Photosynthetic Physiological Responses Of Macroalgae To Different CO₂Concentrations And Their Ecological Effects

Increased atmospheric CO₂concentration can be predicted to result in a change ofthe components of inorganic carbon system. Aquatic plants use CO₂for photosynthesisand some species use HCO₃^-. The CO₂and HCO₃^-concentration in seawater mustinfluence the photosynthetic carbon fixation of aquatic plants and air-sea CO₂flux.Thus, it’s important to study on the photosynthetic physiological responses ofmacroalgae to different CO₂concentration and their ecological effects. This paperreviewed the responses of macroalgae, trying to explore the ecophysiologicalcharacteristics in three macroalgae Gracilaria lemaneformis （a red macroalga）, Ulvapertusa （a green macroalga） and Saccharina japonica （a brown macroalga） and theirecological effects of the fixation carbon, in order to provide theoretical basis forresponse of macroalgae to different CO₂concentrations and controlling the CO₂concentration by the macroalgae cultivation. The main results were as follows:1. Effect of light intensity on the photosynthetical responses of G. lemaneiformisin non-aerated and aerated cultures. At the same light intensity, non-aerated culturecould decrease the photosynthetic rate and increase respiration rate of algae, comparedto non-aerated culture. But aerated culture doesn’t affect obviously on. Yield andpigments contents. In the moderate light intensity group, there was the maximumphotosynthetic rate and the minimum respiration rate for algae, as well as Yield andpigments contents were progressively decreased with increasing light intensity. Theresults suggested that reduced growth was mainly due to the reduced photosynthesisrate in case of low light intensity, or could be ascribed to the increased respiration ratein case of high light intensity. Aerated culture could relieve the negative effect on thegrowth rate of alga for low light or high light.2. Effects of carbon dioxide and light intensity on the photosynthetical responsesof three macroalgae. Under the normal CO₂concentration condition, light intensity hada notably significant effect on the growth of macoalgae, and the low light or high light inhibited growth of thalli. In the low light, elevated CO₂concentration had nosignificant influence on the growth of three algae, and decreased the photosyntheticrate of three algae, but increased the P/R, where the growth of U. pertusa wassignificant lower than in the normal CO₂level; in the mediate light, increased CO₂concentration could promote the relative growth rate of G. lemaneformis, have nosignificant effect on U. pertusa, and decrease the growth of S. japonica; in the highlight, elevated CO₂concentration increased the growth of G. lemaneformis and U.pertusa, but inhibited the growth of S. japonica. Compared to the normal CO₂level,moderate light and high light decreased photosynthetic rate and pigments contents of G.lemaneformis and U. pertusa, but increased photosynthetic rate of S. japonica whenthe thalli were cultured under enriched concentration CO₂condition. In the high light,the pigments contents and Yield of the thalli were significantly decreased.3. Effects of different CO₂and DIN concentrations on the photosyntheticresponses of three macroalgae. The results showed that N supply could significantlyincrease the relative growth rate, photosynthetic rate and pigments contents ofmacroalgae under the normal CO₂concentration condition. When N was limited,elevated CO₂concentration could significantly increase the relative growth rate ofmacroalgae. Under the control N level condition, increased CO₂concentration couldpromote the growth of G. lemaneformis more effectively than U. pertusa and S.japonica. When both CO₂and N were enriched, the growth of G. lemaneformis didn’tincrease significantly, and increased respiration rate of U. pertusa and S. japonica andlow P/R resulted their decreased growth rate. In the any of CO₂concentration, thephotosynthetic rate and photosynthetic efficiency of S. japonica were significantlylower than G. lemaneformis and U. pertusa. There was no significant different betweenenriched NO₃^-and NH₄⁺supply on growth and photosynthesis of macroalgae.4. Effects of DIN enrichment on inorganic carbon system of S. japonica culturewater. The results indicated that inorganic carbon system of culturing S. japonica wascorrelated with the concentrations and forms of DIN. The concentrations of DIC, HCO₃^-and pCO₂generally decreased with the increasing of nitrogen concentrationunder the concentration of4.73^-52.78μmol L^-1（NO₃^--N） and2.56-34.66μmol L^-1（NH₄⁺-N） and were most remarkably affected by the NO₃^--3and NH₄⁺-3groups,corresponding with the lowest values2054and2112μmol L^-1,1776and1869μmolL^-1,86and114μatm, respectively. However, when the concentration of nitrogen wasbetween52.78-427.29μmol L^-1（NO₃^--N） and34.66-268.33μmol L^-1（NH₄⁺-N）, thefalling trends of DIC, HCO₃^-and pCO₂were weakened with nitrogen increasing, butthe concentrations of DIC, HCO₃^-and pCO₂were also lower than the control group.Influence of NO₃^--N addition group on inorganic carbon system of seawater was largerthan NH₄⁺-N addition group. And the ability of carbon fixation in NO₃^--N additiongroup was significantly higher than in NH₄⁺-N addition group.5. Carbon dioxide fixation by the seaweed G. lemaneiformis in integratedmulti-trophic aquaculture with the scallop Chlamys farreri in Sanggou Bay, China. Incontrol groups, continuous dissolution of CO₂produced by scallops into the seawaternot only caused an ongoing increase of partial pressure of CO₂（pCO₂）,5.5timeshigher than that of natural seawater, but also acidified seawater by0.8units after42hof culture. However, in all seaweed-scallop groups, the higher the algal density, themore CO₂was absorbed; pCO₂was lowest in the maximum density of macrolagae.The results showed that a ratio of bivalve to seaweed was1:0.96（FW）, the seawatercould produce the lowest pCO₂. Overall, the integrated culture of seaweed and scallopcould provide an efficient and environmentally friendly means to reduce CO₂emissions from bivalve mariculture.