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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).
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