18.5 Strategies to Enhance Microbial Hydrogen Production

295

18.5.4

Application of Ultrasonication

The literature survey showed that there are rare studies on the enhancement of bio-

hydrogen production by the application of ultrasonication. The majority of these

studies have focused on the application of ultrasonication in other pretreatment

processes like biomass/substrate and inoculum pre-treatment [52]. An increase of

38% H2 production has been reported with the application of ultrasonication over

non-sonicated palm oil mill effluent (POME) pretreatment [53]. For fermentative

production of biohydrogen, the application of ultrasonication has been reported only

twice. Taguchi method was applied to optimize the ultrasonic intensity and the time

of exposure [54]. It was concluded that ultrasonication affects the H2 production

rate and efficiency significantly, triggering 19.11% enhancement in production effi-

ciency under optimal conditions. Other similar reports showed a marked increase

in the yield of biohydrogen by 40% and 50% in the consumption rate of glycerol on

application of controlled sonication cycles during fermentation [55].

18.5.5

Strain Development

Microbial strain development by metabolic engineering or genetic modification

is a promising tool for improving the yield of fermentative H2 yield by enhancing

substrate consumption rates or blocking the production of by-products of the

pathway. Metabolic engineering approaches can overcome the limitations related to

lower yield either by deletion of competitive pathways or by over-expression of the

genes specific to H2 production. With the help of MFA, researchers have elucidated

the role of essential genes involved in the biohydrogen metabolic pathway and other

competitive pathways. In recent years, various investigators have reported several

ways to increase the yield of H2 by application of genetic engineering techniques

such as over-expression of heterologous or homologous genes, knockout of compet-

itive pathways, and reconstruction of the metabolic pathway, thereby channeling

the carbon flow solely toward molecular H2 production [56]. Also, several genetic

engineering approaches have been successfully attempted on the hydrogen pro-

duction pathway of E. coli to boost biohydrogen production by over-expression of

hydrogenase3, formate hydrogen lyase, and hydrogenase gene in C. paraputrificum

[57, 58]. Recent studies on the development of genetic engineering toolkits for

efficient H2 producers such as Enterobacter sps. and Clostridium sps. have motivated

many researchers to explore them for enhanced H2 production [33, 58, 59]. Sarma

et al. (2019) have reported 1.5 times enhancement in H2 yield compared to the

wild-type strain of C. pasteurianum by over-expression of hydrogenase and glycerol

uptake enzymes [33].

It can be inferred from Table 18.3 that various metabolic engineering strategies

have been applied to different H2 producing bacteria to improve yield and pro-

duction rates [32, 33, 56, 58, 60–63]. The studies suggested hydA as the key gene

involved for H2 production in Clostridium perfringens. The deletion of hydA gene

blocked the H2 gas production completely in the organism. A comparative study on

C. butyricum and C. acetobutylicum reported higher specific activity of hydrogenase

in C. butyricum, and negligible hydrogenase activity was reported for C. aceto-

butylicum [60]. It was also reported that lactate and succinate inhibit H2 production.