Insight Into The Coupling Effect Of Organic Matter And Nutrients On Algae Growth In Water Environment

In water environment,algal growth and reproduction may be affected not only by the concentration of total nitrogen(TN)and total phosphorus(TP)in water environment,but also the coexisting total organic carbon(TOC).However,research to date have generally focused on the effect of nutrient levels on water eutrophication and algae growth,while the impact of TOC and the coupling effect of organic matter and nutrients in the process of algal synthesis are still unclear and lacking in-depth exploration.In order to reveal the coupling effect of external organic carbon,nitrogen and phosphorus on algal growth in water environment,experimental studies were conducted using Chlorella vulgaris as the representative microalgae.Microalgal cultivation experiments under three varied modes were carried out,namely,conventional batch cultivation,stably controlled TOC and nutrient levels and chemostat cultivation mode in actual water environment,respectively.The metabolic law of TOC and nutrients for algae growth under different conditions,as well as the influence of TOC,TN and TP levels on algal biomass characteristics were systematically studied.This study also investigated the dynamic relationship between the consumption of substrates and algal metabolism together,the coupling effect of organic matter and nutrients in water environment on microalgal reproduction was explored.The main results and conclusion of this study were listed as below:(1)For the purpose of exploring the metabolic rule of organic carbon(TOC=0-40mg/L)on algal growth characteristics in water environment,the conventional batch cultivation was conducted.It was found that the promotion of TOC on the growth and reproduction of microalgae is most significant when the nutrient supply is sufficient(TN=20 mg/L,TP=5 mg/L),the growth potential,specific growth rate,photosynthetic characteristics,assimilation and transformation efficiency of substrates increased significantly with elevated TOC concentration.With the increase of TOC levels in water environment,the specific growth rate of microalgae under nutrient limited condition(TN=2 mg/L,TP=0.4 mg/L)reached the maximum at TOC=30 mg/L,the maximum photochemical efficiency(Fv/Fm)and the electron transport rate(ETR)corresponding to the photoreaction stage decreased significantly with the increase of TOC supply,the photosynthetic pigments also decreased,the ability of nitrogen assimilation and transformation efficiency of microalgae is reduced while the absorption and utilization of phosphorus increased with increased TOC concentrations.The addition of TOC showed no significant effect on growth,photosynthesis and nutrient metabolic characteristics of microalgae in the absence of nutrients(TN and TP are nearly exhausted).(2)With the goal of investigating the combined effect of organic matter and nutrients in water environment on microalgal growth and biomass characteristics,in-depth analysis of TOC and nutrients consumption and assimilation process were explored from a molecular perspective.Results showed that the addition of TOC(30 mg/L)increased the specific growth rate of microalgae to 0.52±0.002 d^-1 at the same nutrient background,elevated by up to 0.20 d^-1.Due to the inhibitory effect of external organic carbon on microalgal light-harvest ability,the photosynthetic pigments were reduced by 44.69%while the maximum photochemical efficiency(Fv/Fm)and the electron transport rate(ETR)were reduced by 6.19%-36.05%compared with the same nutrient supply under autotrophic cultivation(without TOC addition).With the coupling effect of TOC and nutrients,the biomass dry weight of microalgae increased with the nutrient supply at lower TN-TP content and then decreased,the microalgal dry weight reduced by 2.25-60.15%with the addition of TOC under the same nutrient level.The microalgal consumption and assimilation of nitrogen reduced by 7.96-40.63%and the utilization of phosphorus intensified by 9.38%-50%compared with autotrophic conditions.The addition of TOC reduced the maximum amount of carbon assimilation of microalgae from58.19 mg/L to 28.74 mg/L,confirming the promotion of microalgal catabolism by external organic carbon.(3)The semi-continuous cultivation achieved by stably controlled TOC and TN-TP levels was adopted for exploring the dynamic relationship between algae growth,TOC and nutrient metabolic rule and biomass characteristics using TOC/TN and TN/TP as the coupling indicators.The results of comprehensive dynamic model of microalgal growth showed that the limits of TOC,TN and TP levels that restrict microalgal growth are 3.41mg/L,0.22 mg/L and 0.033 mg/L.The specific growth rate and photosynthetic activity of microalgae reach the maximum when TOC/TN=15,and was less disturbed by light response of photosynthesis.The conversion efficiency of organic matter and nutrients reached the maximum value of 65.04±1.36%,67.67±1.23%and 60.12±1.58%when the supplied TOC/TN/TP ratio is 75/5/1.The increased TOC/TN promotes the accumulation of microalgal biomass by converted pigments and proteins into carbohydrates and lipids.The proportions of carbohydrates and lipids increased to 51.55±0.21%and 4.93±0.03%respectively when TOC/TN=15.The distribution of monosaccharides in microalgal biomass is dominated by glucose,followed by galactose and rhamnose,while the main components of fatty acid methyl esters(FAMEs)are linoleic acid(C18:2)and oleic acid(C18:1)and palmitic acid(C16:0).According to the elemental composition of microalgal biomass,the general chemical formula of the microalgae in the water environment is C108.22-117.43H214.32-241.10O67.53-80.94N9.37-14.63P0.61-_1.50S_0.40-1.34,The main factor affecting the coordination number of carbon(C),nitrogen(N)and phosphorus(P)in the stoichiometric formula is TOC/TN.(4)The chemostat cultivation of microalgae in actual landscape water verified the promotion effect of increased TOC/TN on growth,reproduction of microalgae and the absorption,transformation of organic matter and nutrients,the regulatory effects of intracellular biochemical components have also been verified.In particular,TOC/TN ratio is still the most important indicator that affects the microalgal growth,the absorption and transformation of organic matter and nutrients,the accumulation of biomass,and the impact of intracellular biochemical components.Controlling TOC backgrounds and reducing TOC/TN limits the excessive reproduction of microalgae,and plays an important role in the control of water eutrophication.The general chemical formula of microalgae obtained in stable experiment is also applicable to the scene of chemostat cultivation of microalgae in landscape lake,and the coordination number of each element in the general formula mainly depends on the TOC/TN ratio.Results showed that in addition to nutrients,the level of organic pollution also directly affects the elemental composition of microalgae,which is a main factor of water quality that cannot be ignored in water environment governance.Based on the theoreticalanalysis of the results of those three varied microalgal cultivation modes,this study further clarified the TOC promotion effect on microalgal growth and reproduction and explored the absorption mechanism centered on intracellular protein components and chloroplast corresponding to the action of TOC and TN,and the synthesis mechanism centered on the ribosome RNA corresponding to the action of TP as the coupling mechanism of organic matter and nutrients.Since there are many algae species that may breed and grow in water environment,the applicability of the results obtained in this study based on the Chlorella sp.cultivation to other algae species needs to be further considered.However,even for micro-polluted water,the TOC concentration as an indicator of organic matter is usually not negligible.Therefore,paying attention to the effect of organic matter on the growth and reproduction of algae in water environment may become a new direction in the control of water eutrophication.