**2.2. EPS associated genes in** *Synechocystis* **PCC 6803**

the pentoses- xylose, arabinose and ribose; the hexoses -galactose, glucose (found in 90% of polymers) and mannose (found in 80% of polymers); and derived hexoses rhamnose (found in 80% of polymers), fucose, glucuronic and galacturonic acids, with occasionally methyl and amino sugars [32]. The presence of glucuronic and galacturonic acids associated with EPS

Amongst the cyanobacteria, there is quite a wide variation in the quantity of such polymeric material produced, varying from 144 mg. L-1 day-1 in the case of *C. capsulate* ATCC 43193 to 2 mg. L-1 day-1 in the case of *Synechocystis* PCC 6803 [45], which is amongst the lowest producer of this exopolysaccharide material. There have been few studies on whether compositional change occurs in such material. In *Synechocystis*, it has been shown that strains can modify the composition of this material, particularly as a function of culture age and in response to nutrients. In many cyanobacteria, the exopolysaccharide material may be a subcomponent of the extrapolysaccharide material that remains attached to the cyanobacterial cell itself and leaks or is physically broken away due to the habitat location, flow conditions or activity of the organism. This may occur as the extrapolysaccharide layers enlarge and may become solubilized or fractured as a natural part of the growth cycle. In the case of *Synechocystis*, the small amounts released suggest that it occurs at a low level and is not a common behaviour. In *Synechocystis*, neither addition of up to 0.5M of sodium chloride, nor glyoxylate, nor altering the light intensity during growth affected release of exopolysaccharides [45] suggesting that leakage occurred rather than an active release process. The components in *Synechocystis* PCC 6803 and PCC 6714 appeared different from other cyanobacteria. Substituent group analysis showed an absence of acetate or pyruvate with some sulfate substituent detected and a rather high protein content [46]. It had been thought that sulfated exopolysaccharide material was only found in eukaryotic algae so its presence in *Synechocystis* is somewhat unusual. Such sulfated material is thought to have antiviral activity and may be a defence strategy in natural environments [32]. Acetate groups have been proposed to hinder cation binding so their absence in *Synechocystis* is of significance in its potential role in metal binding. Extrapolysac‐ charide material with high levels of charged groups (such as sulfated derivatives) are impor‐ tant in forming polymeric matrices [47] with metallic ions and indeed natural clay particles

further accounts for the anionic nature of many such polymers.

56 Advances in Bioremediation of Wastewater and Polluted Soil

being a prerequisite for good metal binding.

**Type of Microbial Compound Chemical Group and biosorption** Peptidoglycan Carboxy groups cation binding Gram positive surface groups Phosphate groups cation binding

Microbial surface proteins Charged Amino acid groups

Algal cellulose Hydroxyl groups Fungal chitins, glucans, mannans Amino groups of chitin

**Table 2.** Types of microbial surface compounds associated with biosorption.

EPS and related polysaccharide components Polysaccharide groups uronic acid and sulfate

Archael glucoproteins Carbohydrate groups + charged amino acids

In a study to determine key genes associated with the production of EPS in *Synechocystis*, four *Synechocystis* PCC 6803 genes, slr1875 (exoD), sll1581 (gumB), sll0923 (gumC) and sll5052 (gumD), sharing sequence homology with non-photosynthetic bacteria (in brackets), were determined [31]. The expression of these genes was shown to be dispensable for cell growth under standard laboratory conditions. In the wild type PCC 6803 strain, analysis of the EPS showed it formed a thick layer that enclosed the cell, while in the slr1875 and sll1581 deletion mutants, this layer decreased as did the tolerance of *Synechocystis* to salt and heavy metal stresses. The surface charge of *Synechocystis* PCC 6803, which plays a major role in cell interactions with other cells or surfaces, was determined by measuring the zeta potential with electrophoretic mobility. The zeta potential of the wild type strain and mutant strains were −33 mV and between −20 mV and −25 mV respectively indicating that the zeta potential can be correlated with the total amount of EPS and the resulting density of ionic surface charges produced by the cells [31].

Genome comparison of *in silico* translated genes from *Synechocystis* and *E. coli* was used to locate genes in *Synechocystis* that may modulate the cell surface moieties. The *Synechocystis* genes slr0977 and slr0982 (located in a cluster of transport genes) were shown [48] to encode homologs of the *E.coli* proteins Wzm and Wzt, the permease and oligosaccharide binding proteins functioning as an ABC-transporter. Mutation of these genes in *Synechocystis* also resulted in flocculating strains with modulated adherence properties and altered EPS.
