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- 1. Rajani Prabhu Msc part-1 MI-1414 MICROBIOLOGY MARINE MICROBIOLOGY ISA-2 GOA UNIVERSITY MICROBIAL LOOP -An introduction
- 2. HISTORY Although marine bacteria were known to exist,they were not thought to be significant consumers of organic matter (including C) However, during the 1970s and 1980s Pomeroy and Azam, suggested the alternative pathway of carbon flow from bacteria to protozoans to metazoans. we know little about the relationships between microbes and their environment because only one tenth of one percent of the bacteria have ever been cultured .Developments in technology for counting bacteria have led to an understanding of the significant importance of marine bacteria in oceanic environments. In the 1970s, the alternative technique of direct microscopic counting was developed by Francisco et al. (1973) and Hobbie et al. (1977). Development of the bacterial productivity assay showed that a large fraction (i.e. 50%) of net primary production (NPP) was processed by marine bacteria. In 1974, Larry Pomeroy published a paper in Bio-Science entitled The Oceans Food Web: A Changing Paradigm, where the key role of microbes in ocean productivity was highlighted. The term microbial loop was introduced in early 1980s in the journal Marine
- 3. INTRODUCTION Life in the ocean is microbe based- microbes dominate earth's carbon, oxygen, sulphur, and nitrogen cycles. Organisms comprising the microbial food web: The microbial web includes bacteria and archaea and very small eukaryotes, ranging in size from 0.01 micrometers for viruses to 20 micrometers for ciliates, living in a dilute broth of carbon-based matter ranging from individual molecules to tangled polymers to colloids to clumps of dead matter called marine snow.
- 4. Organisms comprising microbial loop
- 5. The final link in all food chains is made up of decomposers, those heterotrophs that break down dead organisms and organic wastes. A food chain in which the primary consumer feeds on living plants is called a grazing pathway. That in which the primary consumer feeds on dead plant matter is known as a detritus pathway. Both pathways are important in accounting for the energy budget of the whole ecosystem Recent discoveries show that very small plankton -- bacteria and micrograzers -- are key to maintaining the flux of carbon and energy within marine ecosystems. Bacteria consume dissolved organic material (DOM) that cannot be directly ingested by larger organisms. DOM includes zooplankters' liquid wastes and jellylike substance ("cytoplasm") that leaks out of phytoplankton cells. When these marine bacteria are later eaten by micrograzers such as flagellates and ciliates, the formerly "lost" carbon and energy are recycled back into the marine food web. Better still, as bacteria absorb DOM, they release nutrients that facilitate phytoplankton growth.
- 6. DOM and MICROBIAL LOOP Dissolved organic carbon (DOC) is introduced into the ocean environment from: bacterial lysis the leakage or exudation of fixed carbon from phytoplankton (e.g., mucilaginous exopolymer from diatoms) sudden cell senescence sloppy feeding by zooplankton the excretion of waste products by aquatic animals the breakdown or dissolution of organic particles from terrestrial plants and soils (Van den Meersche et al. 2004). Bacteria in the microbial loop decompose this particulate detritus to utilize this energy-rich matter for growth. Since more than 95% of organic matter in marine ecosystems consists of polymeric, high molecular weight (HMW) compounds), only a small portion of total dissolved organic matter (DOM) is readily utilizable to most marine organisms at higher trophic levels. This means that dissolved organic carbon is not available directly to most marine organisms; marine bacteria introduce this organic carbon into the food web, resulting in additional energy becoming available to higher trophic levels. Recently the term "microbial food web" has been substituted for the term
- 7. MICROBIAL LOOP AND FOOD WEB In marine ecosystems, the Microbial Loop can be distinguished from the Microbial Food Webs in that the former likely consists of the pathways relating heterotrophic bacteria to bacterivorus protests (zooflagellates) and Dissolved Organic Matter (DOM), and the latter includes all microbial communities below 100 m including all the 10 m primary producing organisms. Therefore, as much as it is true that the Microbial Loop can mainly act as a carbon sink, the Microbial Food Web is the crucial link for the whole ecosystems. The sink aspect is mostly due to the fact that a considerable amount of Particulate Organic Matter (POC) passes through bacterial production that end up in DOM pools.
- 8. MICROBIAL LOOP This important part of marine food chains involves DOM leaked from diatoms and other phytoplanktons that is consumed by bacteria. The bacteria are then consumed by flagellates and ciliates. These members of the plankton also leak DOM that is in turn absorbed by the bacteria. The bacteria are decomposers and release inorganic nutrients for uptake by phytoplanktons. It is estimated that 60 percent of the energy flowing through marine food chains passes through the microbial loop
- 9. references Azam F, Fenchel T, Field JG, Gray JS, Meyer-Reil LA, Thingstad F (1983) The ecological role of water- column microbes in the sea. Marine Ecology Progress Series 10: 257-263. Barber, R. T. and A. K. Hilting (2000). Achievements in biological oceanography. In: 50 Years of Ocean Discovery. Ocean Studies Board. Washington DC, National Academy Press: 1121. Lawerence R. Pomeroy,Peter J,Farooq Azam And John E. Hobbie(2007) The microbial loop. Oceanography Vol.20 NO.2 Pomeroy LR, Williams PJ, Azam F (2007) The Microbial loop. Oceanography vol.20 no.2
- 10. THANK YOU..
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