The effect of ulvoid macroalgae on the inorganic carbon utilization by an intertidal seagrass Thalassia hemprichii

Blooms of ulvoid macroalgae (mainly Enteromorpha and Ulva) have covered 80% of the intertidal seagrass bed at Wanlitung, southern Taiwan, effectively shading the seagrasss species Thalassia hemprichii resulting in a decrease in photosynthetic performance and low inorganic carbon (C_i) uptake. We looked for evidence of C_i limitation and investigated the C _i utilization characteristics of ulvoid-free and ulvoid-covered T. hemprichii. The rapid light curve (RLC) function of the Diving-PAM (Diving-PAM, Walz, Germany) was used to measure in situ photosynthetic performance (based on the effective quantum yield of PSII [Y] values) of intact seagrasses that were placed in small incubating chambers. Significantly, a lower RETR_max (maximum relative electron transport rate) and E_k (light intensity at the onset of saturation) were noted in the ulvoid-covered compared to the ulvoid-free T. hemprichii, suggesting that the former has acclimatized to the low light environment becoming a "shade type" plant. The ulvoid-covered T. hemprichii showed some evidence of C_i limitation since a significant increase in RETR_max (up to 46%; P < 0.05) was noted after an increase in the concentration of NaHCO₃ from 2.2 (normal seawater) to 6.2 mM. In terms of C_i utilization characteristics, T. hemprichii could directly absorb HCO₃^- as the major C_i source but partially depended on the extracellular carbonic anhydrase (CA) to convert HCO₃^- to CO₂ prior to uptake in the ulvoid-free, high light-adapted populations. A wastewater stream with a high nutrient load coming from the urbanized area may have caused the frequent blooms of ulvoid macroalgae.