Except
in
a
few,
cells
are
contained
in
a
rigid
or
semi-rigid
cell
wall
structure
which
gives
a
constant
form.
Peptidoglycan
forms
the
major
component
of
the
cell
wall,
whenever
present.
Members
which
are
photosynthetic,
do
so
only
under
anaerobic
conditions
and
use
electron
donors
other
than
water
and
do
not
produce
oxygen
during
photosynthesis.
They
do
not
contain
chlorophyiis
a
or
phycobilic
proteins
but
have
three
or
four
types
of
bacterial
chlorophylls.
Chemosynthetic
members
may
be
aerobic,
anaerobic
or
facultative
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