RECONSTRUCTED ANCESTRAL ENZYMES SUGGEST THAT EARTH'S PHOTIC-ZONE TEMPERATURE MARKEDLY DECREASED OVER GEOLOGIC TIME

Stable isotope (δ¹⁸O and δ³⁰Si) studies of marine cherts suggesting that Earth’s oceans cooled from 55-85 °C in the Archean to the present ~15 °C have been subject to question due to uncertainties about oceanic isotopic compositions, diagenetic or metamorphic resetting of isotopic signatures, and the depositional environments of the rocks analyzed. Analyses of the thermal stability of reconstructed ancient enzymes provide an independent method by which to test the hypothesis of a hot early Earth. Although previous studies have shown that such enzymes of bacterial and archaeal taxa exhibit thermostabilities compatible with high Archean temperatures, these ancestral species may have inhabited local thermal environments not indicative of ambient surface conditions. To resolve such problems, we have analyzed reconstructed, enzymatically active NDK (nucleoside diphosphate kinase) – a ubiquitously occurring enzyme that converts nucleoside diphosphates to nucleoside triphosphates – and restricted our study to phototrophs, inhabitants of the photic-zone. Our findings indicate that host optimal growth temperatures, shown to be strongly correlated with NDK thermostability, of ancestral NDK in cyanobacteria (~2.9 Ga; 73 ± 9 °C), cyanobacterial nostocaleans (~2.2 Ga; 46 ± 10 °C), Viridiplantae (e.g., algae, ~775 Ma; 43 ± 19 °C), and Embryophyta (i.e., land plants, ~410 Ma; 39 ± 19 °C) decrease markedly with increasing, fossil record-based divergence age. We interpret our data to evidence a gradual cooling of Earth’s near-surface environment over geological history. Further studies will test this interpretation using additional reconstructed enzymes and/or taxa. These molecular techniques appear to hold great promise in investigating traditionally geological-paleontological problems and illuminating the long-term evolution of the Earth’s biosphere.