Characteristics Of Photoautotrophy, Mixotrophy And Heterotrophy Of Phaeodactylum Tricornutum

Trophic modes of microalgae are diversified.Over the last decades,significant attention has been drawed to mixotrophy and heterotrophy.Culturing microalgae in mixotrophic or heterotrophic conditions could potentially yield higher biomass concentration and algal products of high value.Meanwhile,many studies on photosynthetic mechanisms have been carried out with mixotrophic and heterotrophic strains.Up to now,few microalgae species have been found to possess the ability to grow mixotrophically and heterotrophically,and the reasons for the restrictions are still unknown.In this paper,a typical diatom ofPhaeodactylum tricornuturn was used as the material,characteristics of photoautotrophy,mixotrophy and heterotrophy were studied,parameters of cell growth,nutrient absorption,biochemical components, photosynthetic activity,and cell structure were examined,and mechanism of the light requirement for heterotrophic growth was also investigated.The main results are listed as follows:1.Effects of sources and concentration of organic carbon sources and light intensity on the growth of P.tricornutum were investigated.The results showed that P. tricornutum could grow mixotrophically and photoheterotrophically,and the utilization of organic carbon sources was selective.Under mixotrophic condition,the biomass of P.tricornutum was greatly enhanced.But under photoheterotrophic condition,the biomass maintained in the lower concentration level.Effects of organic carbon concentrations on the growth of P.tricornutum were in normal distribution. The mixotrophic growth was not inhibited by the high light.The saturation irradiance of mixotrophic and photoautotrophic growth was almost at the same level,which was 50μmol·m^-2·s^-12.To study the nutrient absorption of P.tricornutum under different trophic modes,concentrations of organic nutrient and inorganic nutrient in the culture were examined.The results indicated that the absorption of organic nutrient and inorganic nutrient were corresponding to the growth state.During the exponential growth phase, the growth rate was faster,and the concentration of organic carbon decreased dramatically.Under mixotrophic conditions,the absorption of nitrogen and phosphorus were more than those under photoautotrophic condition.However,the nitrogen and phosphorus absorption were lower under photoheterotrophic condition.3.The biochemical components of P.tricornutum under different trophic modes were significantly different.Compared with photoautotrophic condition,under mixotrophic and photoheterotrophic conditions,the content of soluble protein was obviously decreased,whereas the content of soluble carbohydrate and total lipid were increased.The proportions of saturated fatty acids and mono-saturated fatty acids were enhanced,however the proportion of polyunsaturated fatty acids and EPA (Eicosapentaenoic acid) were lowered.The EPA content and yield were 6.23%and 36.59 mg·L^-1 during mixotrophic culture with 100 mM acetate,which were 1.10-fold and 1.40-fold of those obtained under photoautotrophic condition,respectively. During mixotrophic culture with glycerol or glucose,and photoheterotrophic culture with glycerol,acetate,or glucose,the EPA content and yield were lower than those obtained under photoautotrophic condition.4.The experiment was conducted to determine the photosynthetic characteristics of P.tricornutum under different trophic modes.Parameters of photosynthetic pigment content and composition,absorption spectra,77 K fluorescence spectra, photosynthetic O₂ evolution rate,respiration rate,maximal photochemical efficiency of PSâ…¡(Fv/Fm),and electron transport rate(ETR) were examined.The results showed that under mixotrophic and photoheterotrophic conditions,the cell photosynthetic pigment content,values of Chl a/Ch1 c were reduced.The relative fluorescence intensity of PSâ… ,PSâ…¡and the values of F685/FT10 and F685/F738 were decreased.The net maximum photosynthetic O₂ evolution rate was also depressed. These were in consistent with Fv/Fm and ETR.Therefore,organic carbon sources reduced the photosynthesis efficiency of P.tricornuturn.However,organic carbon sources enhanced the respiration rate dramatically.5.Microscopy and transmission electron microscopy were conducted to investigate the cell volume and structure of P.tricornutum under different trophic modes.Under mixotrophic and photoheterotrophic condition,the cell volume was increased.The cell structure was finely triangular under photoautotrophic condition, but the cell center became bulgy,and the radiation was dumpy.Under photoautotrophic condition,the thylakoids were predominantly arranged in pairs. Chloroplast thylakoid pairs were less packed under mixotrophic and photoheterotrophic conditions.6.Organic carbon sources were mostly used as carbon sources ofP.tricornutum, and the main source of energy came from light.Effects of sodium malonate and sodium azide on the growth of P.tricornutum confirmed that there were succinate dehydrogenase and cytochrome oxidase system in the cell.Under chemoheterotrophic growth with glucose,the value of pH dramatically decreased,and there were formic and acetic acids in the culture.It can be assumed that an inoperative terminal oxidase system would adjust metabolism toward anaerobic metabolism pathway.