inches per year at a temperature of 350°C! This
study prompted further studies to analyse marine microbial
associations. To cite a few: (i) photosynthetic algae
in corals; (ii) intracellular bacteria in Noctiluca
scintillans, a cause of red tides; (iii) luminous vibrios
in light organ of squids, flash light producing fishes,
and various mammals; (iv) nitrogen-fixing bacteria in
boring mollusks; (v) endosymbiotic bacteria with marine
nematodes; (vi) coral exudates greatly stimulating microbial
growth with bacterial biomass. |
|
Biodiversity of marine
microbes:A
recent estimate suggests that the sea may support at
least 2 billion different bacteria. But our ability
to culture and study them under laboratory conditions
is relatively difficult and only 0.01 to 0.1% is culturable.
Reason for this may be that the microbes growing in
oligotrophic condition of the ocean are grown under
high nutrients conditions in laboratories, which might
have caused dormancy of the microbes. The biggest challenge
is how to culture the unculturable microbes.The
obvious questions are (1) what are the microbes present
in the ocean? (2) What are their activites? (3) How
do they interact? (4) How do these microbes affect higher
marine forms? The International Census of Marine Microbes
(ICoMM) was therefore initiated involving of 300 scientists
from 53 countries in the year 2003. Their mission was
to accelerate discovery, understanding, and awareness
of the global significance of marine microbes by the
year 2010. |
|
Despite their
various crucial roles in the biosphere, the catalytic
potential of microorganisms in nature is enormous and
they are able to degrade various pollutants and wastes
which cause serious environmental degradation. Using
microbes in environmental clean up activities is of
significance. They are called as 'anti-polluters' that
metabolize toxic substances and remove recalcitrant
compounds present in the pollutant. They are also required
for the biogeochemical cycles that recycle chemicals
between living organisms and the physical environment.Main difficulties
in identification of marine microbes are due to lack
of microbial taxonomists and non-availability of proper
monographs. Genetic tools are available for a negligible
number of the known marine microbes. The use of rRNA
sequencing to identify marine bacteria provided significant
information on the phylogeny of marine taxa, especially
those of the ecologically important Vibrionaceae group.
Immunofluorescent-epifluorescent techniques are useful
in sensitive detection of human pathogens in coastal
environments. |
|
Marine microbes &
environmental changes:Microorganisms
were reported to have been involved with the most important
global change in the history of the earth: the transformation
of the atmosphere into an oxygen-rich environment about
three billion years ago. The marine microbes are responsible
for half of the CO2 fixation in the planet and most
of the respiration in the oceans. Bacteria transform
organic nutrients and dissolved organic matter into
living biomass and particulate carbon. Three microbial
functions are important: (1) photosynthesis, (2) respiration
(3) release of dimethyl sulfide (DMS) to the atmosphere
by phytoplankton and this process regulates climate.
In the atmosphere, DMS is oxidizes and produces acidic
particles that disperse radiation and condenses water
molecules in the troposphere over the open oceans. The
condensed water molecules form clouds. The incidence
of solar radiation on earth and rainfall are determined
by the phytoplankton activity in releasing DMS. An increase
in DMS production by the plankton would result in a
decrease of the solar radiation received at the surface
of the ocean due to cloud formation. This, in turn,
reduces biological activity, eventually decreasing the
DMS emission to the atmosphere. |
|
Interestingly,
viruses that control marine environment were not known
until l989. They are extremely abundant (2.5 x 108/
ml of sea water!) especially in oligotrophic waters.
However, viral infection is higher in nutrient-rich
environment and it could be significant in the ecological
control of planktonic microbes. It is estimated that
as much as 10-20% of marine bacteria is lysed daily
with 2-3% of primary production lost through viral activity.
The viruses play significant roles in the sea: (i) increase
the carbon cycle; (ii) enhance the oceanic respiration;
(iii) cause climatic change; (iv) control the microbial
species diversity; (v) exchange genetic materials among
marine bacteria following phage attack; (vi) operate
the marine food chain; and, (vii) disseminate the cholera
toxins by killing bacteria and this outbreaks of cholera
has close correlation with sea-surface warming brought
about climatic changes such as El Nino. |
|
Bioprospecting of marine
microbes:The
marine microbes occur in extreme conditions at a temperature
of 350°C, salinity of 320 g/l, pressure of >600
atmospheres, pH of <2 and low temperature of -8°C.
These are conditions in which no other life can exist.
These extreme living conditions force the microbes to
produce a vast number of enzymes with unique activities
and pharmaceutically active compounds of new structures.
It can not be denied that with 3.5 billion years of
biosynthetic experience, microbes remain nature's best
source of chemicals.Traditionally,
antibiotic-producing organisms were isolated from the
terrestrial sources, and only recently the potentials
of marine microbes are known. An intensive search for
antibiotics from marine microbes is now felt necessary
for treating infectious diseases that are caused by
pathogenic bacteria and fungi and that pose a serious
challenge for current pharmacology. Two issues are currently
addressed for the discovery of novel drugs from marine
microbes and they are (i) rapid development of antibiotic
resistance by many pathogens and (ii) toxicity of some
of the currently used antibiotics. Future line of research
for the production of novel anti-infective agents will
be (i) to clone the genes governing antibiotic biosynthesis
of marine microbes, (ii) to characterise and manipulate
them with the aim of producing novel pharmaceuticals,
or enhancing the productivity. |
|
The
microbes associated with marine invertebrates especially
sponges are proved to exhibit remarkable potential for
anticancer, antiviral and antioxidant drugs. Of several
compounds patented, Sorbicillacton A is one. Extracted
from a fungus associated with a marine sponge, it is
in an advanced stage of development for medical treatment.
Marine fungi have yielded several antioxidant products
such as acremonin from Acremonium species, xanthone
derivatives from Wardomyces anomalus and 4, 5, 6-Trihydroxy-methylphthalide
from Epicoccum species. These antioxidants prevent oxidative
damage associated with diseases such as atherosclerosis,
dementia, and cancer. They may thus be useful as therapeutics
or food additives. Among the antibiotic-producing microbes,
marine actinomycetes especially Salinospora group within
the family Micromonosporaceae are very promising. These
microbes are found to be a potent source of anticancer
agents that target proteasome function and their industrial
potentials have been validated by Nereus Pharmaceuticals.
Recently our research team has successfully extracted
agricultural fungicides, biofertilizers, shrimp feed
supplement, cholesterol-reducing drugs from marine microbes.
All these researches are limited to those marine microbes
which are easily culturable. The genome sequencing makes
it possible to visualize potential metabolic and biochemical
capabilities of even unculturable marine microbes. One
of the future research trends will be focused on bio-active
substances derived from non-culturable marine microorganisms. |
|
The
economic values of bio-prospecting are remarkable. Pharmaceutical
industry has benefited through drugs developed from
natural compounds. The current half of the best selling
pharmaceuticals is natural or related to natural products.
The combined market worldwide for pharmaceuticals and
agrochemicals is over $400 billion annually. Hence,
there is a great scope for bio-prospecting of marine
microbes for their utility as thermostable and cold
active enzymes in food preservation, leather processing
and cleaning industries, bio-polymers and bio-surfactants
in waste management and in nanotechnology for a broad
range of applications. The greatness of smallness deserves
much attention. |
|
About
the authors: Dr. Kathiresan, M.Sc., Ph.D.,
D.Sc. is a Professor, Centre of Advanced Study in Marine
Biology, Annamalai University with 25 years of research
experience. He has published over 200 papers and 15 manuals/books
to his credit, executed 10 major research projects and
organized 3 international and 6 national training programmes.
He has successfully guided 15 Ph.D., scholars. He is a
recipient of International "NAGA-2001" award
from the World Fish Centre, Malaysia and Tamil Nadu Scientist
award ("TANSA-1995") from Government of Tamil
Nadu. |
|
Dr. A. Duraisamy,
M.Sc., Ph.D., is the Director, Ozone Cell under Ministry
of Environment & Forests. Government of India, involving
actively in global programmes to phase-out Ozone Depleting
Substances). |
|
|
|
|
|
|
|