206
limitation. Generally, cell division rate was higher in the case of surface immobilization than for entrapped cells.
Adsorptive microalgae immobilization results in maximum interface area
between the attached cell surface and the liquid medium resulting in the lowest
mass transfer limitation. In case of cell entrapment, molecular diffusion of compounds through an immobilized cell matrix can be facilitated by the use of microstructured immobilization matrix as well as by capillary or active transport (Eroglu
et al. 2015).
The process of polymer-based entrapment can impose harsh stress on microalgae
reducing immobilized cell population, although this effect is species-specific
(De-Bashan and Bashan 2010; De-Bashan et al. 2005). Thus, the cells of Skeletonema
costatum and Heterocapsa sp. did not propagate in alginate beads, while the growth
rate of other microalgae entrapped in alginate was similar to suspended cells
(Moreno-Garrido et al. 2005). Other modifications in the immobilized cell microenvironment than nutrient/oxygen depletion include accumulation of toxic waste
products and buildup of osmotic pressure (Inanç et al. 1996) that might explain the
reduced growth rate for immobilized microalgae as well.
In certain cases, immobilized microalgae showed faster cell division rate than
suspended cultures. This was the case when Dunaliella bardawil, Pavlova lutheri,
Chlorella minutissima, and Haematococcus pluvialis were entrapped in 2% carboxymethylcellulose gel (Joo et al. 2001). The growth-promoting action of immobilization has been explained by protective effects of the support materials during
cultivation of microalgae in unfavorable conditions. The diffusion resistance in case
of gel-entrapped cells may be also beneficial by decreasing of the local concentration of inhibiting substrates. On the contrary, immobilization may provide a better
bioavailability of nutrients in oligotrophic conditions by their concentration near the
carrier.
It should be noted that the culture growth is often estimated via chlorophyll accumulation rate, which is able to increase in attached cells as compared to suspended
ones, possibly as an adaption to self-shading. Therefore, chlorophyll measurements
would probably overestimate cell number and thus lead to a certain error in the
estimation of immobilized cell productivity.
Alterations in size and shape of attached or entrapped microalgae are also extensively recorded in the literature (Cassidy et al. 1996; Bailliez et al. 1986) as well as
changes in size of trichomes and microalgal colonies (Mallick 2002). Immobilized
colonies of Botryococcus were found to be 2.5 times larger than those of free-living
cell colonies (Chevalier and de la Noue 1985), while the cells of Chlorella entrapped
in calcium alginate tend to form small colonies (8–30 cells) (Trevan and Mak 1988).
Immobilization increases tolerance of microalgae to changes in temperature, pH,
or ionic strength of the cell surroundings (Sinitsin et al. 1994). For example, the
chitosan-immobilized cells of Synechococcus sp. have higher resistance to NaOH as
compared to the suspended cells (Aguilar-May et al. 2007). Immobilized cultures
better withstand different toxicants. Thus, toxicity of chromium and nickel ions to
the cells of a nitrogen-fixing cyanobacterium Aulosira fertilissima was reduced considerably upon their entrapment in alginate beads (Banerjee et al. 2004). This effect
S. Vasilieva et al.
limitation. Generally, cell division rate was higher in the case of surface immobilization than for entrapped cells.
Adsorptive microalgae immobilization results in maximum interface area
between the attached cell surface and the liquid medium resulting in the lowest
mass transfer limitation. In case of cell entrapment, molecular diffusion of compounds through an immobilized cell matrix can be facilitated by the use of microstructured immobilization matrix as well as by capillary or active transport (Eroglu
et al. 2015).
The process of polymer-based entrapment can impose harsh stress on microalgae
reducing immobilized cell population, although this effect is species-specific
(De-Bashan and Bashan 2010; De-Bashan et al. 2005). Thus, the cells of Skeletonema
costatum and Heterocapsa sp. did not propagate in alginate beads, while the growth
rate of other microalgae entrapped in alginate was similar to suspended cells
(Moreno-Garrido et al. 2005). Other modifications in the immobilized cell microenvironment than nutrient/oxygen depletion include accumulation of toxic waste
products and buildup of osmotic pressure (Inanç et al. 1996) that might explain the
reduced growth rate for immobilized microalgae as well.
In certain cases, immobilized microalgae showed faster cell division rate than
suspended cultures. This was the case when Dunaliella bardawil, Pavlova lutheri,
Chlorella minutissima, and Haematococcus pluvialis were entrapped in 2% carboxymethylcellulose gel (Joo et al. 2001). The growth-promoting action of immobilization has been explained by protective effects of the support materials during
cultivation of microalgae in unfavorable conditions. The diffusion resistance in case
of gel-entrapped cells may be also beneficial by decreasing of the local concentration of inhibiting substrates. On the contrary, immobilization may provide a better
bioavailability of nutrients in oligotrophic conditions by their concentration near the
carrier.
It should be noted that the culture growth is often estimated via chlorophyll accumulation rate, which is able to increase in attached cells as compared to suspended
ones, possibly as an adaption to self-shading. Therefore, chlorophyll measurements
would probably overestimate cell number and thus lead to a certain error in the
estimation of immobilized cell productivity.
Alterations in size and shape of attached or entrapped microalgae are also extensively recorded in the literature (Cassidy et al. 1996; Bailliez et al. 1986) as well as
changes in size of trichomes and microalgal colonies (Mallick 2002). Immobilized
colonies of Botryococcus were found to be 2.5 times larger than those of free-living
cell colonies (Chevalier and de la Noue 1985), while the cells of Chlorella entrapped
in calcium alginate tend to form small colonies (8–30 cells) (Trevan and Mak 1988).
Immobilization increases tolerance of microalgae to changes in temperature, pH,
or ionic strength of the cell surroundings (Sinitsin et al. 1994). For example, the
chitosan-immobilized cells of Synechococcus sp. have higher resistance to NaOH as
compared to the suspended cells (Aguilar-May et al. 2007). Immobilized cultures
better withstand different toxicants. Thus, toxicity of chromium and nickel ions to
the cells of a nitrogen-fixing cyanobacterium Aulosira fertilissima was reduced considerably upon their entrapment in alginate beads (Banerjee et al. 2004). This effect
S. Vasilieva et al.