Exudates algal

Aneer G (1987) High natural mortality of Baltic herring (Clupea harengus) eggs caused by algal exudates. Mar Biol 94 163-169... 

DOM Release The Viral Shunt, Algal Exudates, and Sloppy Feeding... 

The most commonly used approach to quantify the concurrent flux of algal exudates to heterotrophic bacteria is to combine the 14C-tracer method with a differential filtration step in which free-living bacteria are physically separated from phytoplankton and excreted DOM (reviewed by Baines and Pace, 1991). The data presented in this review indicate that, on average, 46% of the excreted DOM is incorporated by bacteria. One limitation of this approach is that bacteria attached to particles are largely excluded from the analysis, but this can, to some extent, be overcome by monitoring the distribution of heterotrophic bacteria in both size fractions (Sondergaard et al., 1985). 

In conclusion, the release of exudates appears to be controlled mainly by the total primary production (photosynthetic activity), and although very little is known about the detailed chemical composition of algal exudates, existing studies have shown that carbohydrates, organic acids, and dissolved and free amino acids may be significant constituents. However, to be able to fully understand the role of algal sources of DOM, there is clearly a need for more studies on the release and chemical composition of algal exudates, particularly as the availability of various fractions of the released DOM to bacterial utilization seems to vary extensively (Norrman et al., 1995). 

Marsalek, B., and R. Rojickova. 1996. Stress factors enhancing production of algal exudates A potential self-protective mechanism Zeitschrift fur Naturforschung C51 646-650. 

Gorbi, G., Corradi, M.G., Invidia, M., Rivara, L. Mid Bassi, M. (2002) Is Cr(VI) toxicity to Daphnia magna modified by food availability or algal exudates The hypothesis of a specific chromium/algae/exudates interaction, Water Research 36 (8), 1917-1926. 

Langlois, G., Effect of algal exudates on substratum selection by the motile marine telotroch Vorticella marine, J. Protozool., 221, 115, 1975. 

High precursor concentrations, coupled with an abundance of phenolic materials, suggest that humification should be readily observable in estuaries. There are indications that humification may occur in the dissolved phase, particularly if algal exudates are abundant. Macrophytic debris may be an important site for humification. Sediment humification processes have been suggested on the basis of downcore increases in high-molecular-weight DOC, along with stable isotope data. However, the actual or relative importance of all these sites of humification in estuaries has yet to be demonstrated. 

The algal extract used contained 73 Xg/mL of microcystin-LR and a dissolved organic carbon content of 2200 pg/mL. The organic matter present in addition to microcystin-LR was presumably a complex mixture of algal exudates. 

In comparison with the Langmuirian behavior of MLR adsorption, the rate of MLR loss from solution is observed to decrease on increasing the concentration of added toxin (and, concomitantly, other algal exudate). Thus, as shown in Figure 6, the pseudo-first order rate constants (k ) obtained from best fit exponential functions to the pH 3.5 results exhibit a decrease from over 0.2 min at an initial... 

Both microcystin-LR and other algal exudate are considered to sorb reversibly to Ti02 surface hydroxy groups i.e. 

Figure 7. Adsorption isotherms for uptake of organic carbon (principally algal exudate) onto 1 g/L Ti02 from solutions of pH 3.5 and 8.6. (Reproduced from Feitz et al., Environ, Sci. TechnoL submitted).

The rate of recombination of these surface hydroxyl radicals with conduction band electrons is expected to be relatively slow (3) enabling interaction with other readily available (adsorbed) oxidizable species including toxin and algal exudate i.e. 

Note that while we have written equations 7 and 8 as H abstraction reactions, the processes could also occur by electron transfer however no information is available enabling differentiation between these two mechanisms. The algal exudate radicals so generated at the semiconductor surface might be expected to either react with other oxidizable species at the surface (such as adsorbed MLR) or desorb to solution ... 

The observed decrease in toxin removal rate that is observed in the acidic pH range on increase in exudate concentrations (Figure 6) suggests that release of exudate radicals to solution (equation 10) is a more significant process than reaction with MLR at the surface (equation 9). While reaction of the released exudate radicals with oxygen is likely (equation 10a) the extent of algal exudate peroxyl radical formation is unknown. 

MLR radicals generated at the surface (or in solution) are likely to be unstable and apparently readily decompose (on the basis of our analytical results). Feitz et al. Environ. Sci. TechnoL submitted) have proposed that the relatively long-lived algal exudate radicals released to solution (which may include exudate peroxyl radicals) are deactivated by reaction with other readily oxidizable species such as dissolved toxin (equations 12 and 12a), by reaction with other components of the ill-defined alg d exudate (equation 13) or simply by decomposition (equation 13a) i.e. 

Figure 8. Effect of varying affinity of MLR and algal exudate for Ti02 surface sites on rate of removal of toxin from solution. The cases modeled are identical to those defined in Table I and to the initial conditions and rate constants detailed in the Appendix.

Figure 9. Comparison of the time dependence of toxin removal under conditions of a) strong and b) weak adsorption of toxin and algal exudate to the semiconductor as defined in Table I. a) 58.1% and 21.7% MLR and AE adsorbed to Ti02 respectively (Case 1) and b) 0% and 7.7% MLR and AE adsorbed respectively (Case 6).

Figure 10. Effect of toxin and algal exudate concentrations on pseudo-first order rate constant for removal of toxin from solution.

ACUCHEM input file for hypothetical system involving Ti02 catalysed photodegradation of trace contaminant (toxin, MLRH) in the presence of a higher concentration of organic material (algal exudate, AEH). 

---