[en] Diatoms are experiencing striking fluctuations in seawater carbonate chemistry in the natural marine environment, especially in coastal seawaters. Here, we show that the diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, which utilize different carbon acquisition mechanisms, respond differently to short-term changes in seawater carbonate chemistry. Our results showed that T. weissflogii showed significantly higher photosynthetic oxygen evolution rates than that of P. tricornutum at low levels of CO₂ or HCO₃⁻. This suggests that T. weissflogii had higher affinities for CO₂ or HCO₃⁻ when their concentrations were not sufficient to support saturated growth and photosynthesis. While the activity of Rubisco in P. tricornutum positively correlated with carbonic anhydrases (CA), we observed negative relationship between Rubisco and CA activity in the diatom T. weissflogii. These contrasting physiological responses of diatoms with varied carbon acquisition mechanisms indicate different abilities to cope up with abrupt changes in seawater carbonate chemistry. We propose that the ability to respond to varying carbonate chemistry may act as one determinant of the diatom distributions and phytoplankton community structures.

[en] Intertidal macroalgae suffer different environmental conditions and mat densities during growing period. In the present study, Ulva lactuca Linnaeus were collected from high, intermediate, and low tidal zones at Nan’ao Island, China. These algal photosynthetic pigments and photosynthesis behaviors with different mat densities were measured. The aim is to examine how the physiological responses and acclimation match the representative tidal distribution and algal mat density. The photosynthetic pigment (chlorophyll a and carotenoid) contents and irradiance-saturated maximum photosynthetic rates (P_max) were greater in low zone–grown U. lactuca compared with the algae grown at high and intermediate zones. Under low algal mat density, the P_max, apparent photosynthetic efficiency (α), and dark respiration rate (R_d) of U. lactuca grown at low zone were increased, whereas the irradiance saturation points (I_k) were decreased, compared with the algae grown at higher zone. However, the P_max of high and intermediate zone–grown U. lactuca at high algal mat density were greater than at low density. Moreover, the pH compensation point of low zone–grown thalli (9.98) was lower than the higher zone–grown thalli (more than 10.15); however, the chlorophyll fluorescence parameters (reflect photosynthetic system activity) of the thalli collected from the three different zones were similar. Therefore, we proposed that the effects of varied densities on the photosynthetic rates of these three tidal zone–grown U. lactuca thalli were different, which might be related with different capacity of HCO₃⁻ utilization of macroalgae at their zonations.