Significance The membrane lipids of marine Archaea form the basis of the temperature proxy called TEX 86 , which is used for paleoclimate reconstructions from the Jurassic to the present. To date there remains no satisfactory explanation for how planktonic Archaea are able to record water column temperatures, because TEX 86 does not correlate well with in situ growth temperatures in the modern ocean. Here we show that the TEX 86 lipid ratio changes in response to cellular growth rate, which is controlled by the ammonia oxidation rate. This implies that variation in the TEX 86 ratio with water depth is influenced by the metabolic activity of Thaumarchaeota in the water column. Archaeal membrane lipids known as glycerol dibiphytanyl glycerol tetraethers (GDGTs) are the basis of the TEX 86 paleotemperature proxy. Because GDGTs preserved in marine sediments are thought to originate mainly from planktonic, ammonia-oxidizing Thaumarchaeota, the basis of the correlation between TEX 86 and sea surface temperature (SST) remains unresolved: How does TEX 86 predict surface temperatures, when maximum thaumarchaeal activity occurs below the surface mixed layer and TEX 86 does not covary with in situ growth temperatures? Here we used isothermal studies of the model thaumarchaeon Nitrosopumilus maritimus SCM1 to investigate how GDGT composition changes in response to ammonia oxidation rate. We used continuous culture methods to avoid potential confounding variables that can be associated with experiments in batch cultures. The results show that the ring index scales inversely ( R 2 = 0.82) with ammonia oxidation rate (ϕ), indicating that GDGT cyclization depends on available reducing power. Correspondingly, the TEX 86 ratio decreases by an equivalent of 5.4 °C of calculated temperature over a 5.5 fmol·cell −1 ·d −1 increase in ϕ. This finding reconciles other recent experiments that have identified growth stage and oxygen availability as variables affecting TEX 86 . Depth profiles from the marine water column show minimum TEX 86 values at the depth of maximum nitrification rates, consistent with our chemostat results. Our findings suggest that the TEX 86 signal exported from the water column is influenced by the dynamics of ammonia oxidation. Thus, the global TEX 86 –SST calibration potentially represents a composite of regional correlations based on nutrient dynamics and global correlations based on archaeal community composition and temperature.