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
Photoheterotrophs; 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.
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
Heliobacterium 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.
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
photosynthesisers and fix nitrogen under anaerobic growth conditions.
anoxygenic
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.
Anabaena spiralis
Nostoc
Nostoc colonies
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.
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.
Oscillatoria
Prochloron
Phototrophic Archaebacteria
Halobacterium
Flagellated, 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: