The literature, and previously unpublished data from the authors’ laboratories, shows that the δ13C of organic matter in marine macroalgae and seagrasses collected from the natural environment ranges from –3 to –35‰. While some marine macroalgae have δ13C values ranging over more than 10‰ within the thallus of an individual (some brown macroalgae), in other cases the range within a species collected over a very wide geographical range is only 5‰ (e.g. the red alga Plocamium cartilagineum which has values between –30 and –35‰). The organisms with very negative δ13C (lower than –30‰) are mainly subtidal red algae, with some intertidal red algae and a few green algae; those with very positive δ13C values (higher than –10‰) are mainly green macroalgae and seagrasses, with some red and brown macroalgae. The δ13C value correlates primarily with taxonomy and secondarily with ecology. None of the organisms with δ13C values lower than –30‰ have pyrenoids. Previous work showed a good correlation between δ13C values lower than –30‰ and the lack of CO2 concentrating mechanisms for several species of marine red algae. The extent to which the low δ13C values are confined to organisms with diffusive CO2 entry is discussed. Diffusive CO2 entry could also occur in organisms with higher δ13C values if diffusive conductance was relatively low. The photosynthesis of organisms with δ13C values more positive than –10‰ (i.e. more positive than the δ13C of CO2 in seawater) must involve HCO3- use.

Summary• Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes.• The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties.• The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil.• These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere.

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We present a theory describing how the d 15 N values of the nitrogen (N) pools in a vascular plant depend on that of its source N (nitrate), on 15 N/ 14 N fractionations during N assimilation, and on N transport within and N loss from the plant. The theory allows measured d 15 N values to be interpreted in terms of physiological processes. The d 15 N values of various N pools are calculated using three rules: (1) when a pool divides without transformation, there is no change in the d 15 N values of the N entering the resulting pools; (2) when nitrate is assimilated by nitrate reductase, the d 15 N values of the resulting pools (product and residual substrate) are described by a Rayleigh equation; (3) when two N pools mix, the d 15 N value of the mixture is a weighted average of the d 15 N values of the component pools. The theory is written as a spreadsheet and solved numerically. Potentially, it has multiple solutions. Some contravene physiological reality and are rejected. The remainder are distinguished, where possible, using additional physiological information. The theory simulated independent measurements of d 15 N in N pools of Brassica campestris L. var. rapa (komatsuna) and Lycopersicon esculentum Mill. cv. T-5 (tomato).Abbreviations: a = rate constant for 14 N-nitrate reduction/rate constant for 15 N-nitrate reduction; d 15 N [(R sample /R standard ) A1] á 10 3 &, where R sample is the 15 N/ 14 N ratio of a sample and R standard is that of atmospheric N 2 (0.0036765); NR nitrate reductase

Summary 1.Two studies using the stable-isotope 13 C have shown that large amounts of carbon can move between plants linked by arbuscular mycorrhizal fungi. Quantities comparable to the carbon cost of the symbiosis for an individual plant may be transferred. 2. We measured C transfer between linked plants of the grass Cynodon dactylon (C 4 , δ 13 C ≈ -14‰) and the herb Plantago lanceolata (C 3 , δ 13 C ~ -28‰). To test the hypothesis that the carbon transferred between plants remained in fungal structures at all times, plants were grown for two harvests; at the first harvest they were clipped to ground level, so that shoot re-growth required the transport of carbon from the roots. We also tested the influence of the direction of growth of the fungus, to determine whether C was transported out of or into a newly colonized root, and of growing plants in elevated CO 2 , to increase the availability of carbon compounds in the roots. 3. Large amounts of C were transferred between linked plants, more so into Plantago than into Cynodon roots. Transfer occurred whether root systems were separated by a 20 µm mesh, that excluded roots but not hyphae, or a 0·45 µm mesh, intended to act as a barrier to hyphae as well. We believe that the high root densities achieved in the experiment allowed hyphae to cross the finer mesh between the two dense root mats. 4. Clipping the plants did not result in any movement of C from roots to shoots, thus confirming the prediction that all C transferred remains in fungal structures. 5. The direction of growth of the fungus did not affect the direction of transfer, nor did the CO 2 concentration in which the plants were grown. 6. The amount of C transferred was a positive correlate of the frequency of vesicles in the roots but a negative correlate of the frequency of hyphae. If C were moving into developing colonization units, thus effectively giving the plant a 'free' symbiosis, the correlation with internal hyphae should be positive. The positive correlation with vesicles suggests that C is moving into fungal storage structures. 7. We propose a mycocentric view of the phenomenon of interplant C transfer, in which the fungal colonies within roots are seen as parts of an extended mycelium between which the fungus moves resources depending on the dynamics of its own growth. We do not believe that the transfer has an impact on plant C budgets or fitness, but that it may be a major element in the understanding of fungal C budgets.

To integrate the complex physiological responses of plants to stress, natural abundances (delta) of the stable isotope pairs 15N/14N and 13C/12C were measured in 30 genotypes of wild barley (Hordeum spontaneum C. Koch.). These accessions, originating from ecologically diverse sites, were grown in a controlled environment and subjected to mild, short-term drought or N-starvation. Increases in total dry weight were paralleled by less negative delta 13C in shoots and, in unstressed and droughted plants, by less negative whole-plant delta 13C. Root delta 15N was correlated negatively with total dry weight, whereas shoot and whole-plant delta 15N were not correlated with dry weight. The difference in delta 15N between shoot and root varied with stress in all genotypes. Shoot-root delta 15N may be a more sensitive indicator of stress response than shoot, root or whole-plant delta 15N alone. Among the potentially most productive genotypes, the most stress-tolerant had the most negative whole-plant delta 15N, whether the stress was drought or N-starvation. In common, controlled experiments, genotypic differences in whole-plant delta 15N may reflect the extent to which N can be retained within plants when stressed.

Much evidence suggests that life originated in hydrothermal habitats, and for much of the time since the origin of cyanobacteria (at least 2.5 Ga ago) and of eukaryotic algae (at least 2.1 Ga ago) the average sea surface and land surface temperatures were higher than they are today. However, there have been at least four significant glacial episodes prior to the Pleistocene glaciations. Two of these (approx. 2.1 and 0.7 Ga ago) may have involved a 'Snowball Earth' with a very great impact on the algae (sensu lato) of the time (cyanobacteria, Chlorophyta and Rhodophyta) and especially those that were adapted to warm habitats. By contrast, it is possible that heterokont, dinophyte and haptophyte phototrophs only evolved after the Carboniferous-Permian ice age (approx. 250 Ma ago) and so did not encounter low (