Biomedical Engineering Reference
In-Depth Information
biotechnological applications, the strain and physiological state of the algae play
critical roles in determining output.
In addition to production rates, variations in biochemical profiles must be con-
sidered for optimization of harvesting. Carbohydrates, proteins, lipids, and fats are
known to vary with the medium used among seven species of marine microalgae
(Fernández-Reiriz et al., 1989) and in sixteen species of microalgae commonly used
in aquaculture (Brown, 1991). The medium used (Walne, ES, f/2, and Algal-1) for
cultivation also influenced the biochemical profiles of four species (Fernández-
Reiriz et al., 1989).
Through biochemical manipulation, lipid synthesis can be regulated; this involves
imposing a physiological stress such as nutrient starvation to channel metabolic pro-
cesses toward lipid accumulation. In experiments by Li et al. (2008), cultures of
Neochloris oleoabundans
were supplied with sodium nitrate, urea, and ammonium
bicarbonate as the nitrogen source; only at lower levels of sodium nitrate did cel-
lular lipid increase. Co-limitation for inorganic phosphorus and carbon dioxide in
Chlamydomonas acidophila
Negoro resulted in high photosynthetic rates and also
in a mismatch between photosynthesis and growth rates in phosphorus-limited cul-
tures (Spijkerman, 2010). In
Monodus subterraneus
when phosphate was decreased
from 175 to 52.5, 17.5, or 0 μM, cellular lipid increased (Khozin-Goldberg, and
Cohen, 2006). Limitation of nitrogen in cultures of the green alga
Scenedesmus
obliquus
resulted in an increase of lipid from 12.7% to 43% of cell dry weight (DW)
(Mandal and Mallick, 2009); a deficiency of phosphate increased lipid to 29.5%
(DW). Lipids in nitrogen-limited
Chaetoceros mulleri
increased five- to sevenfold
compared to nitrogen-replete cultures (McGinnis et al., 1997). Results obtained
with
Nannochloropsis oculata
and
Chlorella vulgaris
(Converti et al., 2009) con-
firmed such an impact of nitrogen limitation. Lipid production is enhanced to 90 kg
ha
−1
d
−1
by a two-stage culture system that involves raising high-density cultures
under optimal conditions initially and then transferring them to a nitrogen-deficient
medium (Rodolfi et al., 2008).
In
Dunaliella salina
cultures, an increase in CO
2
from 2% to 10% increased lipid
production by 170% in 7 days (Muradyan et al., 2004). In
Chlamydomonas vulgaris,
the addition of 1.2 × 10
−5
M Fe
3+
not only suppressed cell growth initially, but also
enhanced the accumulation of lipids up to 56.6% DW. Furthermore, the accu-
mulation of lipids occurred earlier during the stationary phase (Liu et al., 2008).
To enhance the yield of microalgal biomass, rigorous experiments should be carried
out to establish the impact of several micronutrients, such as selenium and boron.
Optimized growth of commercial algae should account for the effects of manipula-
tions in nutrients, temperature, and chemical composition of media.
13.7 HARVESTING
Because of their geometry, secretion of mucilage and variations in cell weight, micro-
algae must be harvested in a species-specific, nongeneric manner (Benemann, 2008).
Considerable process-oriented research is needed to optimize methods of algal har-
vest, lipid extraction, and purification of by-products. Flocculation is widely used
for algal harvest using various salts (Grima et al., 2003) such as aluminum, iron,
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