205
tricornutum, C. vulgaris, and Botryococcus braunii resulted in 90% of flocculation
efficiency.
However, co-cultivation of microalgae with bacteria used in this kind of flocculation approaches results in contamination of the biomass precluding its applications
for food (Vandamme et al. 2012). However, the added microorganisms may contribute to the increase in lipid yields and fatty acid contents in microalgae cells, and this
is beneficial for biofuel application of the biomass (Salim et al. 2011; Chen
et al. 2011).
It was demonstrated that algal-fungal flocculation is the efficient way of planktonic algal cell harvesting (Muradov et al. 2015), but the subsequent separation of
the fungi and algal cells challenges its scale-up process. Algal-algal-based flocculation does not demand the separation process of the harvested biomass, and energy
consumption is negligible (Alam et al. 2014). Recently, the naturally flocculating
diatom Skeletonema was used to form flocs of Nannochloropsis (Schenk et al.
2008). Autoflocculating microalgae, e.g., Ankistrodesmus falcatus, Scenedesmus
obliquus, or the marine species Tetraselmis suecica, can be used for harvesting nonflocculating microalga. In comparison with other applied bio-flocculation techniques, the algal-algal self-immobilization does not require different cultivation
conditions and prevents unwanted contaminations (Salim et al. 2011).
7.4 Physiology of the Immobilized Microalgal Cells
Over the last decade, numerous studies have identified regulatory pathways and
genetic responses of the heterotrophic bacteria and yeast immobilized at carrier
surfaces and within biofilms (Kuchma and O’Toole 2000), but corresponding
reports for microalgae are obviously lacking in the literature. Most of the published
reports are focused on the physiological responses which are considered in this
section.
The cells in naturally occurring immobilized systems such as biofilms (Coserton
et al., 1995) display a variety of physiological changes as compared to free cells of
the same species. Part of these modifications starts during the first stages of immobilization following the cell-support contact, where microorganisms develop
surface- sensing responses (O’Toole et al. 2000). The physiology of living algal cells
upon immobilization changes because the attached or entrapped microalgae cells
are subjected to a different microenvironment compared with the free-living cells
(Junter et al. 2002).
Growth rate is probably the most studied parameter of immobilized microalgae
indicative of the metabolic processes within these systems. Contradictory results
have been published, showing decreased (Moreno-Garrido et al. 2005; Pane et al.
1998; Robinson et al. 1986; Hameed and Ebrahim 2007) or enhanced growth rates
(Aguilar-May et al. 2007; Joo et al. 2001; Rai and Mallick 1992; Huang et al. 2000)
of immobilized microalgae as compared to the free cells. The most evident hypothesis to explain a decrease in the growth rate of the immobilized cells is mass transfer
7 Biotechnological Applications of Immobilized Microalgae
tricornutum, C. vulgaris, and Botryococcus braunii resulted in 90% of flocculation
efficiency.
However, co-cultivation of microalgae with bacteria used in this kind of flocculation approaches results in contamination of the biomass precluding its applications
for food (Vandamme et al. 2012). However, the added microorganisms may contribute to the increase in lipid yields and fatty acid contents in microalgae cells, and this
is beneficial for biofuel application of the biomass (Salim et al. 2011; Chen
et al. 2011).
It was demonstrated that algal-fungal flocculation is the efficient way of planktonic algal cell harvesting (Muradov et al. 2015), but the subsequent separation of
the fungi and algal cells challenges its scale-up process. Algal-algal-based flocculation does not demand the separation process of the harvested biomass, and energy
consumption is negligible (Alam et al. 2014). Recently, the naturally flocculating
diatom Skeletonema was used to form flocs of Nannochloropsis (Schenk et al.
2008). Autoflocculating microalgae, e.g., Ankistrodesmus falcatus, Scenedesmus
obliquus, or the marine species Tetraselmis suecica, can be used for harvesting nonflocculating microalga. In comparison with other applied bio-flocculation techniques, the algal-algal self-immobilization does not require different cultivation
conditions and prevents unwanted contaminations (Salim et al. 2011).
7.4 Physiology of the Immobilized Microalgal Cells
Over the last decade, numerous studies have identified regulatory pathways and
genetic responses of the heterotrophic bacteria and yeast immobilized at carrier
surfaces and within biofilms (Kuchma and O’Toole 2000), but corresponding
reports for microalgae are obviously lacking in the literature. Most of the published
reports are focused on the physiological responses which are considered in this
section.
The cells in naturally occurring immobilized systems such as biofilms (Coserton
et al., 1995) display a variety of physiological changes as compared to free cells of
the same species. Part of these modifications starts during the first stages of immobilization following the cell-support contact, where microorganisms develop
surface- sensing responses (O’Toole et al. 2000). The physiology of living algal cells
upon immobilization changes because the attached or entrapped microalgae cells
are subjected to a different microenvironment compared with the free-living cells
(Junter et al. 2002).
Growth rate is probably the most studied parameter of immobilized microalgae
indicative of the metabolic processes within these systems. Contradictory results
have been published, showing decreased (Moreno-Garrido et al. 2005; Pane et al.
1998; Robinson et al. 1986; Hameed and Ebrahim 2007) or enhanced growth rates
(Aguilar-May et al. 2007; Joo et al. 2001; Rai and Mallick 1992; Huang et al. 2000)
of immobilized microalgae as compared to the free cells. The most evident hypothesis to explain a decrease in the growth rate of the immobilized cells is mass transfer
7 Biotechnological Applications of Immobilized Microalgae