HETEROTROPHIC SLIMES IN FLOWING WATERS

Field: Environmental Science (Physical Sciences)

Summary 1. While much attention has been paid to the ecology of macro‐invertebrates in flowing water, the microbial ecology of such systems has been largely ignored and our knowledge of heterotrophic slimes in particular remains far from complete. Slime‐forming organisms are ubiquitous in their distribution and are part of the normal riverine flora. Slime outbreaks occur in all types of organically enriched flowing fresh waters, regardless of their chemical nature. Slimes are predominantly of heterotrophs which require a constant supply of (i) a suitable carbon source, (ii) inorganic nutrients and in particular nitrogen and phosphorous, and possibly (iii) growth factors such as vitamins. Phosphorous is not a limiting factor for growth, with slimes developing in rivers with < 0·02 mg P l ‐1 . Other inorganic nutrients such as nitrogen can be used in various forms and are usually present in adequate amounts, even in unpolluted streams. Therefore occurrence appears to be most closely correlated with the presence of a source of available carbon. 2. The severity of outbreaks are not closely associated with soluble organic carbon content although there is a tendency for heavy growths to occur more frequently in more severely polluted waters. Low‐molecular‐weight sugars are clearly the causative agents of Sphaerotilus natans dominated slimes with higher molecular weight material such as starches not immediately effective as growth promoters. Mono‐ to penta‐saccharides are mainly used by bacterial slimes while fungal components utilize fatty acids up to C 8 . It is not possible to adopt a nationwide BOD 5 standards to control slime outbreaks as even small increases in river BOD 5 (< 1·0 mg l ‐1 ) can support slime growth. There is a need to develop new methods of assessing the slime‐promoting capability of effluents such as measuring the readily degradable low‐molecular‐weight carbon compounds, so that threshold concentrations of soluble organic carbon below which slime will not develop can be determined. 3. The effect of effluent enrichment on slime growth diminishes downstream as there is a tendency for the soluble carbohydrate to mix and dilute. The slime also metabolizes the carbohydrate, reducing the concentration by up to 60 % depending on the stage of slime development, thus limiting its own proliferation. This is the typical pattern of self‐purification in flowing waters. 4. The taxonomy of all the slime‐forming species are poorly understood as is the ecology of slimes. Species composition of slimes vary temporally and spatially within individual rivers. The primary factors affecting composition are nutrient type and water velocity, although pH determines whether a slime is either predominantly fungal or bacterial. The rate of transfer of oxygen and nutrients is dependent on water velocity with zoogloeal forms predominating as velocity falls to < 0·05 m s ‐1 . More details of the effects of water velocity and other environmental factors on all the slime‐forming organisms is required. 5. The effects of specific environmental factors on slime growth have been determined primarily from laboratory‐based studies, often using pure cultures on solid medium or batch culture methods. Clearly it has been difficult to relate these results to what is happening in the field. Little quantitative information on the productivity of slimes exist and the energy budgets or role of slimes in the self‐purification process of rivers is largely unknown. The effects of temperature, pH, dissolved oxygen concentration, water velocity, solar energy input and grazing on the relationship between organic nutrient concentration and slime growth needs to be fully understood. Therefore there is a need for both field‐based flow channel and field studies using mixed slime populations to accurately model the effects of environmental factors on slime development. Formation of such models is a prerequisite to the development of control strategies. 6. Due to slower oxidation rates at lower temperatures and reduced sloughing resulting in more luxuriant growths, slimes are generally considered to be more frequent and extensive in winter. However this is clearly not the case with outbreaks far more abundant in the summer due to the smaller flows reducing the dilution factor of effluents, and the enhanced temperature and suppressed oxygen concentration of the water, reducing competition and grazing. 7. The presence of slimes is not always detrimental, the major effect is unsightly appearance and reduced amenity value. Slime‐forming organisms are predominantly aerobic and the rate of oxygen consumption of slimes is directly related to the dissolved oxygen concentration of the water. It would appear that slimes rarely cause deoxygenation, although sudden increases in water temperature which lowers the solubility of oxygen or enhanced chemical oxygen demand of the effluent due to reduced dilution, may cause total oxygen utilization. 8. The effects of slime growths on the aquatic environment are numerous and cumulative, especially in relation to salmonid fisheries. Nearly all pollutants that are released into the environment will enter surface waters at some stage, so the ability of heterotrophic slimes to rapidly accumulate heavy metals and perhaps other pollutants by various sorption processes will result in metals being concentrated and transferred along the food chain. This will eventually result in increased residual metal concentrations in fish or toxic concentrations of metals being accumulated in macro‐invertebrates normally eaten by fish. Metals can be transported out of the polluted zone via sloughed floes and be released back in to solution downstream of the site originally affected. 9. The severity of problems associated with outbreaks increase with the length of the slime growth, with the majority of the longer outbreaks (> 5 km) resulting in oxygen depletion, increased siltation, alteration in flow pattern, increase in sloughed biomass, reduction in species diversity, destruction and reduction in habitat diversity and the elimination of fisheries. Case studies on the effects of slime growth, especially those causing fish kills, need to be carefully analysed and published. 10. The potential of heterotrophic slimes in biotechnology and wastewater treatment has yet to be fully realized. The ability to grow rapidly, producing considerable biomass rich in protein could be utilized. The sorption of heavy metals by all the slime‐forming organisms but especially by the iron and manganese bacteria, could be used for the removal of low concentrations of metals in wastes, treatment of metal‐rich effluents or for metal recovery. The property of removing phosphorous and nitrogen from solution should also be further considered. 11. No adequate control measures are available except for full treatment of effluents prior to discharge. Even traces of low‐molecular‐weight carbon compounds will result in slime development. Inadequate partial treatment may enhance slime growth by partially breaking down the effluent and releasing slime‐promoting compounds. Intermittent discharge can reduce the standing crop of slime per unit surface area but as the total biomass supported by an effluent will remain the same, the slime will be extended over a greater length of river. Bacterial slimes are assemblages of filamentous and dispersed bacteria, and are far more common than fungal or algal dominated slimes. The two slime‐forming organisms S. natans and zoogloeal bacteria are the major components of the majority of heterotrophic slimes, therefore any attempt at control should be aimed at these two species.