Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 118

4.3.2.2 Fibrillar Polysaccharides

In freshwaters, a number of organic compounds, in most cases polysaccharides and
proteins, but also nucleic acids, peptidoglycan, lipids, lignins, and so on are produced
in the water column by exudation (extracellular polymeric substances, EPS) or
degradation of phytoplankton, aquatic bacteria and macrophytes. Multiple environ-
mental factors, operating on a large scale, can impose stress (e.g. nutrient depriva-
tion, high toxicant levels) upon some algal and bacterial species in the environment.
The stress can lead to biological response such as secretion of organic macro-
molecules (biopolymers) to alleviate the stress (Leppard, 1995). EPS represent the
most abundant organic compounds in the biosphere and constitute the largest frac-
tion of cells. They are important in processes such as mineral dissolution (Welch
et al. , 1999), biomineralization (Chan et al. , 2004), sediment stabilization (Dade
et al. , 1990), bacterial adhesion (Marshall et al. , 1989 ), biofi lm formation (Vandevivere
and Kirchman, 1993) and pollutant distribution (Wolfaardt et al. , 1994 ). This section
considers mainly fi brillar polysaccharides. Other biopolymers (e.g. protein and
peptidoglycan) are not considered as they represent a minor fraction of EPS or
have a short turnover time (protein degrades within hours to days and peptidogly-
can degrades with days to weeks of their release into the water column) (Nagatal
et al. , 2003 ; Smith et al. , 1992 ).
Fibrillar polysaccharides can be released from phytoplankton cells during all
stages of growth (Strycek et al. , 1992). Large amounts of polysaccharides are
released during phytoplankton blooms and may comprise 80-90% of the total
extracellular release (Myklestad, 1995). They may represent a signifi cant proportion
of NOM in freshwater, varying seasonally from about 5 to 30% in surface waters
of lakes (Wilkinson et al. , 1997a) and likely account for higher proportions (up to
80%) of NOM in marine systems (Aluwihare et al. , 1997 ; McCarthy et al. , 1998 ;
Santschi et al. , 1998 ; Verdugo et al. , 2004). Polysaccharides are refractory enough to
be found in the deep ocean and have a turnover time of hundreds of years (Guo
and Santschi, 1997). Polysaccharides are generally rigid due to the large quantity
of strongly bound hydration water (up to 80%), their association into double or
triple helices that may be stabilized by hydrogen or calcium bridges or helices
aggregation (Morris et al. , 1980 ; Norton et al. , 1984 ; Rees, 1981 ). Transmission elec-
tron microscopy (TEM) and atomic force microscopy (AFM) analysis of freshwater
and marine polysaccharides suggest that they are a few nanometres in thickness
with a length greater than 1

m and variable conformation as a function of pH and
ionic strength (Leppard et al. , 1990 ; Perret et al. , 1991 ; Santschi et al. , 1998 ).
Fibrils are important in a variety of environmental functions, such as fl oc forma-
tion via bridging mechanisms which enhance particle sedimentation (Buffl e et al. ,
1998), formation of the matrix component of biofi lms and facilitation of microbial
adhesion to surfaces (Leppard, 1997), and binding of metal contaminants (Lamelas
et al. , 2005, 2006; Plette et al. , 1996). Due to the complexity, which is both species-
specifi c and a function of environmental conditions (Myklestad, 1995), and diffi cul-
ties in isolating environmental polysaccharides, the study of their colloidal properties
is usually performed on model polysaccharides purchased from chemical compa-
nies, or ideally produced in bacterial and algal culture within research laboratories

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