Abstract
Atmospheric CO 2 concentration is predicted to nearly double by the end of this century.There are a large number of reports on the effects of elevated atmospheric CO 2 concentrations on seaweeds.However,the investigation concerning the impacts of combined effects of elevated atmospheric CO 2 concentrations and temperature on seaweeds is very limited.In the present study,the red seaweed Porphyra haitanensis(Bangiales,Rhodophyta) was cultured under four different conditions:(1) 15 ℃ + 390μmol/mol CO 2,(2) 15 ℃ + 700μmol/mol CO 2,(3) 25 ℃ + 390μmol/mol CO 2,(4) 25 ℃ + 700μmol /mol CO 2,in order to examine the combined effects of the elevated CO 2 concentrations and temperature on growth,some biochemical components and chlorophyll fluorescence in this commercially important species.The results indicated that the growth response of P.haitanensis to elevated CO 2 in seawater was temperature dependent,with the enhancement of growth resulting from elevated CO 2 being remarkable greater at lower growth temperature(15 ℃) with respect to 25 ℃.The CO 2 conditions in culture exerted a much pronounced influence on the contents of Chlorophyll a(Chl a) and Carotenoid(Car) than temperature did.15 ℃-grown P.haitanensis exhibited much higher maximum relative electron transport rate(rETR max) in comparison with 25 ℃-grown thalli,regardless of the CO 2 concentrations in culture.This indicated that P.haitanensis displayed a higher photosynthetic potential at low growth temperature than high growth temperature.Meanwhile,extra CO 2 in the culture had no significant impact on rETR max in P.haitanensis.The characters of chlorophyll fluorescence were determined under different measurement temperatures using Junior-PAM.The increase of measurement temperature from 10 to 30 ℃ made a negligible effect on rETR max,the photosynthetic light use efficiency of electron transport(α,the initial slope of the rapid light curves) and maximum photochemical quantum yield values(F v / F m),regardless of the growth conditions.However,the values of rETR max 、α and F v / F m were reduced drastically when the measurement temperatures were above 30 ℃,with the decline being much pronounced in 15 ℃-grown algae than 25 ℃-grown algae.Our results suggested that high temperatures(30—40 ℃) in the short-term inhibited the photosynthetic capacity of P.haitanensis,which might be associated with the damage in photosystem Ⅱ(PSⅡ) reaction centre and a concomitant reduction of electrons transport rate.Meanwhile,25 ℃-grown algae showed a higher tolerance to high temperature stress than 15 ℃-grown algae did,since 15 ℃-grown algae displayed greater downtrend in the values of rETR max 、α and F v / F m when the measurement temperatures rose to 30 ℃.The results also showed that the value of rETR max was lower in P.haitanensis thalli grown under CO 2 enriched air with respect to non-enriched air when the algae were subjected to the high temperatures,indicating that,compared to normal CO 2 growth conditions,elevated CO 2 growth conditions might repress the ability of photosynthetic electron transport of algae when subjected to short-term high temperature stress.
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