Formation Of Organic Rich Aggregates

In their review of the microbial ecology of organic aggregates, Simon et al. gave an overview of the present knowledge of macroscopic organic aggregates (>500 ^m).5 These macroaggregates are heavily colonized by bacteria and other heterotrophic microbes and greatly enriched in organic and inorganic nutrients as compared to the surrounding water. The authors point out that during the last 15 years, many studies have been carried out to examine the various aspects of the formation of aggregates, their microbial colonization and decomposition, nutrient recycling, and their significance for the sinking flux. The significance of aggregate-associated microbial processes as key processes and also for the overall decomposition and flux of organic matter varies greatly among limnetic and oceanic systems, and is affected by the total amount of suspended particulate matter. A conclusion from these studies is that the significance of bacteria for the formation and decomposition of aggregates appears to be much greater than previously estimated. For a better understanding of the functioning of aquatic ecosystems it is of great importance to include aggregate-associated processes in ecosystem modeling approaches. Knoll et al. studied the early formation and bacterial colonization of diatom microaggregates (<150 ^m) during the phytoplankton spring bloom and showed that these are colonized by bacterial populations that differ from those in the surrounding water.13 They conclude that the bacterial community on aggregates develops largely from seeds on their precursor microaggregates.

Theoretical analyses of particle coagulation processes predict that aggregate formation depends on the probability of particle collision and on the efficiency with which two particles that collide stick together afterwards (stickiness).14,15 The former is a function of particle concentration, size, and the mechanism by which particles are brought into contact, for example, Brownian motion, shear or the differential settlement of particles. The latter depends mainly on the physicochemical properties of the particle surface and may vary with the particle type. Particle collision does not necessarily result in aggregation, as the stickiness or sticking efficiency is often only 10% or less but can increase up to 60% depending on the particle type involved.4

Depending on its intensity, shear can either increase particle collision or increase particle destruction. This is particularly the case at the base of the surface mixed layer, where internal waves, wind driven shear and tidal shear are pronounced; and within the benthic boundary layer where turbulence is increased again.

In surface waters, changes in particle coagulation efficiency have been attributed to the abundance of single species or as part of the life cycle strategy of cells.16,17 The occurrence of aggregates does not, for example, always coincide with the peak of phytoplankton abundance. Rather, it is often postponed toward the decline of the bloom.18 This has been hypothesized to be due to an increase in particle stickiness.19 A decade ago, a special class of particles was found to be readily abundant during phytoplankton blooms in water and in aggregates as well. These gels, called transparent exopolymer particles (TEP),20 are thought to play a central role in coagulation processes. Laboratory experiments have demonstrated that diatoms produce more gels under nutrient limitation, although little is known about how limitation by different nutrients affects the quantity and composition of the gels and subsequent stickiness. Because of the great abundance in shelf seas and in the open ocean and because of the stickiness of TEP, the probability of particle collisions is enhanced.21 Logan et al. proposed two hypotheses22 to account for the precipitous formation of large, rapidly settling aggregates at the termination of phytoplankton blooms in nature: aggregation due primarily to cell-cell collisions, and aggregation resulting from the presence of TEP. By comparing TEP and phytoplankton half-lives in these systems, it is concluded that the formation of rapidly sinking aggregates following blooms of mucous-producing diatoms is primarily controlled by concentrations of TEP, not phytoplankton.22

Engel conducted measurements of diatom species composition, TEP, bulk particle abundance, as well as chemical and biological variables in order to reveal the determinants of coagulation efficiency.19 The investigation showed that an increase in TEP concentration relative to conventional particles at the decline of the bloom significantly enhanced apparent coagulation efficiencies. High proportions of TEP led to apparent values of stickiness of 1, which indicates that collision rates can be substantially underestimated when the stickiness parameter alpha is calculated on the basis of conventional particle counting only, for example, with the Coulter Counter.

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