Phosphorus limitation drives seasonal enlargement of the coccosphere size in two coccolithophores

Coccolithophores are abundant marine microalgae that produce a cell cover (coccosphere) composed of calcium carbonate platelets (coccoliths), and that play a unique role in the global carbon cycle. Gephyrocapsa huxleyi is the most prevalent coccolithophore in the oceans. Experiments in vitro indicate that the coccosphere size in G. huxleyi increases or decreases during phosphorus (P) or nitrogen (N) deprivation, respectively. To test whether coccosphere size variation occurs in native populations and relates to specific macronutrient availability, we examined communities at the open sea “station A” in the Gulf of Aqaba, northern Red Sea, and at the open sea station THEMO-2 in the Eastern Mediterranean Sea. At station A, the average diameter of G. huxleyi coccospheres was larger during the stratified, oligotrophic season (6.98 ± 1.29 μm), and smaller during the mesotrophic winter (6.16 ± 1.04 μm), and along the deep-chlorophyll maximum during stratified periods (6.36 ± 1.09 μm). A similar seasonal variation in coccosphere diameter was observed in the sister species Gephyrocapsa ericsonii. Complementary bioassays indicate that G. huxleyi was primarily P-limited during the stratified period, while non-limited or weakly P-limited in winter. The seasonal pattern in G. huxleyi diameter was the opposite at the Mediterranean THEMO-2 station. Larger coccospheres were detected in winter and smaller ones during summer. However, this pattern coincided with the prevalent patterns of inorganic macronutrient availability in the Eastern Mediterranean. Therefore, inorganic phosphorus availability is a key driver of coccosphere size variations in marine ecosystems.