Carbon-to-ATP ratios across the kingdoms of life

Estimating biomass is essential for quantifying energy flow, nutrient cycling, and ecosystem functioning across the biosphere. For nearly 60 years, the cellular carbon-to-ATP ratio (C:ATP, g/g) has been used to estimate marine microbial biomass, often assuming a value near 250. Here we compile ∼400 measurements from more than 80 studies spanning bacteria, unicellular eukaryotes, animals, plants, and tissues, and show that C:ATP varies by five orders of magnitude across species, physiological states, and environmental conditions. We then develop a mechanistic model demonstrating this variation arises from differences in ATP production rate, ATP turnover time, and the fraction of metabolically active carbon. The model predicts that median C:ATP is within a factor of two of 250 for bacteria and unicellular marine eukaryotes, while systematically differing in other groups. These results indicate that C:ATP should be treated as a context-dependent physiological quantity in biomass estimation and metabolic modeling across organisms and environments.