Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 99

Table 1 Major contributions to the global H 2 cycle a .
Ranges resulting from summarizing different studies
[Tg H 2 yr 1 ] b

Sources
Fossil fuel combustion

11-20

Biomass and biofuel burning 8-20

Photochemical production 30-77

Biological production 3-12

Sinks
Microbial soil deposition 55-88

Reaction with OH 15-19

a Table adapted from Vollmer et al. [ 9 ].
b The unit 'Tg' refers to teragram, 1

10 12 g.

AfutureH 2 fuel economy may have a significant effect on atmospheric H 2 levels
since any leaks from storage containers or inefficiencies in fuel conversion processes
will give rise to an increase in the amount of H 2 in the air. This is often discussed in
terms of the possible problems that could be caused by dihydrogen's properties as a
greenhouse gas [ 5 , 9 , 10 ]. However, the function of H 2 as a virulence factor in
disease-causing bacteria such as Salmonella is only just being explored [ 11 , 12 ], and
it is worth considering that the role of H 2 as a microbial fuel may also provide
important guidance for future air quality legislation.

2.2 Dihydrogen Cycling in Microbial Communities

The term 'hydrogenase' was coined in 1931 by Stephenson and Stickland to describe
enzymatic activity identified in bacteria isolated from river mud [ 13 , 14 ]. The
H 2 uptake property of different soil microbes remains a focus of research to this
day [ 9 , 15 , 16 ], but H 2 activity is also an essential metabolic reaction in environmen-
tal niches ranging from the human gut [ 11 ] to photosynthetic algal or cyanobacterial
mats [ 17 ], oceanic hydrothermal vents [ 18 ], and sulfidic geothermal springs [ 19 ]. We
now know that hydrogenases are produced by microbes from the three domains of
Life: Archaea ([Fe] and [NiFe] hydrogenases), Bacteria ([NiFe] and [FeFe] hydrog-
enases), and Eukarya ([FeFe] hydrogenases) [ 7 , 20 ].
Given that the evolution of H 2 biocatalysis pre-dates photosynthesis, it is unsurprising
to note that the majority of H 2 -dependent metabolic processes occur under anaerobic
conditions. For example in carbon monoxide (CO)-rich, O 2 -free environments the
bacterial species Carboxydothermus hydrogenoformans functions as a 'hydrogenogenic'
organism, meaning it couples CO oxidation (equation 4 ), catalyzed by a carbon monox-
ide dehydrogenase enzyme (see also Chapter 2 ) with hydrogenase-catalyzed H 2 produc-
tion, reaction ( 5 )[ 21 ]. The net reaction (equation 6 ) is therefore the same as the