World Aquaculture 59 Marine snow: Its formation and significance in fisheries and aquaculture Pragyan Dash1, Dipanjan Kashyap1 and Sagar C. Mandal2* A continuous shower of organic aggregates that drifts down from the surface of the ocean to the abyssal depths is known as “marine snow.” These white fluffy substances look like snowflakes in sea water, which may be the reason for naming it “snow.” In estuarine and nearshore waters, marine snow consists of detritus, mineral grains, phytoplankton and microorganisms bound loosely in a mucous matrix (Eisma 1986, Shanks and Edmondson 1990). Macro-aggregates of marine snow range from 0.5 mm to several cm in diameter (Alldredge and Silver 1988). The structure of marine snow is delicate and flocs shatter when sampled or handled. Marine snow contributes greatly to the transport of particulate carbon and macronutrients through the marine pelagic zone and serves as a food source for many deep-sea fauna. Composition of Marine Snow Aggregates of marine snow are enriched with dead or dying plankton, diatoms, fecal matter, sand, soot and other inorganic dust, degraded larvacean houses, bacteria with refuges, concentrated nutrient sources and semi-stable substrates (Shanks and Trent 1979). The aggregate is held together by sugary mucus known as transparent extracellular polysaccharides (TEPs), which are natural polymers exuded as waste products by bacteria and phytoplankton. Marine snow aggregates can be classified on the basis of composition as dominated by detritus, diatoms, larvacean houses, or fecal pellets. Formation of Marine Snow Aggregates formation occurs when particles collide and stick together. The primary factors involved in aggregate formation are particle abundance, size spectrum of particles, collision between individual particles or aggregates and between aggregates of various sizes during coagulation, and stickiness of particles (Jackson 2001). There are several physical mechanisms that cause suspended particles to collide, such as differential settling, turbulence, and Brownian motion. The rate of collision is dependent on turbulence and animal feeding activity, whereas stickiness is affected by the presence of exopolymeric substances (EPS), consisting of polysaccharide and smaller amounts of protein and DNA, forming a matrix in which bacterial cells are embedded. One type of EPS is TEP, which is considered an important parameter in determining particle stickiness (Passow and Aldredge 1995). When a marine phytoplankton is exposed to sunlight, all the organic carbon produced in photosynthesis is not used by the cell; a portion is exuded as dissolved organic matter into the surrounding water. Particles coalesce by cation bridging and adhere together. Aided by TEP, particles stick together and form aggregates. Gradually small particles bind with other particles to form larger particles. These aggregates grow over time and may reach several cm in diameter, settling for weeks before reaching the ocean floor. Quorum Sensing as a Tool in Formation of Marine Snow Quorum sensing is a phenomenon where microorganisms communicate and coordinate their behavior by the accumulation of signalling molecules. This mechanism is most commonly seen in bacteria that are equipped with molecular signalling systems (Figure 1). Individual bacteria have a Fig. 1. Quorum sensing is a phenomenon where microorganisms communicate and coordinate behavior by the accmulation of signalling molecules.
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