200
7.3.2 Passive Immobilization
Many species of microalgae have a natural ability to attach to solid surfaces and
propagate on them (Robinson et al. 1986). The optimal carrier for passive attachment of microalgae should have a high affinity toward the surface structures of
microorganisms and cause minimal damage.
Reportedly, the passive attachment of cells to inorganic or organic carrier materials is achieved via chemical (covalent binding) or physical (ionic, van der Waals,
electrostatic, hydrophobic) interactions (Sinitsin et al. 1994). It is expected that the
adhesion behavior of viable cells is affected by chemical and physical properties of
the carrier matrix (Characklis et al. 1990). An increased attachment of microorganisms to rough materials as compared to smooth ones was revealed and explained by
physical phenomena, e.g., the increased convection for rough surface in aquatic
media. The cracks and holes on the surface create zones with slower liquid velocity,
so the algal cells can settle on and attach to the surface (Cao et al. 2009). Rough
texture also decreases the shear forces directed on the immobilized cells (Cui et al.
2014). Moreover, the rough surfaces with holes, cracks, and protuberances provide
more area for microalgae immobilization when compared to smooth polymeric
materials. Particularly, the water-carrier material contact angle (Irving and Allen
2011) and surface energy (Christenson and Sims 2012; Genin et al. 2014) have been
widely researched for their influence on the process of cell attachment.
Microalgae cell attachment mechanism is thought to be different in different
groups of microalgae (Sekar et al. 2004). The cell attachment is more efficient in
microorganisms possessing different surface structures such as flagella, pili, epistructures (Dufrêne 2015; Vadillo-Rodríguez et al. 2004). In this case, the repulsive
force between the cells and a carrier decreases due to reduction of the initial contact
area (Sinitsin et al. 1994). Tosteson and Corpe (1975) reported that the attachment
ability of algal cells depends on their capacity to excrete adhesive substances such
as exopolysaccharides. Woods and Fletcher (1991) studying the attachment of four
marine diatoms found that the variation in rates and strength of adhesion was due to
the cell’s capability of rapid synthesis and excretion of exopolysaccharides. Most of
the diatoms attach to the solid surface by the production of extracellular polymeric
substances in the form of stalks, apical pads, mucilage pads, and cell coatings
(Hoagland et al. 1993). It is shown that the lack of nutrients in media triggered the
secretion of extracellular polymer by microorganisms resulting in the increasing of
their immobilization efficiency (Orvain et al. 2003; Zhang et al. 2014). It was
revealed in the recent study (Kreis et al. 2018) that the unspecific adhesion of
Chlamydomonas reinhardtii to surfaces can be reversibly triggered by switched-on
and switched-off illumination in the cultivation system.
Effects of environmental and/or physiological conditions on the attachment
mechanisms of different microalgae have been intensively studied (Sekar et al.
2004). Reportedly, the attachment of Navicula amphibia was directly proportional
to the culture density. The growth stage of microalgae and cultural media composition are also shown to be important for the cell attachment ability. Zaidi and Tosteson
S. Vasilieva et al.
7.3.2 Passive Immobilization
Many species of microalgae have a natural ability to attach to solid surfaces and
propagate on them (Robinson et al. 1986). The optimal carrier for passive attachment of microalgae should have a high affinity toward the surface structures of
microorganisms and cause minimal damage.
Reportedly, the passive attachment of cells to inorganic or organic carrier materials is achieved via chemical (covalent binding) or physical (ionic, van der Waals,
electrostatic, hydrophobic) interactions (Sinitsin et al. 1994). It is expected that the
adhesion behavior of viable cells is affected by chemical and physical properties of
the carrier matrix (Characklis et al. 1990). An increased attachment of microorganisms to rough materials as compared to smooth ones was revealed and explained by
physical phenomena, e.g., the increased convection for rough surface in aquatic
media. The cracks and holes on the surface create zones with slower liquid velocity,
so the algal cells can settle on and attach to the surface (Cao et al. 2009). Rough
texture also decreases the shear forces directed on the immobilized cells (Cui et al.
2014). Moreover, the rough surfaces with holes, cracks, and protuberances provide
more area for microalgae immobilization when compared to smooth polymeric
materials. Particularly, the water-carrier material contact angle (Irving and Allen
2011) and surface energy (Christenson and Sims 2012; Genin et al. 2014) have been
widely researched for their influence on the process of cell attachment.
Microalgae cell attachment mechanism is thought to be different in different
groups of microalgae (Sekar et al. 2004). The cell attachment is more efficient in
microorganisms possessing different surface structures such as flagella, pili, epistructures (Dufrêne 2015; Vadillo-Rodríguez et al. 2004). In this case, the repulsive
force between the cells and a carrier decreases due to reduction of the initial contact
area (Sinitsin et al. 1994). Tosteson and Corpe (1975) reported that the attachment
ability of algal cells depends on their capacity to excrete adhesive substances such
as exopolysaccharides. Woods and Fletcher (1991) studying the attachment of four
marine diatoms found that the variation in rates and strength of adhesion was due to
the cell’s capability of rapid synthesis and excretion of exopolysaccharides. Most of
the diatoms attach to the solid surface by the production of extracellular polymeric
substances in the form of stalks, apical pads, mucilage pads, and cell coatings
(Hoagland et al. 1993). It is shown that the lack of nutrients in media triggered the
secretion of extracellular polymer by microorganisms resulting in the increasing of
their immobilization efficiency (Orvain et al. 2003; Zhang et al. 2014). It was
revealed in the recent study (Kreis et al. 2018) that the unspecific adhesion of
Chlamydomonas reinhardtii to surfaces can be reversibly triggered by switched-on
and switched-off illumination in the cultivation system.
Effects of environmental and/or physiological conditions on the attachment
mechanisms of different microalgae have been intensively studied (Sekar et al.
2004). Reportedly, the attachment of Navicula amphibia was directly proportional
to the culture density. The growth stage of microalgae and cultural media composition are also shown to be important for the cell attachment ability. Zaidi and Tosteson
S. Vasilieva et al.