208
65% (Thepenier et al. 1985), and keto acid production by Anacystis and Chlorella
is reduced by 70–90% (Wikström et al. 1982).
In most cases, the change in the photosynthetic activity of immobilized phototropic microorganisms is defined by alterations of their illumination as compared with
free-living cells (Lebeau and Robert 2006; Jeanfils and Collard 1983; Robinson
et al. 1986). In Chlorella, no difference in oxygen evolution between free and
immobilized cells was observed (Robinson et al. 1986). In the other studies, the
immobilized cells featured a faster oxygen evolution suggesting a dramatic change
of the cell metabolic activity (Bailliez et al. 1985).
There are different hypotheses explaining the observed alteration of photosynthetic activity in immobilized microalgal cells. Immobilization stabilizes proteinchlorophyll complexes in Botryococcus and Euglena: 90% of chlorophyll was
retained by the immobilized cells after 3 months. Photosynthesis can be enhanced
due to possible increase in the certain ion concentrations in the microenvironment
of immobilized cells (Bailliez et al. 1986). The photosynthetic oxygen evolution
and specific growth rate of suspended and immobilized Scenedesmus dimorphus
cells were compared by Wang et al. (2015). The full illumination in the open pond
happened only for 3-day cultivation, and biomass was very dilute (<20 g m
−3
),
whereas in the attached cultivation system, the full illumination of microalgae lasted
to 10 days, and biomass density reached up to 100 g m
−3
. The higher percentage of
photosynthetically active cells might explain the cases of higher biomass productivity of immobilized microalgae as compared to the free cells.
On one hand, photosynthesis rate decreases if the immobilized cells starve of
light energy due to self-shading or blocking of light by the carrier. On the other
hand, the cells attached to the surface of a carrier or confined in the polymeric gels
can be protected from photodamage if the illumination is in excess. The light supply
to the cultures can be facilitated by usage of optical fibers incorporated into the
polymeric matrix (Matsunaga et al. 1996). Long-term stabilization of the photosynthetic activities measured as chlorophyll fluorescence in chlorophyte (Scenedesmus
and Chlorella) cells has also been reported. Chen (2001) showed that cells of
Scenedesmus quadricauda retain their viability and photosynthetic activity even
after 3 years of storage.
Co-immobilization of microalgae with heterotrophic bacteria also helps to lift
the limitation of photosynthesis in the entrapped microalgal cells by insufficient
CO 2 supply. The co-immobilized bacteria efficiently provide microalgae with the
CO 2 originating from their respiration (Moreno-Garrido 2008). Microalgae-bacteria
associations have already been shown to benefit from each other, with microalgae
cells producing oxygen and some organic compounds that are consumed by heterotrophic bacteria. Bacteria can also excrete vitamins and hormones that can enhance
the algal growth (De-Bashan et al. 2005; Gonzalez and Bashan 2000). Mouget et al.
(1995) also proved that Pseudomonas diminuta and Pseudomonas vesicularis bacterial cells isolated from the cultures of Scenedesmus bicellularis and Chlorella sp.
stimulated the growth of the corresponding microalgae.
As it was shown for bacteria, the process of immobilization is able to modify the
membrane composition and permeability by inducing a shift of the membrane lipid
S. Vasilieva et al.
65% (Thepenier et al. 1985), and keto acid production by Anacystis and Chlorella
is reduced by 70–90% (Wikström et al. 1982).
In most cases, the change in the photosynthetic activity of immobilized phototropic microorganisms is defined by alterations of their illumination as compared with
free-living cells (Lebeau and Robert 2006; Jeanfils and Collard 1983; Robinson
et al. 1986). In Chlorella, no difference in oxygen evolution between free and
immobilized cells was observed (Robinson et al. 1986). In the other studies, the
immobilized cells featured a faster oxygen evolution suggesting a dramatic change
of the cell metabolic activity (Bailliez et al. 1985).
There are different hypotheses explaining the observed alteration of photosynthetic activity in immobilized microalgal cells. Immobilization stabilizes proteinchlorophyll complexes in Botryococcus and Euglena: 90% of chlorophyll was
retained by the immobilized cells after 3 months. Photosynthesis can be enhanced
due to possible increase in the certain ion concentrations in the microenvironment
of immobilized cells (Bailliez et al. 1986). The photosynthetic oxygen evolution
and specific growth rate of suspended and immobilized Scenedesmus dimorphus
cells were compared by Wang et al. (2015). The full illumination in the open pond
happened only for 3-day cultivation, and biomass was very dilute (<20 g m
−3
),
whereas in the attached cultivation system, the full illumination of microalgae lasted
to 10 days, and biomass density reached up to 100 g m
−3
. The higher percentage of
photosynthetically active cells might explain the cases of higher biomass productivity of immobilized microalgae as compared to the free cells.
On one hand, photosynthesis rate decreases if the immobilized cells starve of
light energy due to self-shading or blocking of light by the carrier. On the other
hand, the cells attached to the surface of a carrier or confined in the polymeric gels
can be protected from photodamage if the illumination is in excess. The light supply
to the cultures can be facilitated by usage of optical fibers incorporated into the
polymeric matrix (Matsunaga et al. 1996). Long-term stabilization of the photosynthetic activities measured as chlorophyll fluorescence in chlorophyte (Scenedesmus
and Chlorella) cells has also been reported. Chen (2001) showed that cells of
Scenedesmus quadricauda retain their viability and photosynthetic activity even
after 3 years of storage.
Co-immobilization of microalgae with heterotrophic bacteria also helps to lift
the limitation of photosynthesis in the entrapped microalgal cells by insufficient
CO 2 supply. The co-immobilized bacteria efficiently provide microalgae with the
CO 2 originating from their respiration (Moreno-Garrido 2008). Microalgae-bacteria
associations have already been shown to benefit from each other, with microalgae
cells producing oxygen and some organic compounds that are consumed by heterotrophic bacteria. Bacteria can also excrete vitamins and hormones that can enhance
the algal growth (De-Bashan et al. 2005; Gonzalez and Bashan 2000). Mouget et al.
(1995) also proved that Pseudomonas diminuta and Pseudomonas vesicularis bacterial cells isolated from the cultures of Scenedesmus bicellularis and Chlorella sp.
stimulated the growth of the corresponding microalgae.
As it was shown for bacteria, the process of immobilization is able to modify the
membrane composition and permeability by inducing a shift of the membrane lipid
S. Vasilieva et al.