industrial processes for the production of biodiesel, according to the previous
chapters. However, studies aimed at consolidating a suitable process for the production of bioethanol are still ongoing. On the contrary, cyanobacteria strains have
been shown to produce relevant amount of bioethanol. Markou et al. (2013)
evaluated the potential of bioethanol production using carbohydrate-enriched biomass of the cyanobacterium Arthrospira platensis. The biomass acid hydrolysates
were used as substrate for ethanolic fermentation by a salt stress-adapted
Saccharomyces cerevisiae, with highest bioethanol yields of 16.32% ± 0.9 (gram
ethanol per gram biomass) with HNO 3 0.5 N. The production of bioethanol from
microalgae and cyanobacteria is a feasible technological development, as they
showed higher productivity than certain crops such as sugarcane and corn (already
consolidated as feedstocks for bioethanol production). Moreover, microalgae and
cyanobacteria can reach 50% of their dry weight (DW) in carbohydrates, which can
then be hydrolyzed and fermented with high yields.
2 Carbohydrate Accumulation by Microalgae
Microorganisms with potential for bioethanol production in this way are selected
primarily in accordance with their ability to accumulate carbohydrates, which
depends on environmental and nutritional conditions. The main environmental
factors are light intensity, pH, salinity, and temperature, while the nutritional factors
include availability and source type for nitrogen, carbon, phosphorus, sulfur, and
iron (Chen et al. 2013; Markou et al. 2013).
Genera Scenedesmus, Chlorella, Chlorococcum, and Tetraselmis from
Chlorophyta division and Synechococcus among other cyanobacteria have been
extensively studied as feedstock for this type of bioethanol production. In general,
the cultivation in a high light intensity ranged from 150 to 450 µmol/(m
2 s) using a
mix of CO 2 in air between 2 and 5% and mesophilic temperatures (20–30 °C)
achieves around 50% of carbohydrate content under nutrient starvation, mainly
nitrogen, according to Table 2 (de Farias Silva and Bertucco 2016). However,
carbohydrate content could be extremely variable and the productivity depends on
the cell growth too, that is, growing conditions that allow the simultaneous accumulation and growth. According to Rizza et al. (2017), generally microalgal strains
that accumulated the highest levels of carbohydrates did not accumulate lipids
under identical growth conditions.
The positive effect of increasing light intensity on the accumulation of starch and
lipids is feasible only up to a point, usually equal to saturation of photosynthesis
under given conditions in a particular species. Nutritional factors directly or indirectly influence the rate of photosynthesis and biochemical composition of
microalgae. Macroelement (nitrogen, sulfur, or phosphorous) limitation is the most
widely used and so far the most successful strategy for enhancing starch accumulation. For example, availability of nitrogen enhances the synthesis of proteins,
pigments, and DNA, the amount of iron affects the photosynthetic electron
234
R. G. Bastos
chapters. However, studies aimed at consolidating a suitable process for the production of bioethanol are still ongoing. On the contrary, cyanobacteria strains have
been shown to produce relevant amount of bioethanol. Markou et al. (2013)
evaluated the potential of bioethanol production using carbohydrate-enriched biomass of the cyanobacterium Arthrospira platensis. The biomass acid hydrolysates
were used as substrate for ethanolic fermentation by a salt stress-adapted
Saccharomyces cerevisiae, with highest bioethanol yields of 16.32% ± 0.9 (gram
ethanol per gram biomass) with HNO 3 0.5 N. The production of bioethanol from
microalgae and cyanobacteria is a feasible technological development, as they
showed higher productivity than certain crops such as sugarcane and corn (already
consolidated as feedstocks for bioethanol production). Moreover, microalgae and
cyanobacteria can reach 50% of their dry weight (DW) in carbohydrates, which can
then be hydrolyzed and fermented with high yields.
2 Carbohydrate Accumulation by Microalgae
Microorganisms with potential for bioethanol production in this way are selected
primarily in accordance with their ability to accumulate carbohydrates, which
depends on environmental and nutritional conditions. The main environmental
factors are light intensity, pH, salinity, and temperature, while the nutritional factors
include availability and source type for nitrogen, carbon, phosphorus, sulfur, and
iron (Chen et al. 2013; Markou et al. 2013).
Genera Scenedesmus, Chlorella, Chlorococcum, and Tetraselmis from
Chlorophyta division and Synechococcus among other cyanobacteria have been
extensively studied as feedstock for this type of bioethanol production. In general,
the cultivation in a high light intensity ranged from 150 to 450 µmol/(m
2 s) using a
mix of CO 2 in air between 2 and 5% and mesophilic temperatures (20–30 °C)
achieves around 50% of carbohydrate content under nutrient starvation, mainly
nitrogen, according to Table 2 (de Farias Silva and Bertucco 2016). However,
carbohydrate content could be extremely variable and the productivity depends on
the cell growth too, that is, growing conditions that allow the simultaneous accumulation and growth. According to Rizza et al. (2017), generally microalgal strains
that accumulated the highest levels of carbohydrates did not accumulate lipids
under identical growth conditions.
The positive effect of increasing light intensity on the accumulation of starch and
lipids is feasible only up to a point, usually equal to saturation of photosynthesis
under given conditions in a particular species. Nutritional factors directly or indirectly influence the rate of photosynthesis and biochemical composition of
microalgae. Macroelement (nitrogen, sulfur, or phosphorous) limitation is the most
widely used and so far the most successful strategy for enhancing starch accumulation. For example, availability of nitrogen enhances the synthesis of proteins,
pigments, and DNA, the amount of iron affects the photosynthetic electron
234
R. G. Bastos