Ecological investigations of giant <i>Phaeocystis</i> colonies in Viet Nam: II. Photosynthesis—irradiance characteristics and nitrogen uptake

Phaeocystis globosa is a marine phytoplankton species that forms deleterious blooms in temperate and tropical waters. In some locations, "giant" colonies form, although the controls on its size are unknown. During a "giant" colony bloom, measurements were completed to characterize photosynthesis-irradiance relationships, nitrogen uptake kinetics, and nitrogen-irradiance relationships of P. globosa colonies to understand its growth characteristics and their relationship to colony size. The photosynthetic capacity (F_v/F_m) varied from 0.65 to 0.68 among colony sizes ranging from 3.0 to 11.0 mm, indicating that all colonial cells were physiologically robust. The maximum chl a-specific photosynthetic rates ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>P</mml:mi> <mml:mi>max</mml:mi> <mml:mi>B</mml:mi></mml:msubsup> </mml:mrow> </mml:semantics> </mml:math> ) ranged from 0.89 to 1.92 μg C · μg^-1 chl · h^-1, were maximal in the mid-sized colonies (5.5-6.5 mm) and decreased with size. The relatively low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msubsup><mml:mi>P</mml:mi> <mml:mi>max</mml:mi> <mml:mi>B</mml:mi></mml:msubsup> </mml:mrow> </mml:semantics> </mml:math> values may be related to the high cellular chl a of colonial cells and their acclimation to in situ irradiance. Nitrate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>V</mml:mi> <mml:mi>max</mml:mi></mml:msub> </mml:mrow> </mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mrow><mml:msub><mml:mi>K</mml:mi> <mml:mi>S</mml:mi></mml:msub> </mml:mrow> </mml:semantics> </mml:math> values were greater than those of ammonium, although N affinity was greater for ammonium. No differences in light-limited rates in either nitrate or ammonium uptake among colony sizes were observed, and no dark uptake occurred. Both ammonium and nitrate uptake showed a saturation response as a function of irradiance. While the driving forces for the formation of giant colonies remain unknown, their impacts on coastal systems are substantial and a further assessment of their growth is warranted.