| In recent years,red tides occur frequently,causing more and more damage to marine environment and human society.Looking for effective ways to control red tides has become a research hot spot.In this study,we tried to find the potential algicidal microorganisms for the prevention and termination of red tides.As a result,we isolated an actinomycete U3 from Zhangjiangkou Mangrove National Nature Reserve,which had a potent algicidal effect on the harmful alga Heterosigma akashiwo and studied it systematically.Firstly,the taxonomic status of U3 was determined by morphologic and molecular identification.Then the physiological and biochemical properties of U3,as well as its algicial characteristics,were studied in order to understand its features and potential in the control of red tides.Finally,microcosms were constructed to explore the effect of algicidal compounds on the environment and the bacterial community composition.The results and findings are as follows:1.The bacterial strain U3,isolated from the soil samples of mangrove,could lyse H.akashiwo cells in 1 d.Sequencing of its 16S rRNA gene showed that U3 had the closest relationship with Streptomyces somaliensis NBRC 12916T.Combining the morphology characteristics,U3 was finally identified as Streptomyces sp.2.The exploration of physiological and biochemical characteristics of U3 revealed its high sensitivty to more than ten kinds of antibiotics,especially minocycline and neonatoxin.U3 could degrade macromolecules and release alkaline phosphatase,esterase(C4),lipase(C8),leucine aminopeptidase,acid phosphatase,?-glucosidase,N-acetyl-?-glucose Glucosidase.U3 grew rapidly,and entered the exponential growth phase within 8 h.When U3 reached the exponential growth until the early stationary phase,the fermentation broth showed high algicical efficiency.Unlike most of the other known algicidal Streptomyces,U3 showed strong ability of untilizing the algal inclusion as the nutrient source for proliferation.3.The algicidal characteristics of U3 were studied.U3 had good specificity to kill algae and good application value and potential.It could completely lyse the algal cells by producing some active compounds,which were highly sensitive to high temperature and strong alkaline,but resistant to acid.One ?g/mL of crude extracts of the fermentation supernatant could kill 70%of algal cells in 3 d.Unlike most of the other known algicidal Streptomyces,U3 showed strong ability of proliferation with the algal inclusion as the nutrient source.The washed mycelial pellets also gradually exhibited good algicidal effect during the visible growth in the algal culture.It suggests that U3 could efficiently absorb nutrients from algal culture to support its growth and the production of algicidal compounds.So it has a great potential to apply U3 as a long-term and environmentally friendly bio-agent to control the harmful blooms of H.akashiwo.And the decrease of bioavailable DOM and increase of bio-refractory material during the algicidal process of U3 provided new insights into the ecological influence of algicial microorganisms in marine ecosystem.4.By establishing the microcosms of H.akashiwo blooms,we studied thp ecological effects of algicial compounds.The results showed that the addition of algicial compounds caused the rapid death of H.akashiwo,leading to less NO3-comsuption and NO2-production.At the same time,it stimulated the growth of bacteria,which released abundant NH4+ and accelerated the community succession from H.akashiwo to other algae such as diatoms.We applyed Illumina sequencing to analyze microbial community structure.Data analysis showed that the growh of H.akashiwo reduced bacterial diversity significantly and changed bacterial community structure greatly.Although the addition of algicial compounds did not completely undo the damage,the decline of bacterial diversity index was significantly slower after the treatment.The bacterial community structure 2 d later was also more similar with the original seawater,suggesting the potential remediation effect of algicidal compounds on the bacterial community.As for the bacterial species distribution,the abundance of Gammaproteobacteria,such as Methylophaga that provides ammonium for algal growth,increased significantly and the relative abundance of Alphaproteobacteria decreased during the H.akashiwo bloom.However,when the bloom naturally demised or was terminated by the algicidal compounds,Alphaproteobacteria,mostly Rhodobacteraceae,proliferated quickly and became the most dominant population again.VPA analysis also revealed that the bloom and demise of H.akashiwo,as well as the changing environmental conditions,explained 97.53%of the change of bacterial community.In short,Streptomyces sp.U3 can cause rapid demise of H.akashiwo bloom with either its mycelial pellets or algicidal componuds.It could accelerate the conversion of bioavailable DOM to bio-refractory carobon,and somewhat recover the damaged bacterial community.Considering that the mycelial pellets of U3 can sustainably grow in the bloom environment,it suggests that U3 might be a good candidate to be developed as an efficient,safe and economical microbial agent for the control of red tides. |
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