Ecophysiological Responses Of Coastal Macroalgae To Varied Levels Of Light And CO₂ Concentration

Benthic macroalgae,living in the coastal ecosystems adjacent to human living areas,play important roles in carbon cycle and ecological services.Increasingly dissolved CO2 into oceans leads to ocean acidification.A number of studies showed that,ocean acidification either promotes or inhibits or does not affect photosynthetic carbon fixation and growth of macroalgae.These controversial results are not only related to species,but also affected by other environmental factors.Living in the intertidal zone,macroalgae are periodically exposed to high solar radiation along with tidal and solar radiation changes,which may regulate the response of macroalgae to elevated CO2 and/or ocean acidification(OA).This thesis has investigated several macroalgae’s responses to OA,following a broad review of relevant literatures.The results revealed ecophysiological responses of several typical macroalgae to elevated CO2/OA under different levels of light intensity and ultraviolet radiation(UVR).The main results are as follows:In the green alga Ulva lactuca,high-efficiency photoprotection mechanisms were identified to protect both photosystem Ⅱ(PSⅡ)and photosystem Ⅰ(PSⅠ)from photoinhibition under natural sunlight,with sustained high efficiency of carbon fixation.During the diurnal solar variation,both the maximum photochemical efficiency(Fv/Fm)and the effective quantum yield(YII)of PSⅡ were light-dependent,values of Fv/Fm and YII decreased with increasing levels of sunlight in the morning and increased rapidly with decreasing solar radiation in the afternoon.While the effective quantum yield of PSI was insensitive to the diurnal sunlight fluctuations,though a slight decrease was observed at noon time.These results imply that U.lactuca was well protected from photoinhibition.Such sustained photosynthetic performance was subsequently identified to relate to the enhancement of non-photochemical quenching in both PSⅡ(NPQ)and PSI,as well as cyclic electron transport around PSI(CET).These adaptive strategies play an important role in balancing the energy absorption and utilization between the two photosystems,and thus maintain the high stability of energy transfer from light reaction to dark reaction,endowing it with ubiquitous distributions in the global intertidal zones.Examination of the red alga Pyropia yezoensis showed that,solar UV radiation(UVR,280-400 nm)significantly inhibited its photosynthesis and growth.However,the inhibitions were significantly alleviated by OA treatment,indicating an antagonistic effect of OA and UVR.In the presence of UVR,results of "Kautsky" curves(OKJIP curves)showed that the normalized variable fluorescence at the K-step relative to the amplitude of OJ phase increased significantly,indicating that the oxygen-evolving complex suffered inhibition.The donor-side induced photoinhibition would limit the electron transport rate(rETRmax)and eventually result in a decrease in carbon fixation.UV-induced growth inhibition was up to～31%,of which UVA and UVB accounted for～26%and 5%,respectively.As the thallus was exposed to OA,the increase of UVabsorbing compounds,NPQ and CET(cyclic electron tansport)participate in dissipating excessively excitated energy and alleviating the over-reduction of photosynthetic electron carriers.The compensatory up-regulation of rETRmax promotes carbon fixation and growth of P.yezoensis.Based on the above results,further investigations were carried out on the macroalgae near a natural CO2 seep in Shimoda coastal water,Japan,where different pH gradients exist away from the center of the seep.Results showed that increased extent of OA(close to CO2 seep with lower pH)significantly altered the species structure,with decreased abundance of both calcified and non-calcified macroalgae.In the sea area with pCO2 of～300 μatm(pHtotal=8.22),a total of 23 species of macroalgae were identified during the in-situ survey,consisting of 2 species of Chlorophyta,18 species of Rhodophyta and 3 species of Phaeophyta.In the area with pCO2 of-900μatm(pHtotal=7.90),only 10 species,including 3 species of Chlorophyta,4 species of Rhodophyta and 3 species of Phaeophyta,were identified.By investigating the photosynthetic responses to OA treatment,it was found that OA treatment increases light use efficiency,maximum electron transport rate,as well as the carbon fixation rate in the non-calcified macroalgae.In the red algae Gelidium elegans(a dominant algae in the area),the thalli collected in the water near CO2 seep exhibited higher sensitivity to high light intensity,with higher photosystem damage rate and lower repair rate.While in the brown alga Dictyopteris undulata(a dominant algae species in the area near CO2 seep),the thalli collected from the CO2 seep area showed tolerance to high light and maintained high photosynthetic efficiency under low pH condition.Theese results suggested that these macroalgal species can adapt to OA,sustaining their survival and growth.In summary,the green alga(Ulva lactuca)and the red alga(Pyropia yezoensis)exhibited significant interspecies differences to changes of solar radiation and CO2 concentration,but both were high light tolerant species.Although elevated CO2 concentration could alleviate the inhibitory of UV and promote the growth of thallus(foliage phase)in several macroalgae,e.g.P.yezoensis,its microscopic life history stage may be more sensitive to OA and UV.Decrease of the macroalgal abundance in area near the natural CO2 seep(high CO2/OA)under incident solar radiation indicated that many of the macroalgal species could have failed to complete their life history.Therefore,future research about the effects of OA and UVR on different life-cycle stages of macroalgae would help to clarify the mechanisms involved in regulating the community structure under influence of ocean acidification.