**1. Introduction**

Heavy metals because of their chemical nature cannot be biodegraded by microorganisms to non-toxic species and therefore build up in the environment. Many metals undergo a change in chemical state from one form to another but ultimately they accumulate in the environment and potentially enter the human food chain through uptake by plants or animals. Removal of metals by chemical technologies has been widely used but has proven expensive or inappro‐ priate in the case of low level metal contamination. Thus attention has focussed on newer technologies such as metal biosorption as an alternative to chemical removal.

Biosorption can simply be defined as 'the removal of substances from solution by biological material' [1]. The process is energy independent and differs from bioaccumulation which is an energy dependent transport process associated with accumulation or transport of a metal into the cell. Biomaterials and particularly biomass have a bioaffinity for metals via a number of different physico-chemical interactions with the metal. These include sorption (ad- and ab-), ion exchange and surface complexation and precipitation. There has been a large increase in published work on biosorption but so far little by way of exploitation of the process on a large scale other than by traditional sewage treatment methodologies [1]. Most biological material either living or dead can biosorb a variety of materials including metals with the vast majority of the sorption being adsorption to surface groups associated with the particular biological material. Thus far there appears to be no clear winner in terms of the best candidate as a biomass material although many bacteria and algae including cyanobacteria have been examined.

Within the domain bacteria, cyanobacteria are the only organisms to carry out oxygenic photosynthesis and are phylogenetically most closely related to gram positive microorganisms [2]. Amongst the many thousands of genera of cyanobacteria, a number of model organisms have emerged. Amongst these is *Synechocystis* which is classified within the Phyllum Cyano‐ bacteria and is a member of the Order Chroococcales. The genus *Synechocystis* was originally described as a botanical taxon [3], with the type species being *S. aquatilis*. Members of the genera *Synechocystis* are non-nitrogen fixing, unicellular cyanobacteria which primarily inhabit fresh or marine water environments. As shown in (Table 1), more than 20 species have been characterised within the genera *Synechocystis*. These have been sub grouped on the basis of sequence and GC content into 3 groups, a high and low GC freshwater group, and a marine group [4]. *Synechocystis* PCC 6803 [5, 2] belongs to the marine group although originally isolated from a freshwater lake in the US.

*Synechocystis* PCC 6803 is naturally transformable [6] and can grow heterotrophically on glucose [7]. These characteristics make it of interest as a model cyanobacterium for genetic manipulation. The original PCC 6803 strain, designated the Kunisawa strain was isolated in Berkley in 1968 [8], and since then there have been a number of sub-strains derived from this original strain including Kazusa (non-transformable), China (increased settling), Amsterdam, Mu and a number of New Zealand derivatives based on individual lab publication and morphological variation. All strains (original and sub strains) demonstrate 16s rDNA sequence identity but differ phenotypically in certain traits. Phenotypic differences include increased settleability, motility differences, sensitivity to glucose, propagation rate and transformability [2]. Indeed, recent sequencing of a number of these 'sub'strains has revealed a small number of single nucleotide polymorphisms that may be responsible for the number of phenotypic differences observed. This phenomenon in *Synechocystis* has been termed microevolution [2].

**1. Introduction**

52 Advances in Bioremediation of Wastewater and Polluted Soil

examined.

isolated from a freshwater lake in the US.

Heavy metals because of their chemical nature cannot be biodegraded by microorganisms to non-toxic species and therefore build up in the environment. Many metals undergo a change in chemical state from one form to another but ultimately they accumulate in the environment and potentially enter the human food chain through uptake by plants or animals. Removal of metals by chemical technologies has been widely used but has proven expensive or inappro‐ priate in the case of low level metal contamination. Thus attention has focussed on newer

Biosorption can simply be defined as 'the removal of substances from solution by biological material' [1]. The process is energy independent and differs from bioaccumulation which is an energy dependent transport process associated with accumulation or transport of a metal into the cell. Biomaterials and particularly biomass have a bioaffinity for metals via a number of different physico-chemical interactions with the metal. These include sorption (ad- and ab-), ion exchange and surface complexation and precipitation. There has been a large increase in published work on biosorption but so far little by way of exploitation of the process on a large scale other than by traditional sewage treatment methodologies [1]. Most biological material either living or dead can biosorb a variety of materials including metals with the vast majority of the sorption being adsorption to surface groups associated with the particular biological material. Thus far there appears to be no clear winner in terms of the best candidate as a biomass material although many bacteria and algae including cyanobacteria have been

Within the domain bacteria, cyanobacteria are the only organisms to carry out oxygenic photosynthesis and are phylogenetically most closely related to gram positive microorganisms [2]. Amongst the many thousands of genera of cyanobacteria, a number of model organisms have emerged. Amongst these is *Synechocystis* which is classified within the Phyllum Cyano‐ bacteria and is a member of the Order Chroococcales. The genus *Synechocystis* was originally described as a botanical taxon [3], with the type species being *S. aquatilis*. Members of the genera *Synechocystis* are non-nitrogen fixing, unicellular cyanobacteria which primarily inhabit fresh or marine water environments. As shown in (Table 1), more than 20 species have been characterised within the genera *Synechocystis*. These have been sub grouped on the basis of sequence and GC content into 3 groups, a high and low GC freshwater group, and a marine group [4]. *Synechocystis* PCC 6803 [5, 2] belongs to the marine group although originally

*Synechocystis* PCC 6803 is naturally transformable [6] and can grow heterotrophically on glucose [7]. These characteristics make it of interest as a model cyanobacterium for genetic manipulation. The original PCC 6803 strain, designated the Kunisawa strain was isolated in Berkley in 1968 [8], and since then there have been a number of sub-strains derived from this original strain including Kazusa (non-transformable), China (increased settling), Amsterdam, Mu and a number of New Zealand derivatives based on individual lab publication and morphological variation. All strains (original and sub strains) demonstrate 16s rDNA sequence identity but differ phenotypically in certain traits. Phenotypic differences include increased

technologies such as metal biosorption as an alternative to chemical removal.


**Table 1.** *Synechocystis* strains as designated in the AlgaeBase [9] with the discoverer and the year of discovery when known. In some instances these strains may be called after the discoverer such that it is not uncommon to have the 'Sauvageau strain' for *S. aquatilis*.

Cyanobacteria and in particular *Synechocystis* species, have recently received much attention as potential cell factories for the production of a wide variety of compounds of biotechnological interest. Such compounds include isobutanol, [10], 1,2-propanediol [11], isopropanol [12], 2,3 butanediol [13], ethanol [14], 3-hydroxybutyrate [15], free fatty acids [16], fatty alcohols [17], endogenously produced alka(e)nes [15], carotinoids [18], sesquiterpene β-caryophyllene [19], isoprene [20], squalene [21], poly-β-hydroxybutyrate (PHB) [22], polyhydroxyalkanoate [23], ethylene [24], cellulose [25], sucrose and glucose/fructose carbon substrates [26], mannitol [27], lactic acid [28], acetone [29] and H2 [30]. Given the large interest in experimental production systems using *Synechocystis*, there has been an equivalent interest in utilising the biomass produced to add to the economics and increase the industrial potential of such systems. Biosorption of metals to various biomass types has emerged as one potential candidate for biomass utilisation following its use in other production processes. However, although there have been a wide range of biological materials studied [1], there is no clear best candidate as yet. Biomass with Gram negative peptidoglycan or Gram positive surface phosphate groups have clear advantages, while bacterial surface S-layers, proteins and sheaths all contribute to binding, making microbial candidates important. Attention has focussed on the potential of many cyanobacterial species for bioremediation, in particular, the model organism *Synecho‐ cystis*, given its potential availability from engineered production systems as discussed above. The *Synechocystis* genus is of interest in this respect because of its natural freshwater or marine habitat. In addition *Synechocystis* produces EPS (Extrapolysaccharide Substances) which can be either cell attached or released exopolysaccharides (RPS) [31]. This EPS can biosorb a range of material including metals which can be produced as a by-product of other biotechnological processes at scale. Thus while there may be many candidates for biomass biosorption, the sorption efficiency, scale and economics may well be key factors in adoption of a particular technology or species for an *in situ* industrial process.
