Conclusion

The growing need to cater the energy demand coupled with the urge to mitigate GHG footprint in the energy sector has necessitated inventorying of renewable and sustainable energy alternatives. Green seaweeds have the potential to serve as a renewable and sustainable feedstock for bioethanol production. Dilute acid, H₂SO₄ is an appropriate pretreatment method for extraction of sugars from algal biomass. Algal biomass pretreated using dilute acid yields higher sugar during enzyme hydrolysis by exposing cellulose fibres for enzyme degradation. Neutralization using Na₂CO₃ exhibited lower sugar removal for U.lactuca (39.8%) and E.intestinalis (14.7%). Green seaweeds U.lactuca and E.intestinalis are viable feedstock for bioethanol production due to higher carbohydrate content of 62.15% and 40.1% respectively. Saccharification and Fermentation (SSF) process yielded higher conversion efficiency compared to SHF (Separate hydrolysis and fermentation) process and SSF achieved higher efficiency for U.lactuca indicating fermentation of more than one sugar present in the medium. Toddy yeast strain exhibited higher efficacy in fermentation of E.intestinalis hydrolysate in both SHF and SSF process achieving 25.8% and 77.4% efficiency respectively. Macroalgal biomass has potential to produce various value added products of commercial significance in addition to the production of biofuels. The study highlights the scope for bioethanol from macroalgae and also the availability of potential sites for cultivation. The comprehensive knowledge of macro algal resources distribution, seasonality and biochemical composition through field survey and choice of appropriate enzyme and yeast would aid in realizing the path of the sustainable, economical and commercially viable biofuel.

Data and accessibility Data used in the analyses are compiled from the field. Data is anlysed and organized in the form of table, which are presented in the manuscript. Also, synthesized data are archived at http://wgbis.ces.iisc.ernet. in/energy/water/paper/researchpaper2.html#ce, http://wgbis.ces.iisc. ernet.in/biodiversity/
Competing interests We have no competing interests either financial or non-financial.
Authors contribution Ramachandra T V: data analysis and interpretation of data; revising the article critically for important intellectual content; final editing Deepthi H: Isolation and characterisation, Design of the fermentation experiment, Carrying out experiments, analysis and interpretation of data; and Paper writing.
Funding This research was supported with the grant (i) ISRO-IISc Space Technology Cell [ISTC/BES/TVR/389] (ii) Gajanana and Madhavi Hegde [CES/TVR/GJH001] endowment for biofuel research; (iii) Indian Institute of Science.
Research ethics The publication is based on the original research and has not been submitted elsewhere for publication or web hosting.
Animal ethics The research does not involve either humans, animals or tissues.
Permission to carry out fieldwork Our research is commissioned by the Ministry of Science and Technology (NRDMS Division), Government of India and hence no further permission is required as the field work was carried out in the non-restricted areas/protected areas.

Acknowledgement

The authors are grateful to (i) ISRO-IISc Space Technology Cell [ISTC/BES/TVR/389] (ii) Gajanana and Madhavi Hegde [CES/TVR/GJH001] endowment for biofuel research for the financial support and (iii) the NRDMS Division, Ministry of Science and Technology, Government of India [CES/TVR/DST1045]; (iv) Indian Institute of Science [R1011] for the infrastructure support and colleagues at EWRG, IISc field station, Kumta for assistance during the field sampling.

We thank Sreekanth Naik and Gayatri Naik for the assistance during the field data collection. We thank all the stakeholders of Aghanashini estuary for actively taking part in the scientific discussions and cooperation during field data compilation. We are grateful to the official languages section at IISc for the assistance in language editing.