Impact Of Aquatic Bacteria On Karst Carbon Sequestration

The global warming causing by the atmospheric CO₂ increasing since the industrial revolution has become the most popular topics around the world.And the carbon sequestration process is the most important issue among scientists.The karst carbon sink has been underestimated for its instability.However,it has been proved that the weathering rate and the resolution rate of carbonate are higher than that of silicate,and it cannot be ignored for the climate change.The interaction among water,rock,atmosphere and organism is the key to solve the stability of karst carbon sink.Some research showed that the dissolved inorganic carbon which was from karstification is utilized by the photosynthesis of plankton,and deposit as stable organic matters.This process make the karstification a pure carbon sequestration.Most research were focus on the photosynthesis of aquatic plants.90% of the organic matter was utilized during the deposition process while there is less than 0.1% of them really deposited.Jiao put forward the theory of microbial carbon pump,based on the microbial ecological process,and proposed that the microorganism assimilate the labile organic carbon while it remains a refractory carbon pool in the sea for a long term.And the aerobic anoxygenic photosynthesis bacteria plays an important role because of its distinct ecological function.Nevertheless,there were less research about the aquatic bacterial effect on the karst carbon sequestration than its about the effect of aquatic plants on the karst carbon sequestration and about the bacterial effect on marine carbon sequestration.Hence,the Liujiang river was chosen as the study area to discuss the bacterial effect on karst carbon sequestration.Layered samples of three sections at Liuzhou section and its downstream the Honghua hydropower station-were collected every three month from April 2016 to December 2016,in order to discuss the seasonal variation of physical and chemical properties,bacterial abundance,carbon fixation rate of bacteria and their influence factors.The results showed that the hydrochemical type of Liujiang was HCO₃-Ca.The physical and chemical properties and the concentration of dissolved ion showed seasonal characteristics,as the EC and the concentration of dissolved ion was higher in winter while its lower in summer,and the DO in summer and autumn was higher than the other seasons.The Ca²⁺,Mg²⁺ and HCO₃-were mainly from the carbonate weathering,and the Na+,NO₃^-,SO₄^2-were mainly from the silicate weathering,nitrogenous fertilizer and participation,respectively.And the sulfuric acid had involved in the carbonate weathering in Liujiang basin.Compared to the isotope value of dissolved inorganic carbon generated by edaphic CO₂ weathering the carbonate,the value of study area is more positive because of the participation of sulfuric acid in the carbonate weathering.And there were significant differences of organic carbon sources among different seasons.The organic carbon was mainly affected by algae and bacteria in summer,and only a part of it was from the terrigenous plant material.But the terrigenous material was the main source of organic carbon in Liujiang basin.Seasonal variation also showed on the bacterial abundance in the study area.The abundance was higher in spring and winter while its lower in the summer and autumn,affected by DO and Eh.Algae and bacteria were able to grow simultaneously while there are sufficient nutrient,furthermore,bacteria preferred the organic carbon that produced by algea.Thus,the bacterial abundance was higher in spring and winter when the concentration of dissolved ion and chlorophyll were higher.On the other hand,the algae contend with bacteria while there was lack of nutrient,resulted in the lower bacterial abundance in summer.The carbon fixation experiment showed that the fixation rate was the highest in the autumn seasonally,and the highest at 5m underwater,vertically,which was limited by the illumination.The bacterial fixation was about 14.93% of the total planktonic fixation,and mainly affected by temperature,EC and DOC,instead of the organism abundance.Moreover,the organic carbon in the water decreased constantly with the bacterial activity,and it occurred mainly within the first month.In addition,it showed that the organic carbon with lighter isotope values is more likely to be utilized by bacteria.There was about 2 mg·L-1 organic carbon remained in the water at the end of the experiment,which would be considered as the semi-labile carbon or refractory carbon.The correlation analysis between the organic carbon decomposition and the relative abundance of AAPB showed that the AAPB reduce the decomposition rate of organic carbon.More organic carbon might have been detained because of AAPB,which extend the period of karst carbon cycle.