Physiological Ecology Studies On Nitrogen-fixing Cyanobacteria In Small-scale Artificial Lake

Nitrogen-fixing cyanobacteria is one of dominat cyanobacterial water blooms.Nitrogen and phosphorus inputs have been dramatically enhanced during recent years.Previous studies have found that the increase of nitrogen and phosphorus is the direct cause of cyanobacteria bloom.There are many kinds of cyanobacterial forming algae,such as,nitrogen-fixing cyanobacteria,Aphanizomenon,Anabaena;non-nitrogen-fixing cyanobacteria,Microcystis,Planktothrix.It is usually appeared nitrogen-fixing cyanobacteria bloom and non-nitrogen-fixing cyanobacteria bloom in the eutrophication water.Most of cyanobacteria are able to release cyanotoxins,which are harmful to aquatic organisms and humans.Artificial control of the content of nitrogen and phosphorus is helpful to decrease the bloom of cyanobacteria.Many studies have found that reducing the content of nitrogen in water eems unable to reduce the nitrogen-fixing cyanobacteria biomass.Because nitrogen-fixing cyanobacteria have heterocysts for N2 fixation.It is still controversial whether the nitrogen from the nitrogen-fixing cyanobacteria would induce the nitrogen-fixing cyanobacterial blooms..Some studies have found that it was still occurring nitrogen fixation in the eutrophication lakes.Whether the nitrogen fixation process in lakes was caused by nitrogen-fixing cyanobacteria still need more study and evidence.In this study,two urban water bodies(Muye Lake,the People's Park)were selected for investigation.The content of total nitrogen and phosphorus in water were determined and evaluated by potassium persulfate UV spectrophotometric method and ammonium molybdate spectrophotometric method,respectively.The water samples were classified and identified by microscope.The nitrogen-fixing cyanobacteria in water were separated by single algae strain separation technology.After the purification,the simulated experiments were carried out under different temperatures and different nitrogen concentrations.In situ nitrogen fixation rate and indoor nitrogen fixation rate of water sample were measured by GC with acetylene reduction assay.16 S rDNA high-throughput sequencing and nifH gene sequencing were used to assess bacterial species and nitrogen-fixing microbial species in water,and the contribution of nitrogen fixation was discussed.The main results of this study are as follows:(1)The water of Muye Lake and the People's Park are eutrophic.The total nitrogen content of Muye Lake was 5.3 mg/L,the total phosphorus content was 0.2mg/L,and the total phosphorus content in People's Park is 3.86mg/L,0.18 mg/L.This provides favorable conditions for the occurrence of cyanobacteria bloom.(2)Two water samples were observed under microscope.It was found that the main cyanobacteria in Muye Lake and the People's Park are: Microcystis,Nostoc,planktothrix spirodies,Anabaena,Oscillatoria,Clindrospermopsis.(3)The nitrogen-fixing algae were cultured at 5°C,15°C,20°C,and 30°C.Among them,the 5°C and 15°C growth was slow,and the growth rates of the two groups were faster at 20°C and 30°C.The nitrogen-fixing cyanobacteria had the fastest growth rate at 20°C.When indoor culture was conducted under different nitrogen concentrations,the nitrogen-fixing cyanobacteria biomass increased slowly with the decrease of nitrogen concentration,showing the growth state.(4)In situ nitrogen fixation rate and indoor nitrogen fixation rate of water sample were measured by GC with acetylene reduction assay.Gas chromatography was used for nitrogen fixation rate detection and no ethylene peak was detected.The study demonstrated that the nitrogen-fixing cyanobacteria of Muye Lake and the People's Park didn't show nitrogen fixation capacity.(5)There are more species of bacteria in the People's Park than Muye Lake.Two of the dominant species of water body are Proteobacteria,Bacteroidetes,Actinobacteria,and Cyanobacteria Among them,cyanobacteria account for 13% and 10% respectively in Muye Lake and the People's Park.By sequencing the nitrogenase nifH gene,the nitrogen-fixing cyanobacteria in Muye Lake accounted for only 3%.There are many species of nitrogen-fixing cyanobacteria in Muye Lake,and there are fewer species of nitrogen-fixing cyanobacteria in the People's Park.The study showed that nitrogen fixation in water is mainly contributed by nitrogen-fixing microorganisms instead of cyanobacteria.