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Marine biomass toward biofuel production 457

20.3.3.3 Sea lettuce: Ulva lactuca

The green sea lettuce, Ulva lactuca, is used for bioethanol production and it is a
renewable gas fuel. U. lactuca commonly refers to green tides due to the
eutrophication (excess amount of nitrogen secretion) or algae blooms due to high
lipid content (Allen et al., 2013). Sea lettuce generally contains minimal amount of
cellulose from which biomethane is produced by anaerobic digestion (Vergara-
Ferna ´ndez et al., 2008). The shallow basins remain the most susceptible place for
the growth of sea lettuce. Shallow topographs protect the algae from washout and it
also keeps the pollutant, such as urea and nitrogen, from initiating the algal growth.

20.3.4 Other microbial biofuel sources
Microorganisms are versatile living factories that can utilize numerous natural and
synthetic compounds for their growth and convert them into different useful chemi-
cals. Conversion of marine biomass into simple sugars is a necessary step in the
production of biofuel. The major barrier for this process is the presence of high
branched and recalcitrant compounds in marine biomass. However, these com-
pounds can be subjected to pretreatment methods to release chemicals and enzymes
that help in the conversion of biomass into biofuel. Pretreatment methods involve
the application of microbial enzymes that help in breaking down of highly branched
structures. Generally, marine biomass is composed of cellulose, hemicellulose, lig-
nin, minerals, proteins, and oil (Wyman, 1999). Pretreatment of biomass using
microbial enzymes is considered as a cost-effective and environment-friendly
approach (Chaturvedi and Verma, 2013). However, pretreatment methods will vary
according to the proportion of the chemicals present in different biomass. In earlier
days, saccharification and fermentation process was considered attractive because
of the combined addition of hydrolytic enzymes and microbes in the same environ-
ment (Punnapayak and Emert, 1986). So, this process has led to minimal inhibition,
thus resulting in high yield of products at less cost (Wyman, 1999). On the other
hand, solid-state fermentation (SSF) is employed at low pH and high temperature.
But the microbes are subjected to adopt the ability to grow in this extreme condi-
tion. In order to obtain good results, microbes can be genetically engineered and
employed in this process (Zhang et al., 1995).
Later, SSF was replaced by separate hydrolysis and fermentation (SHF) process,
in which the pretreatment of biomass was followed by enzymatic hydrolysis and
then to a fermentation process. In this method, biomass undergoes each process
independently under optimal conditions. This type of treatment of biomass leads to
the additional discharge of waste, which can be used for producing other
value-added products using biocatalysts (Chaturvedi and Verma, 2013). Unlike
SHF, consolidated bioprocessing process combines both cellulose and hemicellulose
fermentations in a single batch process (Lynd et al., 2002). Microbes that have the
ability to produce cellulolytic enzymes can be employed in this process. Glucose
and xylose cofermentation can be attained by employing genetically engineered
microbial consortium (Zhang et al., 1995).