Planktonic adaptive evolution to the sea surface temperature in the Neoproterozoic inferred from ancestral NDK of marine cyanobacteria

The optimum growth temperature of ancestral cyanobacteria inhabiting the sea surface in the Neoproterozoic was estimated based on the thermal stability of experimentally reconstructed ancestral NDK enzymes. Ancestral NDKs of cyanobacteria that diversified ∼1.7, ∼1.0, ∼0.9, ∼0.7, ∼0.6, and ∼0.5 billion years ago were reconstructed and analyzed, and the unfolding midpoint temperatures (T_ms) ranged from ∼65 °C to ∼70 °C. Among the host of analyzed NDKs, the ancestors of marine, planktonic α-cyanobacteria diversified ≤ ∼1.0 Ga are highly likely to have inhabited marine environments during the Neoproterozoic, while ancestral cyanobacteria diversified ∼1.7 billion years ago were possibly marine but the habitat is less constrained compared to the others. According to the calibration curves derived from extant organisms, the obtained T_ms of α-cyanobacteria diversified ≤ ∼1.0 Ga correspond to the range of optimum growth temperatures of around ∼33–48 °C. The temperature range agrees well with the long-term sea temperature trend during Neoproterozoic suggested by δ¹⁸O and δ³⁰Si records from marine cherts. Adaptation to the low temperature during the snowball glaciations in the late Neoproterozoic was not observed, implying that adaptation of optimum growth conditions to the episodic low temperature may not have been necessary. Therefore, ancestral marine plankton must have consistently adapted to the interglacial sea surface temperature in the Neoproterozoic, which was approximately 5–20 °C higher than that is today. They may have survived the glaciations by acquiring cold tolerance and/or by suppressing growth rate.