By comparing different water samples, we should be able to work out which groups of molecules (that tend to appear together) are most important to downstream ecosystems. We will also measure certain isotopes (rare forms of chemical elements) in our samples. When chemicals undergo certain transformations (e.g. inside of cells), chemicals containing these rare isotopes may be more or less likely to be transformed than chemicals containing common isotopes. This means, for instance, that if we start with a group of chemicals where 1% are rare isotopes, and half of them are transformed into new chemicals, then the proportion of rare isotopes in these new chemicals may be more or less than the original 1%. We can therefore use isotopic ratios to understand which processes a particular group of molecules has been through in order to reach that form. In this case, we can use isotopes of carbon and nitrogen to understand where the bioavailable dissolved organic matter is coming from. We will then use a form of radiocarbon dating to measure the age of carbon dioxide that is respired (given out) when bacteria consume DOM in our samples. This will tell us how old the bioavailable DOM molecules were before they were consumed by bacteria. This information is again useful to know when trying to work out the source of bioavailable DOM. For instance, if the DOM comes from tiny particles in the air released by burning fossil fuels (coal, oil and gas), then we would expect it to be very old (because fossil fuels are formed from prehistoric plants). In addition, there is a biological component to our plans. We will analyse a particular kind of genetic material (16S ribosomal RNA) in our natural samples and our incubation experiments. This rRNA is present in all living cells and is very conserved (i.e. it’s very similar in closely related species) because of its importance. By analysing all of the rRNA in a natural sample, we can get an idea of the entire community of aquatic microbes. If some samples contain DOM that favours one group of cells over another, then we should be able to see this in the 16S rRNA composition of the water. This allows us to link water chemistry – the quantity and quality of DOM – with tangible biological impacts. Lastly, we will compare the results with those from different regions (e.g. the Himalaya and the Andes) to see whether or not the same key groups of molecules are found across the world; if not, this indicates that the main source of bioavailable DOM varies between different regions.