## 5 Potential for Concurrent Production of Biofuels
In addition, it has the advantage of requiring almost half the heat of evaporation compared to ethanol, and this is an advantage when starting the engine at sub-zero temperatures. Due to these properties, it can be used, in internal combustion engines, either as a mixture with diesel (good solubility) or alone as fuel. It is easier to store and transport due to the low vapor pressure and is also easier to manage. Biobutanol, apart from being a fuel, also finds many applications in the pharmaceutical and food industries. Its production is undertaken via anaerobic fermentation of acetone, butanol, and ethanol (ABE), with the bacterium _C. acetobutylcium_ and substrate carbohydrates of microalgae biomass. In the first step, butyric and acetic acid are formed, the pH is reduced, and then acetone, butanol, and ethanol are produced in a ratio of 3:6:1. Because this bacterium is a saccharolytic, no pre-treatment step is required for fermentation as is done for ethanol production, and this is a significant economic advantage. Microalgae biomasses such as _Tetraselmis subordiformis, Chlorella vulgaris, Chlorella reinhardtii,_ and _Scenedensulus obliquus_, can be suitable substrates for investigating biobutanol production as they are characterized by high starch content and convertible sugars [37]. Compared to other fuels, hydrogen (H\({}_{2}\)) has the highest energy content (142 kJ/kg), is safer to handle than natural gas, and can be transported via conventional means for use in industry or cities. Furthermore, because it is the only fuel devoid of carbon, all that is produced when it oxidizes is water. Coal, oil, natural gas, and naphtha, along with very little water and biomass, are used as fossil fuels to produce hydrogen. These procedures are expensive and polluting. As a result, there is a growing interest in producing biohydrogen (BioH\({}_{2}\)). Microbial electrolysis converts wastewater into BioH\({}_{2}\). Anaerobic microorganisms can also produce BioH\({}_{2}\) through fermentation processes like dark fermentation, which involves converting sugars into H\({}_{2}\), CO\({}_{2}\), and organic acids. High rates of hydrogen production, the capacity to produce hydrogen from biomass or biowaste, and the accomplishment of efficient process planning and control are significant benefits of this process. Because they can absorb solar energy and CO\({}_{2}\), and transform them into storage chemicals like starch, a fermentable substrate for the production of BioH\({}_{2}\), microalgae biomasses make excellent raw materials for dark fermentation. BioH\({}_{2}\) can be created under more controlled fermentation conditions using particular strains of microorganisms that produce higher amounts of hydrogen, or from mixed cultures of microorganisms where raw materials are waste. These microorganisms include thermophilic bacteria like _Thermotoga neapolitana_ and mesophilic bacteria like several species of _Clostridium_ and _Enterobacter_. In fermentations using microalgae biomass feedstocks like _Anabaena_ sp., and microalgae strains like _Chlamydomonas_ reinhardtii, _Nannochloropsis_ sp., _Chorella vulgaris_, and _Scenedesmus obliquus_, which grew on an artificial substrate and municipal waste, these anaerobic bacteria achieved high hydrogen productivity [38]. The production of hydrogen by microalgae depends on the activity of the enzyme hydrogenase, which is particularly sensitive to the presence of oxygen. To achieve anaerobic conditions for hydrogen production, the oxygen produced must be removed. Therefore, strictly anaerobic conditions are necessary for the efficient production of hydrogen from microalgae. This phenomenon present in natural bacteria-microalgae symbioses can also be achieved in artificial bacteria-microalgae cultures, and lead to higher hydrogen yields. This was demonstrated by the symbiosis of the bacterial strains _Brevundimonas_ sp., _Rhodococcus_ sp., and _Leifsonia_ sp. with the cultivation of _Chlamydomonas_, where hydrogen production was enhanced.