Macroscopic organic aggregates, which are >500 µm and known as marine and lake snow, are important components in the turnover, decomposition and sinking flux of both organic and inorganic matter and elements in aquatic ecosystems. They are composed of various organic and inorganic materials depending largely on the given system and environmental conditions. The sys- tems include the pelagic limnetic, the neritic and oceanic marine region, as well as shallow turbid environments, e.g. rivers, the littoral zone of lakes, estuaries and tidally affected coastal areas with intense turbulence and a high load of suspended matter. Aggregate abundance and size vary greatly among these systems. Macroaggregates are heavily colonized by bacteria and other heterotrophic microbes and greatly enriched in organic and inorganic nutrients as compared to the surrounding water. During the last 15 yr, many studies have been carried out to examine various aspects of the formation of aggregates, their microbial colonization and decomposition, nutrient recycling and their significance for the sinking flux. They have been identified as hot-spots of the microbial decomposi- tion of organic matter. Further, microaggregates, which are <5 to 500 µm in size and stained by different dyes, such as transparent exopolymer particles (TEP) and Coomassie blue-stained particles, have been discovered and shown also to be important in the formation and decomposition of macroaggregates. In this review we give an overview of the present state of the microbial ecology of macro- and microaggregates, including the mentioned points but highlighting in particular the recent findings on the bacterial colonization of aggregates using molecular tools, their microbial decomposition and mineralization, and the significance of protozoans and metazoans for the colo- nization and decomposition of macroaggregates. Today it is evident that not only the aggregates but also their surroundings are sites and hot-spots of microbial processes, with the plume of solutes leak- ing out of the aggregates and greatly extending the volume of the intense decomposition processes. This microheterogeneity has important implications for the spatial and temporal dynamics of the organic-matter field in aquatic ecosystems and for our understanding of how heterotrophic organisms are involved in the decomposition of organic matter. The significance of aggregate-associated micro- bial processes as key processes and also for the overall decomposition and flux of organic matter- varies greatly among the various systems, and is greatly affected by the total amount of suspended particulate matter. A conclusion from the presented studies and results is that the significance of bac- teria 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 impor- tance to include aggregate-associated processes in ecosystem modeling approaches.

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Microbial ecology of organic aggregates in aquatic ecosystems

Interactions between microplastics and phytoplankton aggregates: Impact on their respective fates

Diatom blooms are frequently terminated by mass aggregation of cells into large, rapidly sinking aggregates. It has been hypothesized that transparent exopolymer particles (TEP), abundant particles formed from the polysaccharides exuded by living cells, may be essential for this mass flocculation processes. We investigated the abundance of TEP and their role in the aggregation of diatoms in laboratory cultures and during a natural diatom bloom off California. TEP and dissolved carbohydrates accumulated appreciably over the growth cycle of Chaetoceros gracilis in the laboratory. The flocculation of C. gracilis in a laboratory flocculator was dominated by TEP, not cells, and large flocs, consisting predominantly of particulate polysaccharides, formed at a rate more than an order of magnitude higher than predicted by coagulation theory for cells alone. The frequency of interparticle attachment was three orders of magnitude higher for TEP than for cells. The pattern of flocculation of a natural diatom bloom was similar to that of laboratory cultures. Prior to bloom flocculation the abundance and total quantity of TEP and the concentration of particulate carbohydrates increased, while dissolved carbohydrate concentrations decreased. During the flocculation stage TEP aggregated into fewer, but much larger particles and concentrations of dissolved carbohydrates decreased further. The percentage of diatom cells which were attached to TEP increased during the flocculation period from 3 to 90% and TEP formed the matrix of all the natural diatom aggregates observed. During the late flocculation stage the quantity of TEP and TEP aggregates did not increase further and concentrations of diatoms decreased, presumably because large flocs sank out.

Our findings indicate that TEP should be included in models of particle aggregation in the ocean. The abundance, large size and high sticking coefficient of TEP make them essential to the aggregation of diatom blooms. The extracellular release of polysaccharides by growing cells may be an adaptation for aggregation. The abiotic formation of particulate organic matter (TEP) from dissolved organic matter (DOC) may help to explain the extremely high turnover rates of DOC observed during blooms.

The role of particulate carbohydrate exudates in the flocculation of diatom blooms

By altering the number, size, and density of particles in the ocean, the activities of different phytoplankton, zooplankton, and microbial species control the formation, degradation, fragmentation, and repackaging of rapidly sinking aggregates of particulate organic carbon (POC) and are responsible for much of the variation in the efficiency of the biological carbon pump. A more systematic understanding of these processes will allow the biological pump to be included in global models as more than an empirically-determined decline in POC concentrations with depth that may not adequately represent past or future conditions. Although progress has been made on this front, key areas needing work are the amount of POC flux associated with appendicularians, the mechanisms by which coccoliths and coccolithophorid POC reach depth, and the impact of polymers such as TEP on the porosity of aggregates. In addition, an understanding of the interaction between biological and physical aspects of the pump, such as aggregate loading with suspended mineral particles, is also important for understanding the transmission of biogenic materials through the meso- and bathypelagic realms. Data suggest that variable biogenic silica to POC production ratios in various ocean regions are responsible for the poor correlation observed between silica and POC in deep sediment traps, and that high concentrations of suspended coccoliths in deep waters may be responsible for the homogeneous calcium carbonate to POC ratios observed in these same traps. Sedimentation of foraminiferal calcite does not appear to be as tightly correlated to POC flux as coccolith sedimentation. Suspended calcium carbonate particles, scavenged by sinking organic aggregates, have been observed to both fragment and increase the density of these aggregates. Analysis of the data suggests that scavenging of minerals by aggregates decreases the porosity of the aggregates and may increase their sinking velocities by hundreds of times.

/pdf/factors-influencing-the-sinking-of-poc-and-the-efficiency-of-3ja4clbuvr.pdf

Factors influencing the sinking of POC and the efficiency of the biological carbon pump

Aggregates generated in water and wastewater treatment systems and those found in natural systems are fractal and therefore have different scaling properties than assumed in settling velocity calculations using Stokes' law. In order to demonstrate that settling velocity models based on impermeable spheres do not accurately relate aggregate size, porosity and settling velocity for highly porous fractal aggregates, we generated fractal aggregates by coagulation of latex microspheres in paddle mixers and analyzed each aggregate individually for its size, porosity, and settling velocity. Settling velocities of these aggregates were on average 4−8.3 times higher than those predicted using either an impermeable sphere model (Stokes' law) or a permeable sphere model that specified aggregate permeability for a homogeneous distribution of particles within an aggregate. Fractal dimensions (D) derived from size−porosity relationships for the three batches of aggregates were 1.78 ± 0.10, 2.19 ± 0.12 and 2.25 ± 0.10. ...

Settling Velocities of Fractal Aggregates

Fractal dimensions of aggregates can potentially be used to classify aggregate morphology as well as to identify coagulation mechanisms. Microbial aggregates of Zoogloea ramigera have a cluster fractal dimension of 1.8±0.3 (± SD), suggesting that these aggregates are formed through cluster-cluster coagulation. An analysis of size-porosity correlations for two types of marine snow aggregates yielded fractal dimensions of 1.39±0.06 and 1.52±0.19, which were lower than values describing inorganic colloidal aggregation.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1990.35.1.0130

Fractal geometry of marine snow and other biological aggregates

The fractal nature microbial aggregates is a function of the type of microorganism and mixing conditions used to develop aggregates. We determined fractal dimensions from length-projected area (D(2)) and length-number scaling (D(3)) relationships. Aggregates of Zoogloea ramigera developed in rotating test tubes were both surface and mass fractals, with fractal dimensions of D(2) = 1.69 +/- 0.11 and D(3)= 1.79 +/- 0.28 (+/-standard deviation), respectively. When we grew this bacteria in a bench-top fermentor, aggregates maintained their surface fractal characteristics (D(2) = 1.78 +/- 0.11) but lost their mass fractal characteristics (D(3) = 2.99 +/- 0.36). Yeast aggregates (Saccharomyces cerevisae) grown in rotating tests tubes had higher average fractal dimensions than bacterial aggregates grown under physically identical conditions, and were also considered fractal (D(2) = 1.92 +/- 0.08 and D(3) = 2.66 +/- 0.34). Aggregates porosity can be expressed in term of a fractal dimensions, but average porosities are higher than expected. The porosities of yeast aggregates (0.9250-0.9966) were similar to porosities of bacterial aggregates (0.9250-0.9966) cultured under the same physical conditions, although bacterial aggregates developed in the reactor had higher average porosities (0.9857-0.9980). These results suggest that that scaling relationships based on fractal geometry may be more useful than equations derived from Euclidean geometry for quantifying the effects of different fluid mechanical environments on aggregates morphology and characteristics such as density, porosity, and projected surface area.

Daniel B. Wilkinson

Papers

Fractal geometry of marine snow and other biological aggregates

Fractal dimensions and porosities of Zoogloea ramigera and Saccharomyces cerevisae aggregates.