Inorganic carbon pools in the bloom-forming dinoflagellate Peridinium gatunense

Peridinum gatunense, the main bloom-forming species in Lake Kinneret, acquires a carbon-concentrating mechanism (CCM) during the progression of the annual winter-spring bloom. Previous indications for CCM activity in P. gatunense were based on decreased carbon isotope fractionation and higher activity of carbonic anhydrase (both external and internal). In the present study, CCM activity in low-density, natural, and cultured P. gatunense populations was demonstrated by cells showing higher internal inorganic carbon (C₁) concentrations than those in the external medium. The presence, size, and dynamics of these C₁ pools were measured using pulse-chase experiments of ¹⁴C uptake into photosynthetic acid-stable products under varying C₁, pH, CO₂(aq), and light regimes. Internal C₁ levels in P. gatunense were higher by a factor of ∼20-80 relative to the ambient C₁. The time required to reach isotopic equilibration of the pool ranged from 30 to 90 min, whereas the time required for the assimilation of the pool into cellular carbon ranged from 10 to 50 min. These long timescales are attributed to the larger vol: surface area ratio of this organism. Induction of the CCM was triggered by CO₂(aq) rather than C₁ and was evident at CO₂(aq) concentrations <15 μM. Preincubation in either the dark or with high CO₂(aq) levels diminished the C₁ pool. The data we report represent the steady-state carbon metabolism of the species since the cells were not deprived of C₁ prior to the measurements. The CCM in P. gatunense is an important adaptation mechanism contributing to the high primary production observed in the lake at the peak of the bloom under CO₂-limiting conditions [pH >9.5 and CO₂(aq) <5 μM].