60 June 2012 capability of secreting and detecting auto-inducers (quormones) that can be detected by other nearby bacteria. These signals allow bacteria to communicate and coordinate with each other. When bacterial density reaches a sufficient level and the concentration of auto-inducers reaches a critical threshold, a positive feedback is initiated, such that more signalling molecules are synthesized and receptor sites becomes fully activated. This process also induces the up-regulation of other specific genes. Certain traits are under quorum sensing control, such as exopolysaccharide production, cell aggregation, exoenzyme production and bioluminescence. Several bacterial strains isolated from marine snow particles produce acyl homoserine lactone (AHL) communication signals (Gram et al. 2002). Because of the common involvement of AHLs in antibiotic production and hydrolytic enzymatic activity, it is tempting to suggest that AHL allows marine snow bacteria to express such phenotypes only when occupying snow particles, reaching high bacterial cell densities, and interacting with marine phytoplankton. Collection of Marine Snow Marine snow can be gathered from the sea by marine snow collectors (traps) or by scuba diving. Marine snow collectors are lowered to a certain depth in the water column and closed. Traps are brought on board the ship and samples are left to settle for some hours. Particles that settle into a bottom chamber can then be isolated from the bulk water and removed for detailed analysis. Production of Marine Snow in the Laboratory Marine snow can be artificially produced in the laboratory by using a vertical recirculating tube with an upward flowing turbulent current. Depending on flow rate and floc composition, particle size and shape can be varied. Seawater is continuously circulated in a glass tube and an upward vertical flow velocity of as much as 0.25 cm s-1 is maintained by an oscillating pump. Artificial suspensions of particulate matter collected from sea water are injected into the tube through a stopcock at the bottom. The pump is run for a few minutes at maximum speed to produce an even suspension and adjusted to produce the desired upwelling rate. To aerate the suspension an air diffuser is provided at the top of the tube. Periodically samples are withdrawn from the top of the tube and run through a Coulter Counter to determine concentrations and grain sizes of suspended particles. The suspension can also be optically analyzed using an inverted microscope (Kranck et al. 1980). Significance of Marine Snow in the Ocean Effective and self-sufficient microhabitats Marine snow aggregates are enriched with microbial communities and chemical gradients within which photosynthesis, decomposition and nutrient regeneration occurs at elevated rates. Activities of autotrophic and heterotrophic organisms are closely linked, thereby providing mutual benefit where the product of one becomes a substrate for the other. Bacteria that colonize marine snow play an important role in the remineralization and solubilization of particulate organic carbon and other nutrients (Turley 2002). Primary transporter of surface-derived organic material in the ocean As aggregates slowly sink to the ocean bottom, it results in transport of particulate material from the surface to the deep ocean, leading to sequestration. The sinking of particulate organic matter in the ocean, such as marine snow, is a key mechanism for transporting carbon to depth (Kiørboe 2001). In some areas, marine snow constitutes up to 63 percent of total particulate organic carbon (Alldredge and Silver 1988). Phytoplankton associated with marine snow fix atmospheric carbon in the form of carbon dioxide, which is subsequently transported to the ocean floor, having a major effect on the long-term removal of carbon. Role in primary productivity in ocean Large fluxes of aggregates are typically coupled with primary productivity (Kiørboe 2001) and are associated with the termination of phytoplankton blooms (Alldredge 1995). Foundation of deep-sea mesopelagic and benthic ecosystems Deep-sea organisms are greatly dependent on marine snow as an energy source. They filter marine snow from the water or scavenge it from the seabed. The small percentage of material not consumed in shallower water becomes incorporated into the muddy ooze blanketing the ocean floor, where it is further decomposed through biological activity. Over the past 20 years, National Oceanic and Atmospheric Administration (NOAA) scientists and others have measured the amount of useable material in marine snow and found that there is plenty of carbon and nitrogen to feed many of the scavengers in the deep sea. Light emitters Marine snow contains certain dinoflagellates including Protoperidinium spp. and Noctiluca spp. Bioluminescence of individual dinoflagellate cells appears to be an optical sensory cue that startles zooplankton grazers, thereby decreasing grazing rates on dinoflagellates (Buskey et al. 1983). Bioluminescence-enriched marine snow may avoid consumption and play an important role in coastal carbon cycling and food-web structure (Herren et al. 2004). Significance of Marine Snow in Aquaculture Marine snow is commercially available and is used as source of dietary plankton in marine aquarium species. The diet reproduces biogenic suspended matter found in natural seawater, including marine snow aggregates. It acts as a good site for heterotrophic bacterial production. Larson et al. (1996) investigated the consumption of marine snow by fishes and found that marine snow contributed to the
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