Combined effects of inorganic carbon and light on Phaeocystis globosa Scherffel (Prymnesiophyceae)

Phaeocystis globosa (Prymnesiophyceae) is a globally dominating phytoplankton species. It plays an important role in both the global sulfur and carbon cycles, by the production of dimethylsulfide (DMS) and the drawdown of inorganic carbon. Phaeocystis globosa has a polymorphic life cycle and is considered to be a harmful algal bloom (HAB) forming species. All these aspects make this an interesting species to study the effects of increasing carbon dioxide (CO₂) concentrations, due to anthropogenic carbon emissions. Here, the combined effects of three different dissolved carbon dioxide concentrations (CO_2(aq)) (low: 4 μmol kg⁻¹, intermediate: 6–10 μmol kg⁻¹ and high CO_2(aq): 21–24 μmol kg⁻¹) and two different light intensities (low light, suboptimal: 80 μmol photons m⁻²s^–1 and high light, light saturated: 240 μmol photons m⁻²s⁻¹) are reported. The experiments demonstrated that the specific growth rate of P. globosa in the high light cultures decreased with increasing CO_2(aq) from 1.4 to 1.1 d⁻¹ in the low and high CO₂ cultures respectively. Concurrently, the photosynthetic efficiency increased with increasing CO_2(aq) from 0.56 to 0.66. The different light conditions affected photosynthetic efficiency and chlorophyll-a concentrations, both of which were lower in the high light cultures as compared to the low light cultures. These results suggest that in the future, inorganic carbon enriched oceans, P. globosa will become less competitive and feedback mechanisms to global change may decrease in strength.