Purple Bacteria

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Purple Bacteria Gram negative about 30 species
bacteriochlorophylls a or b in cytoplasmic
membrane invaginations, along with antenna
pigments and ETS. These membrane invaginations
may be vesicular, lamellar or tubular. Two
alternative systems to generate reducing
power 1. Hydrogen is the electron donor and the
first stage involves quinone reduction. The
quinone reduces pyridine nucleotides (NAD(P))
directly. 2. Reduced sulphur (H2S, or sometimes
S, thiosulphate or sulphite) is the electron
donor and the first stage involves quinone
reduction. Electrons subsequently pass through
reverse electron transport to generate a
protonmotive force for reduction of
(NAD(P)). Purple Sulphur Bacteria CO2 is fixed by
the Calvin-Benson cycle reverse electron
transport generates reductant cyclic
photophosphorylation generates ATP. Most can grow
photoautotrophically with H2 as the electron
donor, in anaerobic conditions, using inorganic C
or photoassimilating acetate as a C source. Some
can grow chemoautotrophically, under low oxygen
conditions, using reduced sulphur. Some need
exogenous vitamin B12. Some require small amounts
of H2S as a sulphur source. Some species are
motile by means of flagella, and some possess gas
vacuoles. Most deposit sulphur as intracellular
granules, but Ectothiorhodospira deposit sulphur
extracellularly. Genera Thiospirillum (M),
Ectothiorhodospira (M), Chromatium (M),
Thiocystis (M), Thiocapsa, Lamprocystis (M, GV),
Thiodictyon (GV), Thiopedia (GV) and Amoebobacter
(GV). Purple Non-sulphur Bacteria Photoheterotroph
s most can grow photoautotrophically with H2 and
some with H2S, in low concentrations, or
thiosulphate many can grow aerobically in the
dark by chemoautotrophism using H2, e.g.
Rhodobacter capsulatus some can grow
anaerobically in the dark can photoassimilate
fatty acids, other organic acids, primary and
secondary alcohols, carbohydrates and aromatic
compounds by photoheterotrophism. The same range
of organic substrates photoassimilated may be
respired aerobically in the dark (except benzoate
which can not be respired). Prefer sulphide-poor
freshwater habitats. Some, e.g. Rhodopseudomonas
palustris and Rhodobacter sphaeroides are capable
of denitrification, using organic compounds as
energy sources. Rhodobacter sphaeroides can grow
on nitrate as the sole N source by
denitrification and nitrogen fixation. Most
require the vitamins biotin, niacin and thiamin,
and Rhodocyclus purpureus needs vitamin B12. Most
will also grow better if amino acids are
supplied. Genera Rhodospirillum (M),
Rhodopseudomonas (M), Rhodomicrobium (M,
prosthecate, exospores), Rhodopila (M),
Rhodocyclus (M or I), Rhodobacter (M or I).
Rhodopseudomonas reproduces by budding from the
cell pole, Rhodomicrobium by budding from the
hyphal tip, the rest reproduce by binary fission.
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Green Bacteria Bacteriochlorophylls a, c, d, e in
stalked vesicles attached to the CM, the stalk or
baseplate contains the bacteriochlorophyll a
antenna pigment and the chlorosome
bacteriochlorophyll c, d or e, and the CM
contains the reaction centre and ETS. Have PSI
only. Green Sulphur Bacteria Small, usually
immotile bacteria 5 genera. Strictly anaerobic
photoautotrophs that use H2S, H2 or other reduced
sulphur compounds as an electron donor the
sulphur produced is deposited extracellularly
prior to oxidation to sulphate. Require sulphide
as a source of sulphur and some require vitamin
B12. Many can fix nitrogen. Occur, along with
purple sulphur bacteria, in sulphide-rich
anaerobic illuminated aquatic environments. Can
photoassimilate acetate if CO2 and H2S are also
supplied. Fix CO2 by the reductive (reverse) TCA
cycle. Genera Chlorobium, Prosthecochloris
(prosthecate), Pelodictyon (GV, forms nets),
Ancalochloris (GV, prosthecae) and Chloroherpeton
(gliding filamentous). Green Non-sulphur Bacteria
(Chloroflexus Group) Chloroflexus is a
filamentous gliding bacterium, with filaments up
to 300 mm long and are thermophiles, thriving in
neutral or alkaline springs at 45-70 oC. It is a
photoheterotroph and facultative photoautotroph
or facultative chemoheterotroph. Form orange to
dull-green mats several mm thick. Produce
bacteriochlorophylls under anaerobic conditions
and can not grow at all anaerobically in the
dark. Can grow aerobically, in complex media, in
the light or dark, but have very low
bacteriochlorophyll content, and so appear orange
due to carotenoids. Obtains organic matter from
cyanobacteria, such as Synechococcus, with which
it can be grown in a mineral medium. Heliothrix
is also a gliding filamentous organism. Genera
Chloroflexus (gliding filamentous), Chloronema
(GV, gliding filamentous), Heliothrix (gliding
filamentous) and Oscillochloris (trichomes,
gliding motility, GV). Heliobacterium Heliobacteri
um chlorum are gliding rod-shaped cells can grow
as anaerobic photoheterotrophs requiring biotin
and fixing N2. Contain bacteriochlorophyll g. No
chlorosomes, pigment probably in the CM.
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Cyanobacteria Nitrogen Fixation Cyanobacteria are
the only organisms able to perform both oxygenic
photosynthesis and nitrogen fixation. Low
concentrations of oxygen rapidly and irreversibly
inactivate the nitrogenase enzymes. Most of these
cyanobacteria are filamentaous and produce
specialised nitrogen-fixing cells, called
heterocysts. Heterocyst and nitrogenase synthesis
is repressed when combined nitrogen is already
present. Lack of combined nitrogen stimulates
heterocyst and nitrogenase production, but if N2
is also absent, then development arrests at an
intermediate stage, called the proheterocyst.
About 5-10 of the cells develop into heterocysts
in a 30 hour period. Heterocyst. The heterocysts
have thick outer wall layers and the thylakoids
become concentrated near the cell poles and
special polar connections form where the
heterocyst is attached to vegetative cells.
Chlorophyll a is present, but phycobiliproteins
are absent. PS II is inactive and rubisco is also
lacking, so they can not fix CO2 nor produce O2
in the light. Respiration uses H2 generated
during nitrogen fixation (nitrogenase produces
one H2 for every N2 fixed). The heterocyst
depends upon the vegetative cells to supply
reductant. Heterocysts are formed at regular
intervals in long filaments. Adjacent cells in
the filament are joined by minute channels called
microplasmodesmata that exchange metabolites
between cells. Non-heterocystous nitrogen-fixing
cyanobacteria are facultative anoxygenic
photosynthesisers and fix nitrogen under
anaerobic growth conditions. Anoxygenic
Photosynthesis Oscillatoria limnetica, found in
hypersaline lakes, is capable of
sulphide-dependent CO2 photoassimilation.
Sulphide inhibits PS II and induces enzymes that
allow sulphide to donate electrons to PS I. The
oxidised product is elemental sulphur, which
accumulates as extracellular granules. In the
dark, stored polyglucose can be respired
anaerobically, using sulphur as the electron
acceptor, or else the cells can undergo
homolactic fermentation. Many strains are
facultatively chemotrophic in the dark, but these
maintain constituitve photosynthetic apparatus
and can photosynthesise immediately when light is
introduced. Many phycoerythrin-producing strains
exhibit complementary chromatic adaptation when
grown in green light they have a high
phycoerythrin to phycocyanin ratio, but when
grown in red light they have very little
phycoerythrin. This response appears to be
mediated by a phytochrome-like light-sensitive
pigment.
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Below left a cyanobacterial surface crust
covering an arid soil surface in Utah. These
colonisers add nitrogen and biomass. They rapidly
hydrate and resume growth when moisture is
present, producing the nodules shown below right.
Left green snow in springtime glacial regions
contains cyanobacteria. Cyanobacteria also occur
inside rocks (endolithic) in Arctic and Antarctic
deserts and inside limestone and inside coral
rubble and coral sand. Others deposit limestone
in reefs and hot springs.
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Left stromatolites (stromatoliths) are large
columnar deposits of calcium carbonate built-up
over immense periods of time by cyanobacteria.
The oldest fossil stromatolites are 2.7 billion
years old.
Symbiosis Hornworts (Anthocerophyta) have
internal mucilage-filled chambers that contain
endophytic nitrogen-fixing cyanobacteria.
Hornworts are primary colonisers of wet exposed
areas.
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Oscillatoria
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Prochloron
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Phototrophic Archaebacteria Halobacterium Flagella
ted, phototactic rods. In aerobic conditions the
cell membrane is red, due to carotenoids and the
cells grow as chemoheterotrophs. However, in
anaerobic conditions, purple membrane is laid
down in discrete patches (about 50 of total
membrane area) 75 of which is bacteriorhodopsin,
the remainder being lipid. Bacteriorhodopsin
spans the membrane 7 times and is in contact with
both the external and internal media and has a
retinal carotenoid chromophore linked via a
Schiff base to a lysine residue. Reaction with
(orange) light causes a conformational change in
the protein and deprotonation of the Schiff base.
The Schiff base is reprotonated with protons from
inside the cell and deprotonation releases these
protons to the outside. Thus, bacteriorhodopsin
is a light-driven proton pump that generates a
proton gradient for ATP synthesis. This process
maintains the cell under anaerobic conditions,
but does not allow it to grow, since O2 is needed
for retinal synthesis from beta-carotene.
Photoheterotrophic growth occurs when O2 levels
are high enough to synthesise bacteriorhodopsin,
but not too high to induce chemoheterotrophic
growth. The solubility of O2 is low in the brine
in which these halophiles grow. Halobacteria give
many salt lakes and salt fields their pink colour