Response Of Macroalgae To Cu²⁺ Stress Under Elevated Atmospheric Co₂ Level

Macroalgae are important primary producers in nearshore waters and have rich economic and ecological value.However,the nearshore marine environment is subject to complex changes due to anthropogenic disturbances,such as global elevated atmospheric CO2 levels and regional pollution of water bodies by land-based heavy metals.Little is known about the response of macroalgae to the interaction of elevated atmospheric CO2 levels and Cu2+ stress.In this study,the growth,photosynthetic function and biochemical components of economic macroalgae Sargassum fusiforme,Caulerpa lentillifera and Gracilaria lemaneiformis in response to changes in Cu2+ concentration gradients,and the effect of elevated atmospheric CO2 levels on this response were investigated.We also investigated the effects of different concentrations of Cu2+ and Cd2+ on the growth and photosynthetic functions of the wild macroalgae Caulerpa racemosa,Ulva linza,Sargassum hemiphyllum,Porphyra crispata and Pterocladiella capillacea.The main findings were as follows:1.Two CO2 levels: 400 ppm and 1000 ppm,and four Cu2+ concentrations: 0 mg·L-1,0.025 mg·L-1,0.075 mg·L-1 and 0.15 mg·L-1 were set up with young sporophytes of Sargassum fusiforme(Phaeophyta).The results showed that Cu2+ concentrations above 0.025mg·L-1 significantly inhibited photosynthesis and growth of the algae,reduced the apparent photosynthetic efficiency(?),maximum net photosynthetic oxygen evolution rate(Pmax),maximum photochemical quantum yield(Fv/Fm)and pigment content(Chl a and Car),while promoted superoxide dismutase(SOD)activity.Elevated CO2 levels did not affect the growth and photosynthesis of this algae in the absence of copper addition.However,CO2 enrichment weakened the inhibitory effect of Cu2+ stress on Fv/Fm,reduced the light compensation point(Ic)and alleviated the Cu2+-induced increase in dark respiration rate(Rd)and SOD activity.This indicates that elevated CO2 increased the activity of photosynthetic system and decreased the activity of antioxidant system in young sporophytes of S.fusiforme under Cu2+stress.2.Two CO2 levels: 400 ppm and 1000 ppm,and three Cu2+ concentrations: 0 ?g·L-1,10?g·L-1 and 30 ?g·L-1,were set up with Caulerpa lentillifera(Chlorophyta).The results showed that 10?g·L-1 Cu2+ almost doubled the Rd of the algae,and although ? was significantly increased,Pmax and light saturation point(Ik)decreased slightly while growth was inhibited.30 ?g·L-1 Cu2+ caused severe damage to the photosynthetic pigments(Chl a and Car)of the algae,and Pmax and ? decreased significantly while growth was severely inhibited.At the enriched CO2 level,the Rd of the algae increased by about 50% and the growth was slightly promoted;however,the high concentration of Cu2+ still significantly reduced ?,Pmax and Ik,and the Chl a and Car contents decreased even more.In conclusion,C.lentillifera is highly sensitive to CO2 level and Cu2+ stress,and elevated atmospheric CO2 level facilitates its access to inorganic carbon and stimulates it to resist stress in a less energy-consuming manner.3.Two CO2 levels: 400 ppm and 1000 ppm,and three Cu2+ concentrations: 0 mg·L-1,0.025 mg·L-1 and 0.25 mg·L-1,were set up with Gracilaria lemaneiformis(Rhodophyta).The results showed that the pigment content Car and PC of the alga were damaged,photosynthetic performance was significantly decreased,and total antioxidant capacity(T-AOC)was significantly increased under high concentration of Cu2+ stress.Without Cu addition,elevated CO2 levels caused a slight decrease in Rd and a significant decrease in Chl a,Car,PE and PC content.When CO2 enrichment was coupled with copper stress,the algae improved photosynthetic efficiency and the Cu2+-induced decreases in Fv/Fm and ? were alleviated,while malondialdehyde(MDA)content was higher and T-AOC was lower,indicating that CO2 levels exacerbated Cu2+ stress and had a negative effect on antioxidant system activity.4.A control treatment,two Cu2+ concentrations: 0.0254 and 0.127 mg·L-1,and two Cd2+concentrations: 0.562 and 5.62 mg·L-1,were set up with Caulerpa racemosa,Ulva linza,Sargassum hemiphyllum,Porphyra crispata and Pterocladiella capillacea.The results showed that these five algae were all more tolerant to Cd2+ than Cu2+.Among them,Caulerpa racemosa was the most sensitive to Cu2+ and the most tolerant to Cd2+,and had the most obvious morphological changes in a short period of time.Pterocladiella capillacea was the most tolerant to Cu2+ stress,while Ulva linza and Sargassum hemiphyllum were the least tolerant to Cd2+ stress.The Fv/Fm of algae which are more tolerant to Cu2+ or Cd2+ does not change with the increase of heavy metal concentration,but their r ETRm decreases.Fv/Fm and r ETRm of algae which are less tolerant to Cu2+ or Cd2+ decrease with the increase of heavy metal concentration.In summary,the effects of heavy metals on physiological characteristics such as growth and photosynthesis of macroalgae were not only concentration-dependent but also showed significant interspecific differences and were influenced by elevated CO2 levels.The growth and photosynthesis of macroalgae were significantly inhibited by exposure to seawater with heavy metal levels above a threshold concentration.In contrast,enriched CO2 levels altered the survival strategies of seaweeds under Cu2+ stress,especially energy production and allocation,and the activity of photosynthetic and antioxidant systems,by changing the ability of seaweeds to utilize HCO3-and the acid-base balance at the cell surface.Under the background of global climate change,studying the response of different species of macroalgae to heavy metal stress in coastal waters is helpful to provide a scientific basis for ecological risk assessment of trace metal pollution.