The light and temperature dependence of growth rate and elemental composition of Phaeocystis globosa scherffel and P. Pouchetii (HAR.) Lagerh. in batch cultures

The light and temperature dependence of growth, cellular elemental composition (C, N, P) and chlorophyll content during exponential growth and phosphate starvation of batch cultures of the temperate algal species Phaeocystis globosa were investigated. Temperature-dependent minimum generation time and cell phosphorus content were compared with those of the cold water species P. pouchetii. The variation in growth and cell carbon content was due to the different origin of seawater used for the culture media. Coastal, estuarine seawater enabled shorter doubling times than “offshore” and oceanic-influenced seawater. At nutrient-saturated growth, P. globosa was able to compete with diatoms only if high temperature (14°C) and high irradiances (180 μE·m⁻²·s⁻¹) predominated. P. pouchetti showed markedly shorter generation times than P. globosa did at temperatures below 10°C. At 6°C, the cell carbon content exceeded the value at 18°C by a factor of 1.7. High C/N atomic ratios of 6.9±0.7 and high C/Chl a ratios of ≥ 54.0 during exponential growth could not entirely be explained by the carbon content of the colony mucus. There was no significant light and temperature dependence of the : N : P ratio during exponential growth. At phosphate starvation (subsistence P quota of cells) this was so only at temperatures higher than 10°C. With their stored phosphorus quota P. globosa and P. pouchetii were capable of only two cell divisions. Hence, because of their limited ability to store phosphorus, the two Phaeocystis species cannot compete with planktonic diatoms for growth. The frequent change in nutrient concentrations (particularly of silicate and phosphate) in coastal and estuarine waters at summer temperatures and light conditions causes a rapid alternation of smaller blooms of P. globosa and diatoms.