Supporting Information Notes S1 and Table S1
Notes S1 Possibilities of replacement of phosphorus by other elements – phospholipids
The three major organic phosphorus components of plants are nucleic acids, low molecular mass
phosphate esters (including nucleotides) and phospholipids. Only in one case, the membrane
phospholipids (considered below), there is any evidence of the possibility of replacement of
phosphorus by other elements.
The obvious element for replacement of phosphorus is arsenic, but apart from its toxicity, recent
claims of essentially complete replacement of phosphorus by arsenic in a bacterium from Lake
Mona (Wolfe-Simon et al., 2011) have been contested on a number of grounds (Danchin, 2010;
Hengeveld, 2010). For sulfur, the only reported substitution is of sulfur for a non-bridge oxygen
in the phosphate in one per several thousand nucleotides in bacterial DNA; the substitution
(phosphothioation) is clearly not of phosphorus by sulfur (Zhou et al., 2005; Liu et al., 2010).
Sulfate forms acid anhydride (= ester) bonds with phosphorus in the nucleotides adenosine
phosphosulfate (APS) and phosphoadenosine phosphosulfate (PAPS) (Bick & Leustek, 1998).
APS is involved in sulfate reduction, and APS and PAPS are involved in the formation of sulfate
esters (Bick & Leustek, 1998), and not as a substitute for phosphate in the phosphate esters in
glycolysis, gluconeogenesis and the reductive and oxidative pentose phosphate cycles.
Constraints on membrane lipid composition
Despite the predominance of S-lipid and glycolipid in the thylakoid membrane of oxygenic
photosynthetic organisms it is difficult to find evidence of a particular role for S-lipid in
photosynthesis (Benning, 1998; Dörmann & Benning, 2002). S-lipid-deficient mutants have only
a marginally lower rate of photosynthesis, whereas the small P-lipid content of the thylakoid
membrane has a significant role (Benning, 1998). P-lipid, but not S-lipid, occurs in preparations
of trimers of Light Harvesting Complex II of Chlamydomonas reinhardtii, and growth of the Slipid-deficient mutants of Synechococcus PCC 7942 ceases before that of wild-type when P is
limiting in the growth medium, and P-lipid content is maintained as far as possible under Plimitation in the mutant (Benning, 1998).
The replacement of P-lipid by S-lipid and glycolipid (galactolipid) may have constraints.
Tjellström et al. (2010) showed that replacement of P-lipid by galactolipid in plant plasmalemma
with retention of function is restricted to the cytosolic as opposed to the outer (apoplasmic)
leaflet. By contrast, the galactolipids in the thylakoid membrane are largely confined to the
luminal leaflet, which corresponds topologically to the apoplasmic rather than the cytosolic
leaflet of the plasmalemma. There may be fewer constraints on the replacement of P-lipid by Slipid, at least in cyanobacteria and eukaryotic algae (Van Mooy et al., 2009). The highest Slipid:P-lipid ratio known for P-limited cultured cyanobacteria is 132 and for cultured eukaryotic
algae (diatoms) is 395; the corresponding values for P-replete cultures is 6.2 and 3.0, indicating
very large acclimation potential. The low-P grown cyanobacteria and algae clearly cannot just
have P-lipid replaced with S-lipid in only one of the leaflets in each bilayer fluid mosaic
membrane. Symbiotic diazotrophic rhizobia can have at least some of the P-lipid in the inner
(cytoplasmic) membrane replaced during P deficiency by S-lipid and also by an ornithinecontaining lipid and diacylglycerol trimethylhomoserine (Zavaleta-Pastor et al., 2010).
Membrane permeability as a function of the content of P-lipid, S-lipid and galactolipid
Proton permeability of the lipid components of biological membranes is not readily measured;
what values are available are summarised in Table S1. The galactolipid- and sulfolipid-
dominated thylakoid membranes (Forde & Steer, 1976) have a permeability to protons lower
than do membranes made from thylakoid membrane lipids in the ratio in which they occur in the
thylakoid, noting that there are limits to the extent to which ‘cylindrical’ lipids can be replaced
by the ‘conical’ monogalactosyl diacylglycerol (Fuks & Homble, 1994). However, lower proton
permeability values than for thylakoid membranes are found for digalactosyldiacylglycerol
bilayers. Values for phospholipid-dominated biological membranes are higher than for
thylakoids, and also higher than for phospholipid bilayers.
The compilation by Raven and Beardall (1981), in conductance units (mS m-2) also suggests that
the proton permeability of thylakoid membranes is lower than that of the inner mitochondrial
membrane and the cytoplasmic membrane of chemo-organotrophic bacteria. The data suggest
that the low-P lipid membranes have a lower proton permeability than the high-P lipid
membranes. It should be noted that any of these values, with the possible exception of those of
Berglund et al. (1999) for proton permeability are very difficult to reconcile with the functioning
of organisms growing at very low photon flux densities (Raven et al., 2000). This is because the
rate at which protons leak back through the lipid bilayer from the thylakoid lumen to the stroma
(cytosol of cyanobacteria) down the H+ gradient needed to drive the ATP synthetase exceeds that
rate at which the light-driven proton pumps in the thylakoid membrane can move protons into
the lumen, granted the H+ permeabilities cited above. For any light-driven H+ pump involving
photosystem II (PSII) there is the additional constraint of charge recombination back-reaction
between the reduced QA and QB and oxidised S states of the 4Mn-Ca cluster, which, like H+
leakage, proceeds at a constant rate and so becomes more important at low irradiances when the
forward redox reaction is slow (Raven et al., 2000). The PSII redox back-reaction and H+
leakage are multiplicative effects: less net electron flow through PSII means even less H+
pumping at low irradiances (Raven et al., 2000).
As for permeability to other ions, the only data seem to be those of Hurry et al. (2000) who
examined freezing tolerance, as measured by electrolyte leakage, in wild-type Arabidopsis
thaliana and in the pho mutant. The pho mutant has a lower freezing tolerance (higher
temperature for significant electrolyte leakage) than the wild-type, and also has a significant
replacement of phospholipid in membranes by digalactosyldiacylglycerol and sulfolipids, with a
constant level of monoalactosyldiacylglycerol (Dörmann & Benning, 2002).
To summarise, cyanobacteria, algae and plants show considerable capacity for acclimatory
(when grown at low P) and/or adaptive (for organisms whose normal habitat is low in P)
replacement of some fraction of their phospholipids by sulfolipid and/or galactolipid. The
organisms with low phospholipid content can clearly grow and reproduce. More work is needed
on any decreases in fitness and, for crops, yield associated with the replacement of phospholipid
by other polar lipids. At the membrane level, additional experiments are needed on how
membrane function is altered by substitution of phospholipids which will help to understand any
specific functions of phospholipids in membranes.
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Table S1 Proton Permeability (PH+) as a function of membrane lipid composition. The values
cited for PH+ do not distinguish permeation of H+ and of OH-, i.e. they are what Deamer and
Nicholls (1983) term Pnet.
Membrane
PH+ (m s-1)
References
Liposomes made of phospholipids
10-6
(Deamer & Nichols, 1983)
Phospholipid –dominated: inner
mitochondrial membrane, frog
sarcolemma (= muscle
plasmalemma), sarcoplasmic
reticulum (= muscle endoplasmic
reticulum
10-5
(Deamer & Nichols, 1983)
Thylakoid membranes, dominated
by sulfo-lipids and galactolipids
rather than phospho-lipids.
2-5∙10-7
(Schönfeld & Neumann, 1977; Bulychev et
al., 1980; Schonfeld & Schickler, 1984)
Bilayer made from thylakoid lipids
6∙10-6
(Fuks & Homble, 1994; Fuks & Homble,
1996)
Bilayers made from
digalactosyldiacyl glycerol
5-10∙10-10
(Berglund et al., 1999)