Poster: Nutrient Biology

Abs # 401: Mechanisms for the uptake and metabolism of selenium by a coccolithophorid, Emiliania huxleyi

Presenter:	Obata, Toshihiro , ainame@kun.ne.jp
Authors	Obata, Toshihiro  (A)   Araie, Hiroya  (A)   Shiraiwa, Yoshihiro  (A)
Affiliations:	(A): Institute of Biological Sciences, University of Tsukuba

Coccolithophorids are unicellular marine calcifying algae that produce CaCO₃ crystals on the cell surface. We previously found that nanomolar level of Se is essential for growth of coccolithophorids but micromolar level of it was toxic. Therefore, Se can be a very important growth-regulatory factor for coccolithophorids but its physiological function is remained unsolved. As the algae distribute widely in the ocean and produce the huge amount of biomass, studies on the regulatory factor for growth of the algae is very important for understanding the control of coccolithophorid-bloom in the ocean. In this study, we examined the uptake and concentrating mechanisms of Se by a coccolithophorid, Emiliania huxleyi, using a ⁷⁵Se-radiotracer. The intracellular concentration of Se increased 2400-fold where Se-concentration in the medium was the same as the seawater-level. Time course of Se-uptake clearly showed biphasic pattern: a rapid phase that continued only within the first 30 s, and the subsequent slow phase that continued 10 h and more. The kinetic analyses of the rapid phase revealed that selenite was absorbed by both active and passive transport systems. During the slow phase, 50% and 20% of ⁷⁵Se was incorporated into fractions containing low-molecular-mass compounds (LMC) and proteins, respectively, but no ⁷⁵Se-selenite was detected. Five kinds of ⁷⁵Se-labelled compounds in LMC-fraction were detected by thin layer chromatography, and two of which were produced with higher rate than the others. Six kinds of ⁷⁵Se-labelled proteins were newly found by SDS-PAGE. In conclusion, the coccolithophorid has an ability to accumulate Se as Se-containing LMC and proteins that are essential for their growth, but not inorganic form.