16.2 Basic Biohydrogen-Manufacturing Technologies and their Deficiency

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are transported to hydrogenases and/or nitrogenase enzymes. Microorganisms

liberate the surplus electrons using hydrogenase enzyme in anaerobic or excessive

energy conditions, which convert the hydrogen ions to hydrogen gas. Molecular

hydrogen production takes place, by rejoining the electrons and protons extracted

from the water-splitting reactions, using chloroplast hydrogenase. Crucial issues

with the coproduction of hydrogen and oxygen comprise co-culture equilibrium,

photosynthetic and respiration ability ratios, concentration, and dispensation of

cell biomass. In certain circumstances, a few microorganisms like algae can directly

generate hydrogen. Sulfur-deficient green algae whose energy was achieved from

light on anaerobic environment could sustain the hydrogenase reaction to generate

biohydrogen photosynthetically.

16.2.2

Photofermentation

An anaerobic photosynthesis is carried out by a non-sulfur purple photosynthetic

bacterium, which exploit captured solar energy to produce adenosine triphosphate

(ATP) and high-energy electrons through overturn electron flow that diminish

ferredoxin. Proton decline to hydrogen by nitrogenase and it is driven during ATP

and condensed ferredoxin formation. On the contrary, cyanobacteria and/or green

algae in photolysis technique and photosynthetic purple bacteria cannot gain

electrons from water, and therefore, organic compounds, usually organic acids or

even dihydrogen sulfide, are used as electron donors under anaerobic environment.

This process can be promising in terms of inclusive alteration of substrate to H2 and

CO2, and types of feed to the microbes.

16.2.3

Dark Fermentation

Diverse organic substrates and wastewaters can be utilized as electron donors to pro-

duce biohydrogen at higher rates and lower cost in dark fermentation as compared

to other biological pathways. Carbohydrate-rich substrates can be broken down to

hydrogen and other products such as acids (lactic, acetic, butyric, etc.) and alcohols

(ethanol, butanol, etc.) anaerobically using diverse microbes. The oxidation state of

the substrate, microbial distributions, and environmental conditions such as pH and

hydrogen partial pressure can affect product distribution in this process. Dark fer-

mentation in anaerobic environment emerges to be the most favorable among the

bio-production procedure because there is no need of straight solar input and vari-

ety of waste streams can be indulged for hydrogen generation. Glucose to pyruvate

formation or glycolytic pathway is generally ordinary path established in all major

microorganisms. In this direction, glucose is converted into pyruvate and NAD+

to NADH (nicotinamide adenine di nucleotide) in the course of anaerobic glycol-

ysis Eq. (16.1). The NADH, acetyl-CoA levels, and environmental conditions may

influence the discarding of electrons during pyruvate-ferredoxin oxidoreductase or

NADH-ferredoxin oxidoreductase and hydrogenase reactions. Therefore, NADH uti-

lization to produce some reduced composites (such as lactate, ethanol, and butanol)