224 Marine Macro- and Microalgae: An Overview
et al. 2006). Due to low solubility of CO 2 in water, a high mass-liquid transfer is needed for suitable
CO 2 supply and to stripe dissolved oxygen—which may cause also harmful effects on microalga cells
(Gouveia 2011; Kumar et al. 2011).
As dinoflagellates are so sensitive, agitation and mixing become major issues—because a compromise
is to be reached between slow growth rates by inhibition due to shear forces and increased speed of
growth by improvement of transport features in the bioreactor. The situation becomes more complex
when the (sensitive) cells to be cultivated are photoautotrophic and require illumination for their growth.
In this scenario, mixing and thus fluid dynamics acquire extraordinary relevance, as they are responsible
for transport of cells from less illuminated areas in the center of the photobioreactor to areas near the
surface that are well lit. Hence, the massive and productive cultivation of delicate photoautotrophic cells
may present a triple restriction associated with stirring: cell damage by shear forces, nutrient restriction
due to poor mixing, and poor internal illumination (also due to insufficient mixing).
The sensitivity threshold of microorganisms grown under such aggressive conditions may vary
according to species, or even strain (Gallardo-Rodríguez et al. 2012a). Such phenomena trigger a prompt
range of physiological effects on dinoflagellate cells: from bioluminescence (Chen et al. 2003) to cell
membrane fluidization, production of peroxisomes and reactive oxygen species to interference in calcium
mobilization, morphology alteration (e.g., increasing in cell size), alteration of cell cycle and inhibition
of cell proliferation/growth to metabolite synthesis (Jaouen et al. 1999; Gallardo Rodríguez et al. 2009).
The actual mechanisms by which cells are damaged, or turn susceptible to shear as a result of
hydrodynamic forces are not exactly understood (Hu et al. 2011). The molecular basis remains unclear—
as it is hypothesized that the cortical cytoskeleton (a particular apparatus present on dinoflagellate
cell) may be implicated in the transmission of mechanical stimuli relevant to arrest cell cycle and/or
growth inhibition (Wong and Kwok 2005; Gallardo-Rodríguez et al. 2015). Essentially due to the large
nucleus that occupies half of the volume of cell, there is an increasing probability of transduction of
external mechanical forces to the DNA in the cells. In addition, chromosomes are attached to the nuclear
envelope—and this can potentiate susceptibility to mechanical stimuli, via surface-adhesion receptors
associated to cytoskeleton. For that reason, external mechanical alterations may entail changes in DNA
conformation—and ultimately gene expression alterations (Alam et al. 2014).
The intensity of cell responses to shear stress, and ultimately cell damage, depends on magnitude,
duration, frequency of exposure to shear field, and how the experimental set up is established to generate
the motion regimes (Berdalet et al. 2007). Surprisingly, some responses can be attenuated, or even reversed
in specific conditions without causing cell damage or death (Yeung and Wong 2003). An interesting
study with Protoceratium reticulatum, subjected to “lethal” agitation, reported increase in cell membrane
fluidity within a few minutes—yet the effect was totally reversed within a few hours by stopping agitation
(Gallardo-Rodríguez et al. 2012b). Similar effects of restoration of normal cell characteristics were also
reported in Alexandrium minutum, Prorocentrum triestinum, and Akashiwo sanguinea (Berdalet et al.
2007). At lower levels of turbulence, the cell cycle can be affected—but instead of arrested, it can merely
progress in a slower way (and without causing any cell damage) (Yeung and Wong 2003). The magnitude
of shear stress also appears to influence production of toxins in at least some dinoflagellates (Juhl et
al. 2001; Gallardo-Rodríguez et al. 2011). Juhl et al. have demonstrated that quantified shear toxin
concentration per cell can increase up to three fold that of control cultures in Alexandrium fundyense, a
PSP producer (Juhl et al. 2001). In addition, cell shear tolerance is also apparently influenced by light/
dark cycle. It is plausible that dinoflagellates do not withstand shear forces during the dark cycle because
of the synchronizing cell division during that time (García-Camacho et al. 2007).
No clear link has been established between flow-sensitivity, and size, shape and taxonomy of
dinoflagellate species (Sullivan et al. 2003; Berdalet et al. 2007). Dinoflagellates generally have large
cells—up to 2 mm in diameter. In photosynthetic species associated to HABs, the size ranges from ca. 10
up to 60 μm. Some species are thecate, meaning that they have a cell wall of cellulosic plates. “Armored”
species had been suggested as less susceptible to turbulent conditions than “naked” ones. Theca was
speculated to conferr a sort of protection against shear (Smayda 2010). However, this hypothesis is hardly
supported because both thecate and athecate species are susceptible to turbulent conditions (e.g., White
1976; Berdalet 1992; Juhl et al. 2001; Sullivan et al. 2003). Even among thecate species, difference
et al. 2006). Due to low solubility of CO 2 in water, a high mass-liquid transfer is needed for suitable
CO 2 supply and to stripe dissolved oxygen—which may cause also harmful effects on microalga cells
(Gouveia 2011; Kumar et al. 2011).
As dinoflagellates are so sensitive, agitation and mixing become major issues—because a compromise
is to be reached between slow growth rates by inhibition due to shear forces and increased speed of
growth by improvement of transport features in the bioreactor. The situation becomes more complex
when the (sensitive) cells to be cultivated are photoautotrophic and require illumination for their growth.
In this scenario, mixing and thus fluid dynamics acquire extraordinary relevance, as they are responsible
for transport of cells from less illuminated areas in the center of the photobioreactor to areas near the
surface that are well lit. Hence, the massive and productive cultivation of delicate photoautotrophic cells
may present a triple restriction associated with stirring: cell damage by shear forces, nutrient restriction
due to poor mixing, and poor internal illumination (also due to insufficient mixing).
The sensitivity threshold of microorganisms grown under such aggressive conditions may vary
according to species, or even strain (Gallardo-Rodríguez et al. 2012a). Such phenomena trigger a prompt
range of physiological effects on dinoflagellate cells: from bioluminescence (Chen et al. 2003) to cell
membrane fluidization, production of peroxisomes and reactive oxygen species to interference in calcium
mobilization, morphology alteration (e.g., increasing in cell size), alteration of cell cycle and inhibition
of cell proliferation/growth to metabolite synthesis (Jaouen et al. 1999; Gallardo Rodríguez et al. 2009).
The actual mechanisms by which cells are damaged, or turn susceptible to shear as a result of
hydrodynamic forces are not exactly understood (Hu et al. 2011). The molecular basis remains unclear—
as it is hypothesized that the cortical cytoskeleton (a particular apparatus present on dinoflagellate
cell) may be implicated in the transmission of mechanical stimuli relevant to arrest cell cycle and/or
growth inhibition (Wong and Kwok 2005; Gallardo-Rodríguez et al. 2015). Essentially due to the large
nucleus that occupies half of the volume of cell, there is an increasing probability of transduction of
external mechanical forces to the DNA in the cells. In addition, chromosomes are attached to the nuclear
envelope—and this can potentiate susceptibility to mechanical stimuli, via surface-adhesion receptors
associated to cytoskeleton. For that reason, external mechanical alterations may entail changes in DNA
conformation—and ultimately gene expression alterations (Alam et al. 2014).
The intensity of cell responses to shear stress, and ultimately cell damage, depends on magnitude,
duration, frequency of exposure to shear field, and how the experimental set up is established to generate
the motion regimes (Berdalet et al. 2007). Surprisingly, some responses can be attenuated, or even reversed
in specific conditions without causing cell damage or death (Yeung and Wong 2003). An interesting
study with Protoceratium reticulatum, subjected to “lethal” agitation, reported increase in cell membrane
fluidity within a few minutes—yet the effect was totally reversed within a few hours by stopping agitation
(Gallardo-Rodríguez et al. 2012b). Similar effects of restoration of normal cell characteristics were also
reported in Alexandrium minutum, Prorocentrum triestinum, and Akashiwo sanguinea (Berdalet et al.
2007). At lower levels of turbulence, the cell cycle can be affected—but instead of arrested, it can merely
progress in a slower way (and without causing any cell damage) (Yeung and Wong 2003). The magnitude
of shear stress also appears to influence production of toxins in at least some dinoflagellates (Juhl et
al. 2001; Gallardo-Rodríguez et al. 2011). Juhl et al. have demonstrated that quantified shear toxin
concentration per cell can increase up to three fold that of control cultures in Alexandrium fundyense, a
PSP producer (Juhl et al. 2001). In addition, cell shear tolerance is also apparently influenced by light/
dark cycle. It is plausible that dinoflagellates do not withstand shear forces during the dark cycle because
of the synchronizing cell division during that time (García-Camacho et al. 2007).
No clear link has been established between flow-sensitivity, and size, shape and taxonomy of
dinoflagellate species (Sullivan et al. 2003; Berdalet et al. 2007). Dinoflagellates generally have large
cells—up to 2 mm in diameter. In photosynthetic species associated to HABs, the size ranges from ca. 10
up to 60 μm. Some species are thecate, meaning that they have a cell wall of cellulosic plates. “Armored”
species had been suggested as less susceptible to turbulent conditions than “naked” ones. Theca was
speculated to conferr a sort of protection against shear (Smayda 2010). However, this hypothesis is hardly
supported because both thecate and athecate species are susceptible to turbulent conditions (e.g., White
1976; Berdalet 1992; Juhl et al. 2001; Sullivan et al. 2003). Even among thecate species, difference