Biology Reference
In-Depth Information
differentiation is arrested at an early stage, and the corresponding filaments
exhibit a nitrogenase activity that is very sensitive to oxygen (
Rippka &
Stanier, 1978
). These proheterocysts lack a mature envelope suggesting a
role of the heterocyst-specific envelope in preventing the entry of air into
the heterocyst cytoplasm. Production of oxygen-sensitive nitrogenase was
further correlated with lack of production of Hgl in mutants of
Anabaena
variabilis
(
Haury & Wolk, 1978
). Analysis of mutants of
Anabaena
sp. strain
PCC 7120 has shown that the barrier to oxygen requires both of the het-
erocyst envelope layers (
Murry & Wolk, 1989
). Because mutants lacking
Hgl retain their Hep layer in heterocysts (
Black et al., 1995
;
Fan et al., 2005
)
but mutants lacking Hep tend to show a delamination or fragmentation
of the Hgl layer (
Wolk, 2000
), the idea, based on the possible low perme-
ability of Hgl to oxygen and nitrogen (
Walsby, 1985
), that the Hgl layer is
the principal barrier to gas diffusion into the heterocyst, whereas the Hep
layer protects the Hgl layer from damage and dilution into the surround-
ing medium is widely accepted (
Xu et al., 2008
).
Walsby (1985)
suggested
that the heterocyst envelope might provide the right degree of gas perme-
ability to allow the entry of sufficient nitrogen gas for nitrogenase function
and maintain an adequate micro-oxic environment to avoid nitrogenase
inactivation. However, he has recently proposed that the main gas diffusion
pathway into the heterocyst is through the cell envelope pores present at the
vegetative cell-heterocyst junctions (
Walsby, 2007
). In any case, the hetero-
cyst envelope appears to have the role of limiting the entry of air, including
poisoning oxygen, into the heterocyst.
2.3. The Heterocyst Cytoplasm and Heterocyst Metabolism
Related to its activity of nitrogen fixation, the heterocyst has several notable
metabolic properties including lack of oxygenic photosynthesis and photo-
synthetic carbon fixation, increased respiratory activity and nitrogen assimi-
lation functions.
2.3.1. Carbon metabolism
The heterocysts are less pigmented than the vegetative cells (
Fay et al., 1968
)
and bear low levels of some photosystem II-related pigments (
Thomas, 1970
).
Although they have some photosystem II components (
Braun-Howland
& Nierzwieki-Bauer, 1990
) or even photosystem II complexes (
Cardona
et al., 2009
), there is ample evidence that heterocysts lack the O
2
-evolving
activity of photosystem II while keeping photosystem I activity (see, e.g.
Almon & Böhme, 1980
;
Donze, Haveman, & Schiereck, 1972
;
Tel-Or &
