Abstract. Skeletonema costatum is a common bloom-forming diatom and encounters
eutrophication and severe carbon dioxide (CO2) limitation during red
tides. However, little is known regarding the role of phosphorus (P) in
modulating inorganic carbon acquisition in S. costatum, particularly
under CO2 limitation conditions. We cultured S. costatum under
five phosphate levels (0.05, 0.25, 1, 4, 10 µmol L−1) and then
treated it with two CO2 conditions (2.8 and 12.6 µmol L−1)
for 2 h. The lower CO2 reduced net photosynthetic rate at lower
phosphate levels (< 4 µmol L−1) but did not affect it at
higher phosphate levels (4 and 10 µmol L−1). In contrast, the lower
CO2 induced a higher dark respiration rate at lower phosphate levels (0.05
and 0.25 µmol L−1) and did not affect it at higher phosphate levels
(> 1 µmol L−1). The lower CO2 did not change
relative electron transport rate (rETR) at lower phosphate levels (0.05 and
0.25 µmol L−1) and increased it at higher phosphate levels
(> 1 µmol L−1). Photosynthetic CO2 affinity
(1/K0.5) increased with phosphate levels. The lower CO2 did not
affect photosynthetic CO2 affinity at 0.05 µmol L−1 phosphate but enhanced it at the other phosphate levels. Activity of
extracellular carbonic anhydrase was dramatically induced by the lower
CO2 in phosphate-replete conditions (> 0.25 µmol L−1) and the same pattern also occurred for redox activity of the plasma
membrane. Direct bicarbonate (HCO3-) use was induced when phosphate
concentration was more than 1 µmol L−1. These findings indicate P
enrichment could enhance inorganic carbon acquisition and thus maintain the
photosynthesis rate in S. costatum grown under CO2-limiting
conditions via increasing activity of extracellular carbonic anhydrase and
facilitating direct HCO3- use. This study sheds light on how
bloom-forming algae cope with carbon limitation during the development of red
tides.