Nickel stable isotopes as tracers of modern and ancient biogeochemical processes

Genetic data suggest a possible origin of life on Earth as far back as 4 billion years ago, but tests of this idea, and other aspects of the early evolution of the biosphere, are few and far between. The earliest forms of life were definitely simple - single-celled and, probably restricted to prokaryotes (simple organisms without a cell nucleus) certainly in the Hadean (up to 3.8 billion years ago) and probably until at least the end of the Archaeon eon (at 2.5 billion years ago). The fossil record this far back is sparse, partly because the microfossils concerned are not readily preserved and partly because the rock record itself is sparse and has additionally been subjected to deformation and heating in the intervening period. So testing ideas about this early history of life using the morphological fossil record is extremely difficult. Instead scientists have often turned to geochemical tools. Life, and the few remnants of it that are found in old rocks, is geochemically distinctive. Moreover, it leaves a strong imprint on the environment. So geochemical tools fall into two broad categories: 1) isotopic and organic geochemical analyses of organic remnants that are no longer in the form of morphological fossils; 2) investigation of the physicochemical environment of the surface Earth, aspects of which are fundamentally controlled by the biosphere at the present-day. Examples of the latter are the cycles of many elements at the Earth's surface, notably carbon, oxygen and nitrogen. In this proposal we seek to develop a new tool - the stable isotopes of nickel. Recent research has demonstrated that nickel is an absolutely essential metal for methanogens, organisms that derive energy by reducing either CO2 or simple carbohydrates to methane (CH4). The genetic data suggest that methanogens were among the first organisms to evolve on the early Earth, while models of the Earth's early atmosphere and climate suggest a key role for methane as a greenhouse gas that may have its source in methanogens. If life has evolved on other planets it is highly likely that metabolisms very similar to methanogens were involved. Our proposal is highly multi-disciplinary and combines expertise in state-of-the-art isotope geochemistry with modern microbiological techniques. It also involves input from scientists working on deep-sea hydrothermal vents - one of the likely places for life to have first evolved on Earth and environments where the microbes today are dominated by methanogens. Our specific plans are listed below. 1) We have already produced the first nickel isotope data for the Earth. Our first aim is to expand on this dataset to characterise the key reservoirs on the Earth for Ni, before focusing in on methanogens. In our preliminary dataset, the only samples that have pronounced Ni stable isotope variations are those where biology has played a prominent role, including pure cultures of methanogens. Another objective is to culture a range of organisms, but particularly more methanogens, to investigate how they incorporate Ni and modify Ni isotopes. We will then extend this to natural samples where methanogens are important components of the biomass - lacustrine, estuarine and marine microbial mats, deep-sea sediments and hydrothermal vent settings. Our long-term aim in this research program is to develop Ni stable isotopes as a marker for ancient life. We see the full realisation of this objective as beyond the present proposal but we seek to begin here the search for Ni stable isotope variations in the early Earth through an array of organic-carbon-rich samples available to us via existing and previous collaborations.