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Nine publications showing REE data in groundwaters and river water were examined to understand the general condition of terrestrial water where a negative Ce anomaly develops. It was found that the negative Ce anomaly only appeared when Fe, Mn and dissolved organic carbon (DOC) concentrations were low (Fe and Mn < 5 × 10 -5 mol/L and DOC < 10 mg/L). Assuming that Ce 3+ and Fe 2+ concentrations were limited by the formation of cerianite and ferrihydrite, respective redox potential (Eh) was calculated from each of Ce 3+ and Fe 2+ concentrations. For data sets displaying Ce anomalies, the calculated Ehs (cEhs) showed a 1:1 relationship between Ce and Fe. This relationship indicates that the absolute concentration of Ce in natural water may be thermodynamically determined in situ by Eh and pH when concentrations of Fe, Mn and DOC are poor.

Chemotrophic microorganisms synthesise biomass by utilising energy obtained from a set of chemical reactions that convert resources to by‐products, forming catabolic interactions. The amount of energy obtained per catabolic reaction decreases with the abundance of the by‐product named as the ‘abundant resource premium’. Consider two species, Species 1 and 2, Species 1 obtains energy from a reaction that converts resource A to by‐product B. Species 2 then utilises B as its resource, extracting energy from a reaction that converts B to C. Thus, the presence of Species 2 reduces the abundance of B, which improves the fitness of Species 1 by increasing the energy acquisition per reaction of A to B. We discuss the population dynamic implication of this effect and its importance in expanding a realised niche, boosting material flow through the ecosystem and providing mutualistic interactions among species linked by the material flow. Introducing thermodynamics into population ecology could offer us fundamental ecological insights into understanding the ecology of chemotrophic microorganisms dominating the subsurface realm.

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