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The voltage-gated ion channel CALHM1 is vital to taste-stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells in mice, but does not seem relevant to the recognition of sour and salty tastes. Perception of sweet, bitter and umami tastes relies on the non-vesicular release from taste bud cells of ATP, which acts as a neurotransmitter to activate the nerves sending gustatory signals to the brain. Now teams led by Kevin Foskett and Philippe Marambaud have used gene-knockout mice to show that the voltage-gated ion channel CALHM1 (calcium homeostasis modulator 1) acts as a voltage-gated ATP-release channel required for sweet, bitter and umami taste perception. CALHM1 deficiency affected taste perception without interfering with taste cell development or integrity, implying that it is a key functional component of the peripheral taste system. Recognition of sweet, bitter and umami tastes requires the non-vesicular release from taste bud cells of ATP, which acts as a neurotransmitter to activate afferent neural gustatory pathways1. However, how ATP is released to fulfil this function is not fully understood. Here we show that calcium homeostasis modulator 1 (CALHM1), a voltage-gated ion channel2,3, is indispensable for taste-stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells. Calhm1 knockout mice have severely impaired perceptions of sweet, bitter and umami compounds, whereas their recognition of sour and salty tastes remains mostly normal. Calhm1 deficiency affects taste perception without interfering with taste cell development or integrity. CALHM1 is expressed specifically in sweet/bitter/umami-sensing type II taste bud cells. Its heterologous expression induces a novel ATP permeability that releases ATP from cells in response to manipulations that activate the CALHM1 ion channel. Knockout of Calhm1 strongly reduces voltage-gated currents in type II cells and taste-evoked ATP release from taste buds without affecting the excitability of taste cells by taste stimuli. Thus, CALHM1 is a voltage-gated ATP-release channel required for sweet, bitter and umami taste perception.

It is generally thought that, in order to compensate for lower solar flux and maintain liquid oceans on the early Earth, methane must have been an important greenhouse gas before approximately 2.2 billion years (Gyr) ago. This is based upon a simple thermodynamic calculation that relates the absence of siderite (FeCO3) in some pre-2.2-Gyr palaeosols to atmospheric CO2 concentrations that would have been too low to have provided the necessary greenhouse effect. Using multi-dimensional thermodynamic analyses and geological evidence, we show here that the absence of siderite in palaeosols does not constrain atmospheric CO2 concentrations. Siderite is absent in many palaeosols (both pre- and post-2.2-Gyr in age) because the O2 concentrations and pH conditions in well-aerated soils have favoured the formation of ferric (Fe3+)-rich minerals, such as goethite, rather than siderite. Siderite, however, has formed throughout geological history in subsurface environments, such as euxinic seas, where anaerobic organisms created H2-rich conditions. The abundance of large, massive siderite-rich beds in pre-1.8-Gyr sedimentary sequences and their carbon isotope ratios indicate that the atmospheric CO2 concentration was more than 100 times greater than today, causing the rain and ocean waters to be more acidic than today. We therefore conclude that CO2 alone (without a significant contribution from methane) could have provided the necessary greenhouse effect to maintain liquid oceans on the early Earth.

Microorganisms have flourished in the oceans since at least 3.8 billion years (3.8 Gyr) ago1,2, but it is not at present clear when they first colonized the land. Organic matter in some Au/U-rich conglomerates and ancient soils of 2.3–2.7 Gyr age has been suggested as remnants of terrestrial organisms3,4,5. Some 2.7-Gyr-old stromatolites have also been suggested as structures created by terrestrial organisms6,7. However, it has been disputed whether this organic matter is indigenous or exogenic, and whether these stromatolites formed in marine or fresh water. Consequently, the oldest undisputed remnants of terrestrial organisms are currently the 1.2-Gyr-old microfossils from Arizona, USA8. Unusually carbonaceous ancient soils—palaeosols—have been found in the Mpumalanga Province (Eastern Transvaal) of South Africa9. Here we report the occurrences, elemental ratios (C, H, N, P) and isotopic compositions of this organic matter and its host rocks. These data show that the organic matter very probably represents remnants of microbial mats that developed on the soil surface between 2.6 and 2.7 Gyr ago. This places the development of terrestrial biomass more than 1.4 billion years earlier than previously reported.

THE solubilities of solid compounds in solution, and the kinetics of the reactions that take place between solid and liquid phases, have usually been studied by indirect methods, which are often time-consuming and can be inaccurate. Here we present a new experimental method, using the titration principle and in situ observation of solution–solid reactions on a solid surface, which allows the rapid acquisition of accurate data on the dissolution and growth rates and the solubilities (both stoichiometric and nonstoichiometric) of solids in a variety of aqueous and organic solutions at temperatures up to 350 °C and pressures up to 200 bar. This technique should provide new opportunities for determining the phase diagrams of complex systems and for studying crystal growth.

Ohmoto et al. reply - The idea of a methane-rich Archaean atmosphere has become popular since Rye et al. assumed in their calculation1 that siderite was absent in pre-2.2-Gyr palaeosols. We have concluded that the absence of siderite in some Archaean palaeosols does not constrain the atmospheric pCO2, but the presence of much siderite in sedimentary rocks does2. Sleep's recognition3 that siderite occurs in Archaean palaeosols substantiates our arguments2: although siderite should be absent in well aerated soils of all geological ages, it may form in waterlogged soils where pO2 became less than about 10−60 atm owing to the abundant anaerobic production of H2. In fact, we have reported this in a 2.6-Gyr soil profile at Schagen, South Africa4: abundant ferric-rich minerals formed while the soil was exposed to air, but ferrous-rich carbonate formed while it was apparently submerged under an anoxic pond.

In recent marine sediments, bacterial reduction of seawater sulphate is responsible for the formation of diagenetic sulphides, which are typically strongly depleted in 34S relative to source sulphate, and highly variable in their δ 34S values. In contrast, the δ 34S values of Archaean sedimentary sulphides are generally found to be less variable and nearly identical to those of sulphates in the same sedimentary units. This finding has led previous investigators1–4 to suggest that either sulphate-reducing bacteria had yet to develop in Archaean time, (especially before ∼2.75 billion years ago), and/or Archaean oceans contained much less sulphate ( 1 mM) of sulphate, with δ 34S values of +3%