Introduction

Rio Earth Summit in 1992 witnessed the beginning of the international political responses and adoption of the UN Framework on Climate Change (UNFCCC), which aimed at stabilising atmospheric concentrations of greenhouse gases (GHGs) to avoid dangerous anthropogenic interference with the climate system. Recent conference COP21 (http://www.cop21paris.org) unanimously agreed to restrict the increase in the global average temperature to well below 2°C above pre-industrial levels and also make efforts to limit the temperature increase to 1.5°C by reducing the dependence on fossil fuels and cutting the greenhouse gas emissions. This has given impetus to the exploration of viable renewable energy alternatives to meet the energy demand of the burgeoning population.  Alternative sustainable energy sources are wind solar, geothermal, hydroelectric, biomass and biofuel (Ghadiryanfar et.al,2016).

Earlier attempts in this regard were the manufacture of biodiesel and pure plant oil derived from sugarcane, corn, soybean, potato, wheat or sugar beet (first generation biofuel), which proved to be unsustainable due to the competition with human food resources. This conflict, led to new attempts on biofuel derived from lingo-cellulosic biomass (second generation biofuel). This involved direct and indirect land use changes with the energy crop cultivation inducing a significantly high carbon debt and higher water consumption (Dominguez-Faus. et.al, 2009). Conflict with land for cultivation of biofuel feedstock, led to the exploration of viable alternatives focusing on algal biofuels (third generation biofuels).

Algal feedstock being carbon neutral has proved to be a very promising renewable resource for sustainable energy production. Algae fixes the greenhouse gas (CO2) and have higher photosynthetic efficiency (6-8%) compared to any terrestrial biomass (1.8-2.2%) (Ramachandra et al., 2009; Aresta, 2005; FAO,1997). Also, algae feedstock can be grown in fresh as well as marine waters which reduces the need for higher water consumption. Micro algae grown in marine and freshwater ecosystems and macro algae grown in estuaries have proved to be beneficial feedstock for biofuel production. Microalgae grown in marine ecosystem (with higher salinity and silica) accumulate lipid, while macro-algae, which are multicellular with  plant like characteristics are rich in carbohydrate and net energy (net energy of 11,000 MJ/t dry algae; Aitken et al.,2014 ) and are aptly suited for bioethanol conversion (Jin et al.,2013).

Macro-algae or seaweeds have higher potential to produce sustainable bioenergy and biomaterials and do not require land or freshwater for their cultivation. (Lobban et.al., 1985). Macro-algae are currently used for hydrocolloids, fertilizers and to some extent as animal feed (Bixler and Prose, 2011; McHugh,2003). Despite all these environmental and economic merits of macroalgae, challenges are experienced during extraction of biofuel as macro-algae have unique carbohydrate architecture, distinctively different from terrestrial biomass (Roseijadi et al., 2010; sze, 1993). Though macro-algae are ideally suited for biofuel such as biogas, bioethanol, etc., attempts towards economically efficient technological solutions of biofuel production are still at infancy (Bastianoni S et al., 2008). Marine macro-algae are broadly classified as (i) brown algae (Phaeophyceae), (ii) red algae (Rhodophyceae), and (ii) green algae (Chlorophyceae). Table 2 lists number of species and characteristics, which are distinctly different with regard to their photosynthetic reserve and cell-wall polysaccharides. Abiotic parameters of habitat (namely light, temperature, salinity, nutrient, pollution, water motion, etc.) play a vital role in algae’s growth, pigment and also other chemical constituents. Macro-algae are vertically distributed from the upper zone (close to the sea surface) to lower sub-littoral zone to optimally use natural light and the pigment absorb selectively light at specific wavelength (Guiry 2012).

Acid hydrolysis involves cleaving the polysaccharide’s glyosidic bond to release monosaccharides. But, acid hydrolysis decomposition also releases undesirable compounds, such as Fufural, 5-hydroxymethylfufural(HMF), levulinic acid and caffeic acid, which inhibit subsequent fermentation. These compounds are derived from xylose and galactose in macroalgal biomass, can be detoxified using activated charcoal treatments (Meinita et al., 2012). Metal contents in macroalgal biomass are (0.5-11% wt) higher than terrestrial biomass (1-1.5% wt) (Lee and Lee, 2012; Ross et al., 2008), which inhibits microbial fermentation during pretreatment.  In contrast to this, during enzyme hydrolysis there is no undesirable compounds as enzyme activity is specific to type of polysaccharides (Nguyen et al., 2009). Simple sugar resulting from hydrolysis is subjected to fermentation using various organisms particularly yeasts microorganisms, to produce ethanol. In order to produce bioethanol cost-effective manner, efforts are in progress to screen microorganisms (Table 4) that possess the ability to directly convert polysaccharides (including glucans) into ethanol. Table 4 also lists species wise quantum of ethanol production, while Table 5 lists microorganisms (to convert sugar into ethanol) for different macro algae.