Formation and microbial utilization of amorphous aggregates in the sea: ecological significance

Amorphous organic particles, ("marine snow"), are thought to play a vital role in the transfer of carbon and energy from the euphotic layers of the ocean to the deep sea bed. These particles are mainly derived from phytoplankton extracellular release. As 10 to 40% of the dissolved organic carbon consists of colloidal material, colloids may represent a prominent role in these transfer processes. These colloids adsorb charged molecules efficiently and can coagulate further to form the recently described transparent exopolymer particles and eventually the ubiquitous macroscopic marine snow. The other mechanism proposed leading ultimately to marine snow formation is inefficient top-down control of phytoplankton growth by zooplankton. In the senesce of the phytoplankton bloom the cell surface becomes sticky due to the release of polysaccharides, and single phytoplankton cells then aggregate. Subsequently these aggregates become densely colonized by auto- and heterotrophic microbes, but while the abundance of these microorganisms is from 2 to 4 orders of magnitude higher than in the ambient water, their growth rates are usually not significantly increased in the aggregates. In this review, the reasons for this conspicuous pattern are discussed and recently investigated novel types of particles and their significance in the light of oceanic carbon transfer are presented.