5. THE RESPONSE OF FRESHWATER MICRO-ALGAE TO INORGANIC SALTS
5.1 INTRODUCTION
The previous chapters described how freshwater micro-algal species (Scenedesmus bicaudatus, Chlorella vulgaris and Chlorella sorokiniana) were successfully isolated from samples obtained from local rivers, cultured, and maintained axenically in the laboratory for use in toxicity tests (Chapter 2). Chapter 4 examined the responses of these species to when they were exposed to reference toxicants in order to assess their ability to withstand experimental conditions, and thus assess their potential as toxicity test species. Scenedesmus bicaudatus was not a good toxicity test species because it did not achieve constant, uniform growth during the experimental period (Chapter 4). The response of the remaining two species, C. vulgaris and C. sorokiniana, to reference toxicants was compared to that of the standard species Pseudokirchneriella subcapitata CCAP 278/4 (Appendix B) and the commercial species Chlorella protothecoides ATCC® 30411TM. Furthermore, the sensitivity of the two species, C. sorokiniana and C. vulgaris, to the reference toxicants was compared to that of other algal species, using species distribution curves (Chapter 4).
This chapter begins to address the third and last phase of this study, namely assessing the application and value of using the local freshwater micro-algal species in toxicity testing for water resource management in South Africa. In this phase, the sensitivity of the local micro- algae to a range of carefully selected toxicants is assessed. Toxicants are carefully selected based on their relative importance in the South African context, as well as the practicality of using the local micro-algae to routinely determine the impact of these toxicants on local aquatic resources. The response of the two locally isolated species (C. vulgaris and C.
sorokinaina) to the selected inorganic salts (NaCl and Na2SO4), in comparison to the standard toxicity test species, Pseudokirchneriella subcapitata, CCAP 278/4 and a commercial species, Chlorella protothecoides ATCC® 30411TM is assessed. Because land use in South Africa is characterized mainly by agricultural and mining activities, which potentially leach salts into the natural aquatic resources, selecting salts (NaCl and Na2SO4) as toxicants to be tested in this study is considered to be appropriate as representative of those land-use activities respectively.
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The sensitivity of these algae to the salts will also be compared to that of freshwater macro- invertebrates using species sensitivity distribution (SSD) curves and data obtained from an international and a local database. The SSDs focus on the distribution of sensitivity of test species to selected toxicants, and these test species are supposed to be representatives of a given ecosystem (Schmitt-Jansen et al. 2007). Comparing the sensitivity to salts of both algae and macro-invertebrates is appropriate because, in aquatic communities, algae are an integral part of these communities and many macro-invertebrates rely on algae as their source of nutrition. Therefore, any effect on algae (chemical or otherwise) may directly or indirectly affect the macro-invertebrates in the aquatic environment. Kefford (2006) suggested that the community structure freshwater biota may be affected by salinity (the presence of dissolved solids or salts).
The process of increased salinity (salinization) in natural waters is a recognized environmental problem in some parts of the world, including Australia and southern Africa (Kefford et al. 2004, Kefford 2007). In many industrialized and agricultural regions, salt contamination may become a serious problem. There are aspects of land-use, in-stream and riparian habitat that are correlated with elevated salt concentrations (Kefford et al. 2006).
Salts from agricultural and other land surface activities may be washed out from the land surface into rivers and streams and pose a risk to aquatic biota in their environments. Salts are generally not considered as toxicants, since their presence at low concentrations in aquatic environments may be beneficial to organisms (Kefford 2007). There are concerns that low salt concentrations in some rivers may have sub-lethal effects on aquatic species and ecosystems (Gross 2003). However, at elevated concentrations, salts may affect organisms directly or indirectly by causing biodiversity shifts and disrupting trophic pathways. Macro- invertebrate community structure in rivers has been shown to be related to salinity (Kefford 2000). Macro-invertebrates have a broad range of salinity tolerances and therefore salt sensitive organisms can be excluded from the community, leaving behind the more tolerant species (Gillis 2011).
The effects of dissolved solids, or salts, on algae have been studied extensively in order to synthesize growth media for culturing these algae. However, there is not much work done on the ecological effects of increased salt concentrations on freshwater algae. Salinity or elevated salt concentrations can affect the productivity of algae by increasing the osmotic pressure and subsequently inhibiting photosynthesis in these organisms (Cleave et al. 1981).
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Osmotic pressure is dependent on the ionic strength of the algal growth medium, and therefore, changes in the salinity of the medium may cause inhibition of photosynthesis in some freshwater algae (Cleave et al. 1981).
There is existing literature on the physiological effects of organisms (algae and macro- invertebrates) exposed to changes in salinity when sodium chloride is present. The interest in studies of the effects of NaCl to organisms has possibly stemmed from it accounting for about four-fifths of the total salts in seawater (Batterton and Van Baalen 1971). Only recently has attention been given to the effects of sodium sulphate to organisms, despite it being a common component of saline agricultural drainage waters and some industrial effluents (Soucek 2007). Much of the information on sodium sulphate has examined the acute and sub- lethal effects on freshwater invertebrates such as crustaceans and molluscs (Soucek 2007a, b, and c). Sub-lethal effects of sodium sulphate on macro-invertebrates include reduced rates of growth, reproduction, metabolism, feeding etc. (Soucek 2007, Gillis 2011).
The inorganic salts (such as Na2SO4 and NaCl) are not on the list of water quality constituents of the South African Water Quality Guidelines for aquatic ecosystems (DWAF 1996). This may because the derivation of criteria was based on the available information and cause-effect data at the time (DWAF 1996). Electrical conductivity (EC) is used as a surrogate of salinity in the South African Water Quality Guidelines for aquatic ecosystem.
Electrical conductivity (EC) is a measure of the ability of water to conduct an electrical current. The total dissolved salts concentration is a measure of the quantity of all dissolved compounds in water, and most dissolved salts in water carry an electrical charge, therefore total dissolved salts concentration is directly proportional to the electrical conductivity (EC).
Ions such as chloride, sulphate and sodium carry an electrical charge when dissolved in water.
Both salts were selected for this study because they are common salts and salts of concern in terms of water quality management in South Africa (DWAF 2006a). They serve as indicators of the most prominent land-use activities of South Africa, mining and agriculture. Mining effluent is generally high in dissolved solids with sulphate as the dominant anion, and aquatic resources around some mining areas contain high sodium content (DWA 2004). Salinity generally becomes a serious water quality concern in most mining areas (DWA 2011).
According to resource water quality objectives (RWQOs) in South Africa, electrical
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), and chloride (Cl-) are among the parameters selected as indicators of fitness for use of water resources by certain user groups such as the mining and the agricultural sector. The RWQOs for South African catchments such as the Olifants River and the Vaal River that are heavily impacted typically include water quality standards for Cl- and SO42-, as these are problematic in such mining and agricultural areas. Resource Water Quality Objectives (RWQOs) form the water quality component of the Resource Quality Objectives (RQOs). The purpose of RQOs as defined by the National Water Act (Chapter 3, Section 13) is to establish clear goals relating to the quality of water resources, finding a balance between protecting and sustaining the resources, and the need to use them (DWAF 2006a). As part of compliance monitoring of the water quality standards of water resources, in-stream levels of indicator parameters are compared to RWQOs established for that particular resource (DWAF 2006a).
These two salts feature in the determination of the Present Ecological State of the EcoClassification process of South Africa’s Reserve determination (DWAF 2008b).
EcoClassification refers to the determination and categorization of the Present Ecological State of various biophysical attributes of rivers relative to natural reference conditions (DWAF 2007). Table 5.1 presents the Present Ecological State rating values for the two inorganic salts (Na2SO4 and NaCl) as outlined on the EcoClassification process of Reserve determination (DWAF 2008b).
Table 5.1 Present ecological state (PES) rating values for inorganic salts (Na2SO4 and NaCl) (DWAF 2008)
PES rating Deviation from reference
condition
Water quality Category
Na2SO4 (mg/L)
NaCl (mg/L)
0 No change A 20 45
1 Small change B 33 191
2 Moderate change C 38 243
3 Large change D 51 389
4 Serious change E 64 535
5 Extreme change F >64 >535
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Processes such as photosynthesis, protein synthesis and metabolic activity may be affected by salt stress in plants. A relationship between water and salt uptake, enzymatic salt removal capacity and toxicity of salts has been found in some plants (Trapp et al. 2008). Salt stress in plants has three main effects: it reduces water potential, causes ionic imbalance and induces toxicity (Parida and Das 2005). However, plants and algal cells may be able to develop phenotypic adjustments such as changes in biochemical and molecular mechanisms that enable them to cope with salt stress. Biochemical strategies such as change in photosynthetic pathways, induction of anti-oxidative enzymes, as well as alteration of membrane structures may assist plants and algae to cope with salt stress (Berube et al. 1999, Trapp et al. 2008).
These biochemical processes may act additively or synergistically in plants to improve salt tolerance (Parida and Das 2005). Some micro-algae such as some blue-green algae have been found in some hypersaline environments indicating their tolerance to salts. This may be cause for concern in aquatic environments because some blue-greens may cause algal blooms when they are able to out-compete salt-sensitive algae from other taxonomic groups.