Characteristics Of Bacterial Community Succession During Growth And Litter Decomposition Of Seaweed Gracilaria Lemaneiformis

Gracilaria lemaneiformis is an important cultivated seaweed in China,which has important economic and environmental benefits.However,Gracilaria lemaneiformis may be beaten by wind and waves,or gnawed by fishes and else.The algae falls to the seabed for litter and decomposed,which will affect the environment.Bacteria are the important participants in the decomposition of G.lemaneiformis.In order to analyze the succession changes of bacterial community during the growth and decomposition of the seaweed,clarify the characteristics of the physical and chemical environments,and reveal the bacterial community attached to the seaweed,water bodies and sediment,we studied through in-situ investigation,indoor experiments and in-situ simulation experiments.The main results are as follows:(1)The survey of the seaweed cultivation area in Nan’ao,Shantou showed that the large-scale cultivation of Gracilaria significantly promoted the growth of the bacteria in water bodies.The number of bacteria in the cultivation area was maintained at 10~5copies/m L,while that in the non-cultivation area was 10~4copies/m L.And it also changed the relative abundance of dominant bacteria in the water bodies of the cultivation area.The main dominant bacteria in the cultivation area were Proteobacteria,Bacteroidetes,Cyanobacteria,Actinomycetes and Planctomycetes.Compared with the non-cultivation area,the relative abundances of Proteobacteria and Bacteroidetes in cultivated area were higher,while that of Cyanobacteria,Actinomycetes and Planctomycetes were lower.The species number of dominant bacteria on the surface of the litter increased,and as so as theα-diversity.And the bacterial community structure attach to Gracilaria changed significantly when withering.(2)The indoor enclosure experiment of fresh Gracilaria litter showed that the decomposition period of fresh seaweed was long.The algae still grew slowly in the early stage of the decomposition,and began to wither slowly after 20 days.On day 180th,the decomposition rate reached 79.3%.During the process of Gracilaria withering,the content of dissolved oxygen(DO)in water continued to decline,however the concentrations of carbon,nitrogen and phosphorus in water increased significantly.The content of DO decreased by 92.5%,the concentrations of nitrogen and phosphorus in water increased by 5 to 45 times.During the process of withering,the number of bacteria attached to the seaweed remained about 10~8copies/g.The community structure of dominant bacteria attached to Gracilaria was significantly affected by TOC,TN,TP and DO in water.In addition,the relative abundances of Planctomycetes,Firmicutes,Spirochaetae andδ-Proteobacteria were increasing,while that of Cyanobacteria,Bacteroidetes,α-Proteobacteria andγ-Proteobacteria were decreasing.During the process of the decomposition,the functional gene abundance of metabolism attached to Gracilaria litter continued to decline.(3)The indoor enclosure experiment of dried Gracilaria showed that it decomposed rapidly in a short time,and the decomposition rate reached 83.5%on day 50th.During this process,the concentrations of organic carbon,total nitrogen and total phosphorus in water increased by 70.7%,229.8%and 101.3%,respectively.The DO decreased by 82.9%,which from 1.87 mg/L to 0.32 mg/L.In addition,the number of bacteria attached to Gracilaria remained about 10~7copies/g,while the richness and diversity of bacteria continued to increase during the decomposition processes.The bacteria community structure was significantly affected by TOC,TN,TP and DO in water.The relative abundances of Planctomycetes,Spirochaetae,Firmicutes andδ-Proteobacteria increased,while that of Bacteroidetes,α-Proteobacteria andγ-Proteobacteria decreased.During the process of decomposition,the functional gene abundance of metabolism attached to Gracilaria continued to decline.(4)Experiments of fresh and dried Gracilaria in suit showed that decomposition rate of the dried seaweed reached 69.3%on day 20th and 76.7%on day 50th.The release rates of carbon,nitrogen and phosphorus reached 83.4%,85.7%and 81.3%,respectively.However,the decomposition rate of fresh Gracilaria was only 29.9%on day 20th.It began to decompose quickly from day 35th.The decomposition rate reached 75.5%on day 35th and 85.0%on day 50th.The release rates of carbon,nitrogen and phosphorus were 89.5%,91.2%and 90.1%,respectively.The number of bacteria attached to the two types seaweed was maintained about 10~7copies/g.The richness and diversity of bacteria attached to Gracilaria increased continuously in the process of the decomposition.The relative abundances of Planctomycetes,Actinomycetes and Saccharibacteria were increasing,while that of Bacteroidetes was decreasing,which were significantly correlated with environmental factors such as water salinity,DO and water temperature.During the process of the decomposition,the functional gene abundance of metabolism attach to Gracilaria increased firstly,and then decreased.