Introduction

The global energy demand is projected to escalate beyond 37% by 2040 [1, 2], posing severe threats to fossil fuels reserve; for the foreseeable future, approximately 31% of crude oil remains the most significant energy source [3, 4]. The enhanced greenhouse gas (GHG) footprint [3, 5–7] with the concerns of climate changes have necessitated exploring alternative and renewable sources such as biofuels. Biofuels are produced from plant biomasses, and widely used biofuel comes from the feedstock that involves food crops like corn grains and molasses (first generation). This is followed by lignocellulose biomass such as rice straws, wheat straws, sugarcane bagasse, etc. (Second generation). Both these feedstocks faced the constraints of arable land, water, and higher production cost. In this context, algal biomass gained significance as a potential third-generation feedstock for bioethanol production [8–13]. Production of bioethanol from algae involves (i) degradation of feedstock to release fermentable sugars [14] and (ii) fermentation of variants of sugar using appropriate organisms to produce bioethanol.

Macroalgal genera of Kappaphycus, Gelidium, Gracilaria, Sargassum and Ulva are regarded as potential feedstock for bioethanol production [7]. Macroalgae are composed of structural and storage polysaccharides, which serve as a raw material for bioethanol production [7]. Polysaccharides of macroalgal feedstock constitutes monosaccharide: glucose (26-30%), xylose (10-15%), rhamnose (3.3-12.7%), mannose (0.1-0.29%), galactose (1-6%), arabinose (0-0.08%), uronic acid (20-25%) and glucuronic acid (0-10%) [13, 15–18]. Xylose, glucose, and rhamnose are the two most abundant monomers found in green macroalgae. Maximum ethanol production is achieved by converting all the sugars present in the biomass [19]. Glucose (hexose sugar, C6) is ubiquitous in nature and is readily fermented by yeast strain S. cerevisiae. Xylose (pentose sugar, C5) is the second most abundant sugar in nature and is not fermented by S.cerevisiae, limiting its usage in bioethanol production from lignocellulosic biomass [20]. Rhamnose is a deoxy sugar present in green macroalgae, mostly in the range of 3.3-12.7% [21–23]. Earlier studies indicate that the microorganisms cannot grow on rhamnose as a sole source of carbon as the uptake of rhamnose by these organisms is extremely slow [24, 25]. However, fermentation of these deoxy sugars like rhamnose and fucose is solely investigated to produce a high concentration of 1,2- propanediol [23, 26], which is used to synthesize polymer resins, non-ionic detergents, cosmetics, liquid detergents, biodegradable plastics, etc. The economic value of 1,2- propanediol is estimated over 1 billion pounds [26]. Therefore, the production of 1,2- propanediol by bacteria and yeast using deoxy sugars (rhamnose and fucose) is more economical than by-passing rhamnose sugar for bioethanol production. Therefore, this study highlights the utilization of glucose and xylose efficiently for bioethanol production by isolated wild yeast strains. Several studies have focused on investigating the potential of wild (non-domesticated) yeast strains for bioethanol production [27]. Co-fermentation for fermenting xylose and glucose using two different species of yeast also has been reported. Candida shehatea, Scheffersomyces stipitis (Pichia stipitis), and Pacchysolen tannophilus are the most commonly used yeast species for converting xylose [19]. Scheffersomyces stipitis strain (UFMG-IMH 43.2) proved to be the most efficient yeast strain, as it fermented glucose, xylose, and cellobiose with high ethanol yield and low quantities of co-products [19] with the ethanol yield of 0.91g/g from 30g/L of xylose [28].

Bioethanol production from macroalgal biomass is carried out either by Separate Hydrolysis (acid/enzyme) and Fermentation (SHF) or Simultaneous Saccharification (enzyme) and Fermentation (SSF) process. SHF involves two separate stages; (i) biomass is hydrolysed by acid or enzyme to release sugars and (ii) fermentation of sugars to produce ethanol. In SSF process, acid-pretreated biomass is subjected to enzyme hydrolysis and fermentation in a single reactor. However, both these processes are with relative merits and demerits. SHF is a faster process but encounters the formation of hydroxymethyl furfurals (HMF), an inhibitor during acid hydrolysis of biomass, which is detrimental to yeast organisms. SSF requires a more extended period as it involves enzyme hydrolysis and fermentation. Lower concentrations of inhibitors are formed in the SSF process. Bioethanol production from cellulosic feedstock involves four unit operations: pretreatment, enzyme production, enzyme hydrolysis, and microbial fermentation. Consolidated bioprocess (CBP) combines three-unit operations (enzyme production, enzyme hydrolysis, and microbial fermentation) into a single unit operation. This brings down the cost of bioethanol production from macroalgae. However, wild yeast microorganisms with the capability of high cellulolytic activity and saccharification of lignocellulosic biomass and ethanol production are still unexplored.

The fermentation process is exothermic and causes an increase in temperature during industrial scales. However, higher temperatures are not tolerated by yeast organisms as it shortens the exponential phase of the yeast cell [20], affecting ethanol production. Other stresses such as sugar concentration, changes in pH, etc., inhibits cell growth. Microorganisms tolerant to these stresses naturally occur in nature [29]. Yeast strains are prioritized for the fermentation process based on the characteristics such as (i) rapid and relevant fermentation ability, (ii) genetic stability (iii) osmo-tolerance (iv) ethanol tolerance (v) cell viability, and (vi) thermotolerance, etc. The fermentation process is highly influenced by the type of yeast strain utilized [30, 31], due to which there is a perpetual quest for isolation of a novel, robust and tolerant yeast strain with a potential of fermenting all the sugars available for higher bioethanol production and industrial application through bioprospecting. Catering to the challenges, the present study deals with bioprospecting of ethanologenic wild yeasts with a potential to produce bioethanol and screening of cellulolytic yeast strain and bioethanol production by CBP. The exploration of wild (non-domesticated) yeast strains with desirable characteristics would strengthen yeast strains' current repository for optimal bioethanol production.