53
the amino group is forwarded to keto acid; see Giordano et al. 2005 and their references), followed by a reduction reaction, converting in 4-methylthio-2hydroxybutyrate, using nicotinamide adenine dinucleotide phosphate molecule.
This mechanism is catalyzed by 4-methylthio-2-oxobutyrate reductase, and the
activity of this enzyme is usually high in dimethylsulfoniopropionate-producing
species (Giordano and Prioretti 2016).
The next stage in the reaction is that the S-methylation of 4-methylthio-2hydroxybutyrate to 4-dimethylsulfonio-2-hydroxybutyrate finally is converted to
dimethylsulfoniopropionate through oxidative decarboxylation (Giordano et al.
2005; Giordano and Prioretti 2016). The demethiolation of dimethylsulfoniopropionate form to methanethiol which can be transformed into dimethyl sulfide by methylation (Achyuthan et al. 2017; Curson et al. 2017).
Given the above, the establishment of biochemical pathways can target the production of the specific biomolecules of microalgae metabolism. However, the biogeneration of these molecules is strongly influenced by the cultivation conditions as
well as other key factors that can be improved.
2.4 Culture Systems for Volatile Organic Compound
Production
The CO 2 conversion into VOCs could be excellent and efficient if certain conditions
were considered, two of which are the injection of high CO 2 loads and the design of
appropriate culture system. Among them, there are open and closed systems for
microalgae cultivation, which are operated under different aspects (Jacob-Lopes
and Franco 2010).
The state-of-the-art and argumentative opinions on the use of open systems for
large-scale cultivation as well as engineering requirements began in the 1960s.
Today, different models are being studied: shallow lagoons and ponds, circular
ponds, mixed ponds, inclined systems, and raceway ponds, the latter being the most
accepted for commercial application (Borowitzka 2013). Although they are easy to
build and cheap, open systems rely on operational conditions that oscillate wildly.
They can be rapidly contaminated by external agents and are vulnerable to inclement weather, which directly affects microalgal productivity, limiting the CO 2 conversion (Verma and Srivastava 2018). Another obvious disadvantage is the considerable
increase in evaporation rates. Undoubtedly, these issues make it difficult to increase
the production and collection of gaseous products such as VOCs.
Later, in the 1980s in the United States, work began on closed systems, which are
now commonly called photobioreactors. Many versions have been patented in
recent decades aiming to overcome the bottlenecks that closed systems show
(Borowitzka 2013). Among the designs accepted, the most common are flat-plate
panels, tubular photobioreactors, airlift, and bubble columns. Recently, innovative
models have emerged such as biofilms, membrane, soft-frame, and hybrid photobioreactors (Vo et al. 2019).
2 Biological Conversion of Carbon Dioxide into Volatile Organic Compounds
the amino group is forwarded to keto acid; see Giordano et al. 2005 and their references), followed by a reduction reaction, converting in 4-methylthio-2hydroxybutyrate, using nicotinamide adenine dinucleotide phosphate molecule.
This mechanism is catalyzed by 4-methylthio-2-oxobutyrate reductase, and the
activity of this enzyme is usually high in dimethylsulfoniopropionate-producing
species (Giordano and Prioretti 2016).
The next stage in the reaction is that the S-methylation of 4-methylthio-2hydroxybutyrate to 4-dimethylsulfonio-2-hydroxybutyrate finally is converted to
dimethylsulfoniopropionate through oxidative decarboxylation (Giordano et al.
2005; Giordano and Prioretti 2016). The demethiolation of dimethylsulfoniopropionate form to methanethiol which can be transformed into dimethyl sulfide by methylation (Achyuthan et al. 2017; Curson et al. 2017).
Given the above, the establishment of biochemical pathways can target the production of the specific biomolecules of microalgae metabolism. However, the biogeneration of these molecules is strongly influenced by the cultivation conditions as
well as other key factors that can be improved.
2.4 Culture Systems for Volatile Organic Compound
Production
The CO 2 conversion into VOCs could be excellent and efficient if certain conditions
were considered, two of which are the injection of high CO 2 loads and the design of
appropriate culture system. Among them, there are open and closed systems for
microalgae cultivation, which are operated under different aspects (Jacob-Lopes
and Franco 2010).
The state-of-the-art and argumentative opinions on the use of open systems for
large-scale cultivation as well as engineering requirements began in the 1960s.
Today, different models are being studied: shallow lagoons and ponds, circular
ponds, mixed ponds, inclined systems, and raceway ponds, the latter being the most
accepted for commercial application (Borowitzka 2013). Although they are easy to
build and cheap, open systems rely on operational conditions that oscillate wildly.
They can be rapidly contaminated by external agents and are vulnerable to inclement weather, which directly affects microalgal productivity, limiting the CO 2 conversion (Verma and Srivastava 2018). Another obvious disadvantage is the considerable
increase in evaporation rates. Undoubtedly, these issues make it difficult to increase
the production and collection of gaseous products such as VOCs.
Later, in the 1980s in the United States, work began on closed systems, which are
now commonly called photobioreactors. Many versions have been patented in
recent decades aiming to overcome the bottlenecks that closed systems show
(Borowitzka 2013). Among the designs accepted, the most common are flat-plate
panels, tubular photobioreactors, airlift, and bubble columns. Recently, innovative
models have emerged such as biofilms, membrane, soft-frame, and hybrid photobioreactors (Vo et al. 2019).
2 Biological Conversion of Carbon Dioxide into Volatile Organic Compounds