The arbuscular mycorrhizal (Am) fungal symbiosis is widely hypothesized to have promoted the evolution of land plants from rootless gametophytes to rooted sporophytes during the mid-Palaeozoic (480-360 myr, ago), at a time coincident with a 90% fall in the atmospheric Co 2 concentration ([Co 2 ] a ). Here we show using standardized dual isotopic tracers ( 14 C and 33 P) that Am symbiosis efficiency (defined as plant P gain per unit of C invested into fungi) of liverwort gametophytes declines, but increases in the sporophytes of vascular plants (ferns and angiosperms), at 440 p.p.m. compared with 1,500 p.p.m. [Co 2 ] a . These contrasting responses are associated with larger Am hyphal networks, and structural advances in vascular plant waterconducting systems, promoting P transport that enhances Am efficiency at 440 p.p.m. [Co 2 ] a . our results suggest that non-vascular land plants not only faced intense competition for light, as vascular land floras grew taller in the Palaeozoic, but also markedly reduced efficiency and total capture of P as [Co 2 ] a fell.

Volatile fatty acids (VFAs) are central intermediates of anaerobic metabolism and present in aquatic environments such as marine sediments. Conceptually, the isotopic composition of volatile fatty acids is presumed to be sensitive to the specific biogeochemical processes involved in their production and consumption. However, due to generally low environmental concentrations, our knowledge on isotopic variability of VFAs is limited. We report the development and application of a new protocol for compound-specific carbon isotopic analysis of VFAs in marine porewaters and other aqueous liquids. This new protocol involves reversed-phase separation of volatile fatty acids with an aqueous mobile phase by high performance liquid chromatography (HPLC) combined with chemical oxidation of the effluents by the Finnigan TM LC IsoLink interface (Krummen et al. 2004) and subsequent online transfer of the resulting CO 2 into an isotope ratio mass spectrometer. We obtained reproducible and accurate results for pure Na-acetate dissolved in artificial seawater at concentrations as low as 2 μM, whereas minimum concentrations in marine porewaters were 10 μM. Our technique extends the previously accessible concentration range and the fully automated online operation allows for systematic analysis of large sample sets. These technical improvements make carbon isotopic analysis of selected VFAs a realistic perspective for many sedimentary environments. Our initial survey of acetate in porewaters and fluids obtained from incubations of marine sediments has revealed an unexpectedly large range of isotopic compositions from -5‰ to -85‰ and provides strong support for process-specific information encoded in the isotopic compositions of VFAs.

The saline springs of Gypsum Hill in the Canadian high Arctic are a rare example of cold springs originating from deep groundwater and rising to the surface through thick permafrost. The heterotrophic bacteria and autotrophic sulfur-oxidizing bacteria (up to 40% of the total microbial community) isolated from the spring waters and sediments were classified into four phyla (Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria) based on 16S rRNA gene analysis; heterotrophic isolates were primarily psychrotolerant, salt-tolerant, facultative anaerobes. Some of the isolates contained genes for thiosulfate oxidation (soxB) and anoxygenic photosynthesis (pufM), possibly enabling the strains to better compete in these sulfur-rich environments subject to long periods of illumination in the Arctic summer. Although leucine uptake by the spring water microbial community was low, CO 2 uptake was relatively high under dark incubation, reinforcing the idea that primary production by chemoautotrophs is an important process in the springs. The small amounts of hydrocarbons in gases exsolving from the springs (0.38 to 0.51% CH 4 ) were compositionally and isotopically consistent with microbial methanogenesis and possible methanotrophy. Anaerobic heterotrophic sulfur oxidation and aerobic autotrophic sulfur oxidation activities were demonstrated in sediment slurries. Overall, our results describe an active microbial community capable of sustainability in an extreme environment that experiences prolonged periods of continuous light or darkness, low temperatures, and moderate salinity, where life seems to rely on chemolithoautotrophy.Perennial springs are extremely rare in areas underlain by deep, continuous permafrost, because of the limited opportunity for exchange between sub-and suprapermafrost groundwater systems. The perennial springs found at Gypsum Hill (GH) on west-central Axel Heiberg Island in the Canadian high Arctic originate from deep saline groundwater and rise to the surface through ϳ600-m-thick continuous permafrost in a region with a mean annual temperature of Ϫ15°C (38). They discharge oligotrophic brines (7.5 to 7.9% salt) that are rich in inorganic sulfur compounds, saturated with dissolved gasses (primarily N 2 ), and maintain a constant temperature (Ϫ1.3°C to 6.9°C) throughout the year despite air temperatures that drop below Ϫ40°C during the winter (36, 38). The springs' location at nearly 80 o N exposes them to long periods of continuous illumination or total darkness during the Arctic summer and winter, respectively.Recent reports have examined mesophilic (15) and cold sulfur (14, 22, 23) springs, including cold springs in Germany that harbor a string-of-pearls-like community consisting of microbial filaments of Archaea in close association with sulfideoxidizing bacteria related to Thiothrix (33). Sulfur springs usually support a diversity of sulfur-oxidizing bacteria, including anaerobic anoxygenic phototrophs (purple and green sulfur bacteria) that use reduced sulfur compounds as electron donors...

Belowground interactions between plant roots, mycorrhizal fungi and plant growth-promoting rhizobacteria (PGPR) can improve plant health via enhanced nutrient acquisition and priming of the plant immune system. Two wheat cultivars differing in their ability to form mycorrhiza were (co)inoculated with the mycorrhizal fungus Rhizophagus irregularis and the rhizobacterial strain Pseudomonas putida KT2440. The cultivar with high mycorrhizal compatibility supported higher levels of rhizobacterial colonization than the low compatibility cultivar. Those levels were augmented by mycorrhizal infection. Conversely, rhizobacterial colonization of the low compatibility cultivar was reduced by mycorrhizal arbuscule formation. Single inoculations with R. irregularis or P. putida had differential growth effects on both cultivars. Furthermore, while both cultivars developed systemic priming of chitosan-induced callose after single inoculations with R. irregularis or P. putida, only the cultivar with high mycorrhizal compatibility showed a synergistic increase in callose responsiveness following co-inoculation with both microbes. Our results show that multilateral interactions between roots, mycorrhizal fungi and PGPR can have synergistic effects on growth and systemic priming of wheat.

SummaryMycorrhizal functioning in the fern Ophioglossum is complex and poorly understood. It is unknown whether mature O. vulgatum sporophytes form mutualistic associations with fungi of the Glomeromycota and with what specificity. Are green sporophytes able to 'repay' fungal carbon (C) invested in them by mycorrhizal partners during the initially heterotrophic gametophyte and early sporophyte stages of the lifecycle?We identified fungal partners of O. vulgatum sporophytes using molecular techniques and supplied them with 33 P-orthophosphate and O. vulgatum sporophytes with 14 CO 2 . We traced the movement of fungal-acquired nutrients and plant-fixed C between symbionts and analysed natural abundance 13 C and 15 N isotope signatures to assess nutritional interactions. We found fungal specificity of O. vulgatum sporophytes towards a mycorrhizal fungus closely related to Glomus macrocarpum. Our radioisotope tracers revealed reciprocal C-forphosphorus exchange between fern sporophytes and fungal partners, despite competition from surrounding vegetation. Monocultures of O. vulgatum were enriched in 13 C and 15 N, providing inconclusive evidence of mycoheterotrophy when experiencing competition from the surrounding plant community. We show mutualistic and specific symbiosis between a eusporangiate fern and fungi of the Glomeromycota. Our findings suggest a 'take now, pay later' strategy of mycorrhizal functioning through the lifecycle O. vulgatum, from mycoheterotrophic gametophyte to mutualistic aboveground sporophyte.

Wheat yields have plateaued in the UK over the last 25 years, during which time most arable land has been annually cropped continuously with short rotations dominated by cereals. Arable intensification has depleted soil organic matter and biology, including mycorrhizas, which are affected by tillage, herbicides, and crop genotype. Here, we test whether winter wheat yields, mycorrhization, and shoot health can be improved simply by adopting less intensive tillage and adding commercial mycorrhizal inoculum to long-term arable fields, or if 3-year grass-clover leys followed direct drilling is more effective for biological regeneration of soil with reduced N fertiliser. We report a trial of mycorrhization, ear pathology, and yield performance of the parents and four double haploid lines from the Avalon x Cadenza winter wheat population in a long-term arable field that is divided into replicated treatment plots. These plots comprised wheat lines grown using ploughing or disc cultivation for 3 years, half of which received annual additions of commercial arbuscular mycorrhizal (AM) inoculum, compared to 3-year mown grass-clover ley plots treated with glyphosate and direct-drilled. All plots annually received 35 kg of N ha−1 fertiliser without fungicides. The wheat lines did not differ in mycorrhization, which averaged only 34% and 40% of root length colonised (RLC) in the ploughed and disc-cultivated plots, respectively, and decreased with inoculation. In the ley, RLC increased to 52%. Two wheat lines were very susceptible to a sooty ear mould, which was lowest in the ley, and highest with disc cultivation. AM inoculation reduced ear infections by >50% in the susceptible lines. In the ley, yields ranged from 7.2 to 8.3 t ha−1, achieving 92 to 106% of UK average wheat yield in 2018 (7.8 t ha−1) but using only 25% of average N fertiliser. Yields with ploughing and disc cultivation averaged only 3.9 and 3.4 t ha−1, respectively, with AM inoculum reducing yields from 4.3 to 3.5 t ha−1 in ploughed plots, with no effect of disc cultivation. The findings reveal multiple benefits of reintegrating legume-rich leys into arable rotations as part of a strategy to regenerate soil quality and wheat crop health, reduce dependence on nitrogen fertilisers, enhance mycorrhization, and achieve good yields.