Effects Of Carbon Quantum Dots On The Growth Process Of Photosynthetic Bacteria And Its Mechanisms

As a new kind of fluorescent carbon nanomaterials,carbon quantum dots have attracted much attention due to their excellent physical and chemical properties,and have become a hot research topic in the field of carbon nanomaterials.Over the past ten years,CQDs has been proved to have broad application prospects in many fields.With the rapid development of synthesis and application of CQDs,more and more people pay attention to the toxic effect of CQDs.Considering its nanometer size,abundant surface functional groups and excellent fluorescence and photoelectric transfer properties,CQDs may have a negative impact on organisms and environment.In this paper,the effects of N-CQDs on the growth of photosynthetic bacteria(Rhodopseudomonas acidophila)and its mechanism were studied by using citrate-based nitrogen-doped carbon quantum dots(N-CQDs)as experimental materials and R.acidophila as test organisms.The results are as follows:(1)Nitrogen-doped carbon quantum dots(N-CQDs)were synthesized by hydrothermal synthesis using citric acid as carbon source and ethylenediamine as nitrogen source.It was confirmed that N-CQDs is rich in hydroxyl,carboxyl,amino and other functional groups.The average particle size of N-CQDs is about 6.72 nm,and its aqueous solution can emit strong blue fluorescence under UV irradiation.The relative fluorescence quantum yield is 19.76 %.(2)The effects of N-CQDs on the growth of R.acidophila were studied by analyzing the physiological indexes of R.acidophila.The results showed that N-CQDs inhibited the growth of R.acidophila under 4500 lux,and decreased its biomass in a concentration-dependent manner.N-CQDs increased the content of carotenoids,but decreased the content of bacteriochlorophyll in R.acidophila.N-CQDs induced the increase of total superoxide dismutase(T-SOD)activity,which resulted in the accumulation of malondialdehyde(MDA),the damage of cell membrane,and the leakage of intracellular contents such as photosynthetic pigment,protein,etc.PI staining and SEM images confirmed the contact damage of N-CQDs to R.acidophila.It was found that the biotoxicity of N-CQDs to R.acidophila is mainly caused by membrane contact damage and oxidative stress.(3)The effects of N-CQDs on the growth of R.acidophila under different light intensity(3000 lux,6000 lux,10000 lux)were studied by analyzing the physiologicalindexes of R.acidophila.The results showed that under low light intensity(3000 lux),N-CQDs inhibited the growth and metabolism of R.acidophila,prolonged the time of bacteria entering the stable phase,and decreased the biomass.N-CQDs can not only decrease the content of bacteriochlorophyll and lead to the accumulation of carotenoids,but also induce the increase of total superoxide dismutase(T-SOD)activity and increase the content of malondialdehyde(MDA),which leads to the oxidative stress reaction of bacteria,the damage of cell membrane and the leakage of intracellular protein.The change of total superoxide dismutase(T-SOD)activity showed that R.acidophila was subjected to double stress of weak light and N-CQDs under 3000 lux.However,under high light intensity(6000 lux and 10000 lux),the growth and metabolism ability of R.acidophila was activated.N-CQDs stimulated the synthesis of photosynthetic pigments and intracellular proteins,which can stimulate the growth of bacteria.