| Increased anthropogenic CO2 concentration emissions in the atmosphere lead to ocean acidification.The ocean can absorb more than 25 % of anthropogenic carbon dioxide emissions into the atmosphere every year.Ocean acidification is now ten times faster than in the past,threatening ecosystems’ ability to adapt to the chemical changes in the oceans caused by CO2.Increased seawater acidity affects the formation and dissolution of some marine species,including shellfish.As an essential component of the ecosystem,it is vital to use the food chain to assess ocean acidification’s ecological effects.In this study,a secondary food chain model consisting of Isochrysis galbana and the thick-shelled mussel Mytilus coruscus was used to investigate ocean acidificationmediated changes in the nutrient quality microalgae on the digestion,immunity,and energy metabolism of the thick-shelled mussels.Our findings can inform future studies on the impact of ocean acidification on marine ecosystems and aid in creating healthy and long-term shellfish cultivation.Firstly,we exposed the algae I.galbana to two p H conditions(p H 8.1 and p H 7.7).We found that different physiological responses exhibited in I.galbana under acidified water conditions.p H reduction promoted growth in cell density,actual photosynthetic efficiency,and chlorophyll content in I.galbana.Our study further showed that the total phenols and soluble sugars of I.galbana increased under acidification stress.In contrast,C/N,total lipid content,and polyunsaturated fatty acids significantly decreased,while soluble sugar content did not change significantly.It is evident that seawater acidification promotes algal cell growth but reduces its nutritional value as an essential biological diet.To further reveal the effects on digestibility and gut microorganisms due to changes in the dietary quality,we extracted total DNA amplification from the gut microbiota of thick-shelled mussels in different dietary quality and seawater p H treatment groups.We sequenced them using second-generation 16 S r RNA targeted sequencing technology to analyze the microbiota diversity and microbiota distribution.The results showed that all groups shared 146 OTUs,and the Simpson index of gut samples was significantly lower than that of algae.In contrast,the Shannon index sees an opposite trend,indicating that the gut has a higher microbial communities’ diversity.The abundance and diversity of gut microorganisms did not differ too much among different treatment groups.At the phylum level,the dominant bacteria for both algae and mussel gut were Proteobacteria,Bacteroidetes,and Firmicutes.At the genus level for both were Prevotella＿9,Ulvibacter,and Bacteroides.However,Bacteroides was significantly lower in the gut of thick-shelled mussels exposed to acidified conditions and fed low nutrient quality than the other three groups.As a beneficial bacterium,the reduction in the number of Bacteroides may affect the intestinal barrier of thick-shelled mussels.Digestive enzyme activity reflects the extent to which nutrients are digested and absorbed by the organism.In this study,acidification stress promoted amylase secretion from the thick-shelled mussel’s digestive glands but limited Pepsin,Trypsin,and Lipase’s activity.The reduction in unsaturated fatty acids and soluble sugars in the algae reduced lipase activity and amylase in the mussel’s digestive glands.The mussel meets its energy requirements by secreting more protease in the digestive glands to digest and utilize more protein.Finally,this study investigated the mussel’s physiological response to acidification and nutrient stress in the context of cellular energy allocation(energy reserves and energy expenditure)and immune defense(antioxidant enzyme activities).Our findings indicates that acidification and changes in the algal nutritional quality affect thick-shell mussels’ immune defenses.Acidification and low dietary quality treatments increased ROS levels in the digestive gland of thick-shell mussels.Reduced p H levels and dietary quality boosted the activities of superoxide dismutase(SOD),catalase(CAT),and glutathione peroxidase(GPx).At the same time,malondialdehyde(MDA)and thioredoxin reductase(Trx R),indicators of oxidative damage,did not differ considerably between stress treatments.p H and dietary quality had significant interactions on energy metabolism index,energy storage,and RNA/DNA.Lipids,carbohydrates,and protein were all significantly affected by p H and diet.There was a marked drop and increase in the total energy reserve and energy cost,respectively,inducing a significant decline in energy distribution.Considering the current scenario of ocean acidification resulting from global climate change,the algal primary nutrient,polyunsaturated fatty acids,are significantly reduced,and therefore its nutritional value as a critical diet for shellfish is diminished.The altered nutrition of I.galbana disrupts the antioxidant system of the digestive gland tissues of thick-shell mussels,affecting energy metabolism processes and energy reserve capacity.It may pose a potential threat to the maintenance of their populations if exposure continues over time. |
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