Formation and composition
Marine snow is a mucopolysaccharide matrix in which living and dead organisms are embedded. The mucopolysaccharide is an extracellular product released marine organisms, especially bacteria and phytoplankton but also ctenophores and appendicularia, forming colloidal fibrils which are found at densities up to 109 ml-1 in the upper layers of the ocean with sizes of the order of a few nm. These fibrils coagulate to form transparent exopolymer particles or TEPs, with a size of a few µm. Random collision of TEPs forms marine snow. The abundance and size distribution of transparent exopolymeric particles (TEPs) were monitored in the Kattegat (Denmark) during 1 yr. TEP number concentration ranged from 0.5 x 105 to 3.8 x 105 ml-1 and the volume concentration between 3 and 310 ppm. TEP volume concentration peaked during the spring bloom and again during the summer period. The observed accumulation of TEP during summer is consistent with the recent observation that dissolved organic matter (DOM) concentration has a similar seasonal distribution and suggests that TEP are formed from DOM.
Marine snow is especially abundant during periods of high photosynthetic activity when, in the N. Adriatic, up to 80 % of the photosynthetically produced carbon may be excreted as dissolved organic matter. Concentration may reach 3-8.7 g dry weight m3 in summer.
Transmission electron microscopy shows that marine snow-particles contain fibril-mediated associations between algae, bacteria and embedded organic and inorganic particles. 95% of bacteria in marine snow have a capsular envelope compared to only 63% in seawater.
Reducing microzones can occur within snow particles, sulphide concentrations of 1.3-25 µM have been recorded and methanogens are present.

Prokaryote composition
In the NE Atlantic heterotrophic bacteria were concentrated 100-6000-fold over the density in the water column whilest cyanobacteria were concentrated 3,000-2,500,000-fold.
The diversity of the recovered rDNA clones from marine snow prokaryotes was initially assessed by comparing restriction fragment length polymorphisms (RFLPs) of individual clones. Ninety-five bacterial clones examined yielded 90 different RFLP patterns. Bacterial phyla represented in the library included affiliates of the Planctomyces, the Gram-positive bacteria, the Cytophaga-Flavobacteria-Bacteroides (CFB) lineage, and the alpha-, gamma-, delta-, and epsilon-subdivisions of the Proteobacteria. The results suggest that bacterial colonization of suspended marine macroaggregates can result in diverse and complex assemblages, with specific phyla, such as the CFB, being commonly associated with marine particles. Furthermore, this particle-attached bacterial assemblage was similar to other bacterial assemblages found in marine sediments and terrestrial soils, with respect to the nature of the associated phylogenetic groups.

16S rRNA sequence analysis showed that ammonia oxidisers in marine snow particles from the northwestern Mediterranean Sea were related to Nitrosomonas eutropha while those in the water column were related to a novel Nitrosospira. Marine snow had a high biodiversity with 90 different taxa being found out of 95 prokaryote clones characterised. Groups found included gram positives, Cytophaga, and 4 subgroups of proteobacteria (some related to symbiotic sulphur-oxidisers, others to anaerobes).

Eukaryote composition
The zooplankton community in marine snow also differs from the free-living community. In the N. Adriatic larval stages, particularly of polychaetes, dominated the snow community in the Spring. Both polychaetes and turbellarians were enriched by up to 600-fold compared to the water. In summer harpacticoid copepods and Temora stylifera were common in the snow community, wheras Acartia clausi and Penilia avirostris were dominant in the water.
In the N. Atlantic protozoa ( especially heterotrophic nanoflagellates), the tintinid Dictyosta elegans and large pennate diatoms are common in snow particles.
In the Equatorial Pacific, 50% of nanoplankton spp. are primarily associated with marine snow.

Sedimentation
Mean rates ranged from 16.3 to 25.5 m day-1, with a trend of increasing sinking rate with greater aggregate diameter.
Nitzschia closterium aggregates sink more slowly when bound together with TEP.

Respiration and photosynthesis
1 to 6 mm large marine snow aggregates were net heterotrophic communities at Light intensities <152 ± 64 mu E m-2 s-1, but respiration comprised 75 ±21% of gross photosynthesis at saturating light intensities >500 mu E m-2 s-1. Bacterial densities on aggregates were >2000-fold higher than in the surrounding water. Cytophaga was highly abundant in the aggregate-associated bacterial community as identified by in situ hybridization techniques. Both the respiration rate per aggregate volume and the bacterial densities decreased with increasing aggregate size. The respiration rates, normalized to the number of bacteria in single aggregates, were 7.4 to 70 fmol C cell-1 d-1. The aggregate community respired 433 to 984 ng C d-1 per aggregate in darkness, which yielded a turnover time of 8 to 9 d for the total organic carbon in aggregates. Thus, marine snow is not only a vehicle for vertical flux of organic matter; the aggregates are also hotspots of microbial respiration which cause a fast and efficient respiratory turnover of particulate organic carbon in the sea.

Food web
In the NE Atlantic, examination of faecal pellets from planktonic crustacea showed that marine snow sometimes dominated their diet.
In the NE Pacific ecosystem at at Ocean Station Papa (OSP;
50 degrees N, 145 degrees W), the major trophic pathway is always from
picophytoplankton (0.2-5 µm) to microzooplankton (heterotrophic dinoflagellates and ciliates) to mesozooplankton. This supports the idea of a strong coupling between the microbial and metazoan food webs. Much of the primary production (and bacterial production in late summer) is not grazed and is recycled through the detrital pool.
The euphausiid Euphausia pacifica and the copepod Calanus pacificus feed on marine snow and consume diverse types of field-collected marine snow, including diatom flocs, abandoned larvacean houses, and dinoflagellate aggregates. Ingestion rates of aggregates by E. pacifica increased with increasing marine snow concentration, although in situ concentrations of aggregates were not sufficient to elicit a maximum ingestion rate. Assimilation efficiencies of euphausiids grazing on marine snow were 83 % (dinoflagellate snow) and 64 to 75 % (diatom/larvacean house snow). Ingestion rates, on cells of the diatom Thalassiosira weissflogii, by the euphausiid, Euphausia pacifica were lower in the presence of transparent exopolymer particles (TEP). However, grazing on cells was not inhibited by TEP, rather, TEP-clusters, aggregates which formed from TEP and nano-sized particles normally too small for the filtering apparatus of E. pacifica to retain, served as an alternative food source for E. pacifica, reducing their ingestion of cells. When feeding on TEP-clusters, euphausiids short circuit the food web by feeding on nano- and picoplankton directly, bypassing the microbial loop. Thus, the presence of TEP appears to enhance, rather than depress, macrozooplankton grazing.