Biology Reference
In-Depth Information
viii) Hydrogen metabolism
:
Reduction of dinitrogen to ammonia mediated by nitrogenase is
associated with concomitant evolution of hydrogen. At least 25% of the electrons during nitrogen
fi xation process are allocated for an obligatory reduction of protons (H
+
) to form H
2
(Tamagnini
et
al
., 2002). However, according to Simpson and Burris (1984) nitrogenase generates one mol of H
2
for
every mol of N
2
reduced. Hydrogen evolution has been viewed as an inbuilt trait of diazotrophs.
Photobiological production of hydrogen has been receiving attention since last three decades, since
it can be used as a clean, storable, transportable and renewable energy source. Many nitrogen-fi xing
bacteria (mainly species of
Clostridium
; Calusinska
et al
., 2010), cyanobacteria (Tamagnini
et al
.,
2007) and certain green algae (
Chlamydomonas reinhardtii
,
Scenedesmus obliquus
,
Chlorella fusca
and
Nephroselmis olivacea
; Melis and Happe, 2001) are being exploited for ameliorating the situation in
energy consumption and saving spheres (Lopes Pinto
et al
., 2002; Tamagnini
et al
., 2007). H
2
evolution
and its subsequent consumption provide a means of protection against O
2
damage to nitrogenase.
The oxidation of hydrogen (the oxyhydrogen) in a reaction termed as Knallgas reaction (2H
2
+ O
2
→
2H
2
O) facilitates the removal O
2
from the site of nitrogenase. Walsby (1985) suggested that the molar
fl ux of O
2
into the heterocysts is half of that of N
2
. Besides the high respiratory activity, the Knallgas
reaction provides additional protection. The organism saves part of its reductant as well as ATP for
activity of nitrogenase if H
2
evolved is not allowed to be diffused out. So the hydrogen evolved is
immediately consumed that is catalyzed by an uptake hydrogenase. There is a strong correlation
between the activity of uptake hydrogenase and nitrogen fi xation in a number of heterocystous
cyanobacteria (Lambert and Smith, 1981; Houchins, 1984; Wolk
et al
., 1994; Oxelfelt and
et al
., 1995;
Tamagnini
et al
., 2002, 2007). Uptake hydrogenases function in the direction of H
2
uptake and recycle
or reutilize the H
2
/electrons so as to facilitate metabolism. This is supported by the induction of
uptake hydrogenase activity in heterocystous cyanobacteria by H
2
(Tel-Or
et al
., 1977), production
of ATP via respiratory chain by coupling to O
2
(Peterson and Burris, 1978; Houchins and Burris,
1981a) and acetylene reduction by the supply of electrons (Bothe
et al
., 1977). Stimulation in uptake
hydrogenase activity of
Anabaena
sp. strain PCC 7120 was observed by the inclusion of H
2
or removal
of O
2
in the gas phase of the cultures (Houchins and Burris, 1981b). It seems mere exogenous supply
of H
2
is not suffi cient to maintain uptake hydrogenase activity but it is dependent on the extent of
induction of nitrogenase activity
in vivo
(Lambert and Smith, 1981; Oxelfelt
et al
., 1995).
A comparative study of nitrogen fi xation and H
2
uptake by four cyanobacteria (
A
.
variabilis
,
Nostoc spongiaeforme
,
Westiellopsis prolifi ca
and
Nostoc
sp.) revealed that (i) a concentration of up to
20% H
2
enhanced nitrogenase activity; (ii)
N
.
spongiaeforme
showed higher H
2
uptake activity under
both aerobic and anaerobic conditions and (iii) addition of DCMU did not inhibit H
2
uptake in
case of
N
.
spongiaeforme
(Vyas and Kumar, 1995). In case of
A
.
variabilis
ATCC 29413 exogenous H
2
has little effect on the stimulation of
in vivo
H
2
uptake during early stages of nitrogenase induction
(Troshina
et al
., 1996). Photoautotrophically grown cultures of
Anabaena cycadae
showed enhancement
in the activities of nitrogenase as well as uptake hydrogenase but dark incubation in presence of
glucose caused a reduction in nitrogenase activity to half of that observed in light with out uptake
hydrogenase activity. Fresh isolates of
A
.
cycadae
from its host tissue exhibited higher nitrogenase
activity with no apparent uptake hydrogenase activity (Kumar
et al
., 1986). Nitrogen-fi xing cultures
of
A
.
variabilis
ATCC 29413 showed fructose-dependent H
2
evolution but not those grown in presence
of ammonium. Fructose (up to 10 mM) was dissimilated into H
2
and CO
2
but the presence of DCMU
inhibited H
2
evolution (Reddy
et al
., 1996). Houchins and Burris (1981b,c) demonstrated the presence
of an O
2
-sensitive reversible hydrogenase in
Anabaena
sp. strain PCC 7120 that was independent of
nitrogen-fi xing capacity of the organism. The activity of reversible hydrogenase was enhanced 2-3
fold under microaerophilic conditions but remained unaffected by the addition of H
2
. Due to their
