Ecophysiology of Pavlomulina ranunculiformis (Rappephyceae, Haptophyta): Implications for its fundamental niche

SUMMARY Understanding the physiological traits of dominant primary producers is crucial for characterizing oceanic energy input. Pavlomulina ranunculiformis S. Sym, R. Pienaar & M. Kawachi, the sole described species of the class Rappephyceae, is a large‐sized, globally distributed haptophyte, yet its fundamental niche remains poorly defined. In this study, we investigated the growth characteristics of the type strain (NIES‐3900) under various light, temperature, and salinity conditions and compared these with its global distribution using the Tara Oceans 18S rRNA gene dataset. In culture, P. ranunculiformis grew under photon flux densities of 25–100 μmol photons m −2 s −1 , temperatures of 15–25°C, and salinities of 32–44‰. The maximum specific growth rate was approximately 0.35 day −1 , which is lower than the specific growth rates reported from the smaller, bloom‐forming haptophyte Gephyrocapsa huxleyi (Lohmann) P. Reinhardt (formerly known as Emiliania huxleyi ). Notably, the strain failed to grow at temperatures ≤10°C and ≥30°C. These physiological limits generally align with environmental DNA data showing the species is widely distributed in low‐to‐mid latitudes but absent in the Arctic Ocean, contrasting with the cold‐tolerant species such as G. huxleyi and Chrysochromulina leadbeateri Estep, Davis, Hargreaves & Sieburth. However, environmental DNA assigned to P. ranunculiformis was also detected in waters >30°C (Indian Ocean) and in the aphotic mesopelagic layer (>200 m depth). The detection in the deep water is explained by the passive vertical transport of their sinking biomass. Meanwhile, the presence of its DNA in waters exceeding the thermal limits of the cultured strain points to several alternative hypotheses, including uncultivated regional ecotypes or potential phenotypic plasticity that was not fully captured due to limited laboratory acclimation.