Effects Of Ocean Acidification On The Growth,Physiology And Rhizomephere Of Tropical Seagrasses

Seagrass beds are one of the most productive and biodiverse nearshore marine ecosystems,providing important ecosystem services and functions,but they are under threat due to human activities and climate change,and their area and associated ecosystem functions and services continue to decline,so it is urgent to strengthen research on seagrasses and their conservation and restoration.In this study,we investigated the effects of different levels of ocean acidification on the growth,photosynthesis,material metabolism and structural characteristics of the bacterial community in the rhizosphere of tropical seagrasses through an indoor simulation of ocean acidification.The results showed that to adapt to the complex marine environment and maintain the genetic diversity of the population,the E.acoroides has evolved biological characteristics and ecological strategies such as the ability of fruits and seeds to remain floating in seawater,the conical structure of large seeds and the production of filamentous root hairs.The effects of ocean acidification on seed germination,material metabolism and seedling growth and the physiology of E.acoroides were also investigated.The results showed that ocean acidification accelerates the consumption of soluble sugars and starch,reduces the survival rate of E.acoroides seedlings and narrows the leaves,but significantly promotes the number of leaves,roots and leaf length,increases the photosynthetic rate of seedlings,promotes the accumulation of non-structural carbohydrates,osmoregulatory substances and secondary metabolites,but reduces photosynthetic pigment content.Secondly,the effects of ocean acidification on the growth and physiology of E.acoroides seedlings were investigated.The results showed that under reduced p H,E.acoroides seedlings maintained a high photosynthetic rate with reduced leaf chlorophyll content,increased leaf number,root number and leaf length,increased leaf and root carbon content and reduced nitrogen content.In addition,acidification promoted the accumulation of non-structural carbohydrates,osmoregulatory substances and secondary metabolites in the E.acoroides seedlings.The final study was one of the few controlled indoor experiments using in situ seagrass beds（preserving the intact structure of leaves,roots,rhizomes,epiphytes and sediments）to better assess the effects of future p H levels on seagrass beds.The results showed that ocean acidification has a significant effect on the carbonate system,leading to changes in the DIC balance,where p CO₂,CO₂,HCO₃^-,TA and DIC levels increase with decreasing p H,while CO₃^2-concentrations decrease with decreasing p H.Ocean acidification had no significant effect on the leaf growth rate,leaf length,leaf width and root number of T.hemprichii,while it had a significant effect on above and below-ground biomass and leaf number.Ocean acidification also increased the rate of oxygen release from seedlings,while the rate of oxygen consumption tended to increase but did not differ significantly,and had no significant effect on above-and belowground antioxidant enzyme activity,osmoregulatory substances and non-structural carbohydrates.There were significant differences in species abundance and diversity between acidification levels,affecting the bacterial community composition of seagrass bed sediments and altering the abundance of associated bacteria.Based on the above findings,we hypothesized that there may be small differences in the degree of response to ocean acidification between the two seagrasses since tile grass and caladium have different carbon allocation strategies or that seedlings are not limited by carbon or other nutrients and have different sensitivities to decreases in p H.At the lowest p H levels,there was a tendency for seagrass productivity to increase,while at intermediate and high p H levels productivity was limited or not stimulated.Thus,the benefits of ocean acidification on seagrass growth and physiology appear to be minimal over the next few decades or centuries,although it is likely to be a’winner’in the event of extreme ocean acidification in the future.