Simulation Of The Sedimentation Process Of Particulate Organic Matter In The Mariana Trench And Study Of Microbial Extracellular Enzyme Activity

In the marine environment,high-molecular weight polymers produced by primary productivity cannot be directly used by microorganisms,and can only be absorbed and utilized by heterotrophic microorganisms after extracellular enzyme hydrolysis to form small molecular compounds.Therefore,extracellular enzymes,as the initial drivers of high-molecular organic matter mineralization,play a crucial role in the rate and spatial distribution of marine carbon cycling,and are key participants in the marine carbon cycle.Deep-sea ecosystems are usually oligotrophic areas without light.Microorganisms in the deep ocean rely on the sinking particulate organic matter(POM)for carbon and nutrients.Extracellular enzymes released by particle-attached microorganisms(PAM)participate in the decomposition of granular organic matter to release dissolved organic matter(DOM),which in turn provides organic carbon and nutrients for PAM and free-living microorganisms(FLM)in waters.So particle-attached heterotrophic microorganisms(PAM)need to decompose POM to produce low-molecular organic matter that can be utilized by PAM and FLM in the water column.Studies have shown that the community structure and biodiversity of PAM and FLM change with depth,and the activity of extracellular enzymes produced by them are significantly different.The efficiency of enzymatic reactions has an important effect on the material cycling in deep sea waters,as shown in the fact that the greater the activity of extracellular enzyme enzymes,the more conducive it is to participate in the process of POM decomposition and release of DOM,thereby increasing the amount of organic carbon available for bioavailability,and enhancing the degradation and carbon cycle potential of microorganisms in the ocean for dissolved organic carbon(DOC).Deep-sea microorganisms are mostly pressure-tolerant or piezophilic microorganisms.Therefore,it can be hypothesized that piezophilic microorganisms have high biological activity and can produce abundant extracellular enzymes to degrade various organic substances in the ocean.Thus,microbial extracellular enzymes play a key role in deep ocean carbon cycle.At present,most research on extracellular enzymes in marine microorganisms has focused on shallow or coastal sea areas,and the properties of deep-sea extracellular enzymes are still unknown.Deep-sea extracellular enzyme research can help us understand the method of microorganisms to control the formation,transformation and degradation of organic matter in the ocean and the characteristics of deep-sea microbial extracellular enzyme activity with the depth.Therefore,conducting deep-sea microbial extracellular enzyme studies is of great significance for further understanding the deepsea carbon cycle.In this paper,the activities of four extracellular enzymes(aminopeptidase,alkali peptidase,α-glucosidase,and β-glucosidase)were measured at different simulated depths(50m,1000 m,4000m,6000 m and 10000m)of the Mariana Trench by simulating the POM sedimentation process of the Mariana Trench to study the extracellular enzyme activity under vertical gradients based on the fluorescent substrate method.At the same time,the content of DOC and particulate organic carbon(POC)in the culture system was measured,and the characteristics of their dynamic change with depth were analyzed,so as to explain the characteristics of extracellular enzyme degradation of organic matter by deep-sea microorganisms.By comparing the extracellular enzyme activities released by PAM and FLM,the contribution of PAM enzyme activity and FLM enzyme activity to total enzyme activity under different depth conditions was obtained,and the role of PAM and FLM in the deep-sea carbon cycle was analyzed.Finally,combined with the result of high-throughput sequencing,the relationship between the changes in microbial extracellular enzyme activity and community composition during the sedimentation of POM was analyzed.The main conclusions are as follows:1.First of all is the characteristics of total extracellular enzyme activity changes with the depth: the four extracellular enzymes can detect the enzyme activity in all the deep water layers,and according to the value of the enzyme activity,the rank is aminopeptidase,alkaline phosphatase,α-glucosidase and β-glucosidase;the surface of the four extracellular enzymes Km is greater than the deep layer,indicating that the surface layer can use the substrate concentration is high,and the microbial extracellular enzyme will enhance the affinity of the substrate to cope with the deep sea oligotrophic environment;the aminopeptidase activity has always shown a stable downward trend with the increase of depth,alkaline phosphatase,α-glucosidase and β-glucosidase activities showed a downward trend at depths of less than 6000 m,and slightly increased at depths greater than 6000 m.It shows that the effect of high hydrostatic pressure on aminopeptidase is the most significant.Elevation of some enzyme activities greater than6000 m may be associated with Psychrobacter.2.The characteristics of FLM enzyme activity and PAM enzyme activity change with the depth: in the process of POM from the surface to the deep sea,the FLM enzyme activity of alkaline phosphatase is always greater than that of PAM enzyme activity;aminopeptidase,α-glucosidase and β-glucosidase in the surface PAM enzyme activity is greater than that of FLM enzyme activity,and the FLM enzyme activity gradually dominates when the depth is greater than 1000 m.3.The relationship between the dynamic change of organic carbon concentration with depth and extracellular enzyme activity: DOC shows an upward trend with depth,and POC shows a downward trend with depth.Deep-sea microorganisms break down POM faster than they absorb and utilize DOC.Combined with the results of extracellular enzyme activity,deep-sea enzyme activity is smaller than surface enzyme activity,that is,the metabolic capacity of microorganisms is inhibited,which result in the accumulation of DOC.4.The results of the change of organic carbon concentration correspond to the results of the detection of enzyme activity in the entire water layer,that is,the extracellular enzyme always plays a role in the POM sedimentation process,which confirms that microorganism extracellular enzymes are important participants in the deep-sea carbon cycle.5.POC is positively correlated with extracellular enzyme activity,and DOC is negatively correlated with extracellular enzyme activity with depth.And for the analysis of extracellular enzyme activity,the results of DOC are the most meaningful.6.In this study,the measurement results of various high enzyme activities may be related to bacteria such as Bacteroidetes,Rhodobacterales,Pseudoalteromonas and Psychrobacter.7.FLM may play a more important role in the POM settling process than PAM.