Effects Of Organic Particles On Microbial Community Structure And Function In The Marine Environment

Marine particulate organic matter(POM)encompasses products mainly generated by phytoplankton photosynthesis together with zooplankton debris and feces.These are the main sources of organic matter in the marine environment,so directly affect the growth and metabolism of microorganisms and have an observably significant impact on microbial community composition and function.Additionally,the processes by which various microbial populations transport and metabolize(in)organic matter in the marine environment play an important role in marine carbon sequestration.The marine carbon and nitrogen cycles,the two most important biogenic element cycles,play a key role in maintaining the dynamic balance and stability of marine ecosystems.Marine POM also links the two most important biological organic matter storage mechanisms in the ocean,Biological pump(BP)and Microbial carbon pump(MCP).Moreover,marine microbes release a variety of inorganic nutrients mainly derived from nitrogen during the transportation and utilization of organic matter.In this thesis,the in situ oligotrophic seawater obtained from the Adriatic Sea was mixed with sterilized algae and then cultured in a rolling tank to simulate a nutrient-rich(Nutr-R)and highparticulate matter(HPM)environment,as well as investigated the sinking particles in the South China Sea basin(1000 m),to exploring microbial community composition and function in different trophic environments.Furthermore,samples for microbial community analysis were collected along a gradient from the Pearl River estuary to the South China Sea to investigate the effects of different nutrient and particulate matter concentrations on microbial composition and function.(1)Using metagenomics and metaproteomics,we compared functional metabolic changes of various microorganisms in the presence or absence of phytoplankton to simulate the occurrence of algal blooms in the Adriatic Sea.In the Nutr-R and HPM environment,Vibrionales and Alteromonadales,which belong to ?-proteobacteria,were the two dominant groups and Rhodobacterales(?-proteobacteria)was also present.Vibrionales contained various ATP-binding cassette transporters for amino acids,carbohydrates and inorganic ions,while Alteromonadales mainly contained TonBdependent transporters for(in)organic matter.A variety of central carbon metabolic processes,chemotaxis and motility prompt microbes to utilize and metabolize various organic substrates during algal blooms.In contrast,in a nutrient-poor(Nutr-P)and lowparticulate matter(LPM)environment,Pelagibacterales(SAR11,?-proteobacteria)and Rhodobacterales dominated;they transported(in)organic substrates in oligotrophic seawater to facilitate their survival.Based on the various transporter proteins detected,we analyzed the spatiotemporal transformation of a microbial community from a NutrR and HPM environment to a Nutr-P and LPM environment.Vibrionales was the first functional group to play a role in this Nutr-R and HPM environment by hydrolyzing various types of labile organic matter that were quickly transported and absorbed by Alteromonadales.This allowed Alteromonadales to then become the most dominant group in the culture system.Rhodobacterales was also abundant in this system.If the incubation time was increased,Pelagibacterales and Rhodobacterales,the most important in situ oligotrophic groups,became dominant.At the same time,the organic matter in the culture system was continuously converted from larger particles(labile organic matter)to the initial phase(Adriatic Sea water)via smaller particles(recalcitrant organic matter).When dense organic particles were transported and absorbed,microbial biomass and respiration were enhanced by the metabolism of the most important organic substances,such as carbohydrates and amino acids.A significant increase in translational levels also suggested an increase in microbial biomass.Through secretions,microorganisms are better able to degrade,transform and utilize(in)organic substances.In addition,cellular chemotaxis and motility also implied that the microorganisms are beneficial to the environment.Serial processes revealed the biological mechanism of microbial transformation and utilization of organic matter to some extent.During the entire process on both temporal and spatial scales,microbial community succession and organic matter converted from labile to recalcitrant vividly reflected the carbon storage processes of microbes.(2)Compared with the dense organic matter produced by an algal bloom,the concentration of sinking organic particles was poor.Studies on sinking particles have mainly focused on flux;the amount of particles exported was closely related to microbial metabolic activity.However,these studies were somewhat limited in that they mainly focused on the upper ocean(?500 m water depth).In this study,molecular biological methods and a functional prediction analysis were used to investigate sinking particles collected by a sediment trap at a water depth of 1000 m in the South China Sea basin.The results showed that heterotrophic microbes(?-and ?-proteobacteria,Bacteroidetes,Firmicutes and Actinobacteria)adhered to sinking particles degraded and mineralized aromatics and hydrocarbons during carbon metabolism.In addition,anaerobic nitrate reduction/respiration and sulfur compound respiration were the main energy sources for growth and metabolism.In summary,heterotrophic microbes that reside in the particulate microenvironment maintained their growth and metabolism by degrading and remineralizing relatively recalcitrant organic matter in an anaerobic environment.The rapid decay of particulate organic matter and selective conversion by microorganisms formed refractory organic compounds during carbon conversion.This preserved organic matter in the marine environment for a long period of time.(3)Organic particles are used by microorganisms for growth and metabolism and also are(re)mineralized and decomposed.Subsequently,inorganic nutrients(mainly ammonia)are released and then oxidized by nitrifiers to fix carbon in the marine environment.In this study,a gradient from a Nutr-R and HPM environment(Pearl River Estuary)to a Nutr-P and LPM environment(South China Sea basin)was investigated.We analyzed the diversity and abundances of ammonia-oxidizing archaea(AOA)and?-proteobacteria(AOB)and nitrite-oxidizing bacteria(NOB),as well as nitrification rates.We found AOA were generally more abundant than ?-AOB;however,AOB were more clearly attached to particles in the upper reaches of the Pearl River estuary(eutrophic,hypoxic and with dense particulates).The NOB Nitrospira had higher abundances in the upper and middle reaches of the Pearl River estuary,while Nitrospina was dominant in the lower estuary.AOB and Nitrospira could be more active than AOA and Nitrospina since significantly positive correlations were observed between their gene abundances and the nitrification rate in the Pearl River estuary.There was a significant positive correlation between ammonia and nitrite oxidizer abundances in the hypoxic waters of the estuary,suggesting a possible coupling through metabolic interactions between them.Phylogenetic analysis further revealed that the AOA and NOB Nitrospina subgroups can be separated into different niches based on their adaptations to substrate levels.The particle concentration was an important parameter influencing the substrate availability,which had a significant impact on the composition and function of nitrifying microorganisms.In summary,we found that nitrifiers depend on micro-anoxic habitats and that inorganic nutrients adsorbed onto particles play an important role in carbon fixation.