A Comparative Study On The Phycobilisomes Of Chl F-Producing Cyanobacteria Cultured Under White Light And Far-Red Light

Photosynthesis refers to the process by which photosynthetic organisms convert light energy into chemical energy and synthesize organic matter.It is a direct or indirect source of energy for all living things on Earth.As the oldest group of autophototrophic organism that perform organisms that perform oxygenic photosynthesis,cyanobacteria have similar photo synthetic apparatus as higher plants.Unlike the presence of light-harvesting complexes in the plant photosystems,cyanobacteria mainly capture light energy and transfer it to the photosystems through phycobilisomes.The discovery of chlorophyll d(Chl d)and f(Chl f)in cyanobacteria has made people realize that photosynthetic oxygen releasing organisms can not only absorb visible light,but also the light energy in the far-red light band can be used to drive oxygenation photosynthesis.The adaptation of these red-shifted chlorophyll producing cyanobacteria to the far-red light environment involves the reorganization of both photosystems and phycobilisomes and the synthesis of red-shifted chlorophyll.However,the adaptation strategies of different cyanobacteria strains are not same.The research on the adaptive change of phycobilisomes to far-red light in chlorophyll f-producing cyanobacteria is still scant.In this paper,four cyanobacteria capable of producing Chl f are used as research materials.Four chlorophyll f-producing cyanobacteria were isolated and purified from the subtropical forest ecosystem in Wuhan in the previous research of our laboratory.The following studies were carried out on the adaptation of phycobilisomes to far-red light in the chlorophyll-f producing cyanobacteria:1.The four chlorophyll-f producing cyanobacteria are named as Leptolyngbya sp.CCNUW1,Aphanocapsa sp.CCNUM3,Chroococcidiopsis sp.CCNUC1 and Chroococcidiopsis sp.CCNUC3,respectively.The phycobiliproteins of the strains cultured under white or far-red light(wavelength>700 nm)were separated by sucrose density gradient centrifugation.In order to initially explore the adaptive changes of the phycobilisomes of the four cyanobacteria to the far-red light environment,the absorption spectra and 77K fluorescence emission spectra of each separated phycobiliprotein band were identified.It is found that these four cyanobacteria all can produce far-red-light-adapted phycobilisomes when growing under far-red light conditions,and the proportion of peripheral rod to central core decreased when compared with intact phycobilisomes under white light conditions.However,the adaptation strategies of phycobilisome in different strains to the far-red light are different.The spectroscopic tests showed that two of the cyanobacteria still contained intact white-light-type phycobilisomes when cultured under far-red light,while the other two cyanobacteria only had far-red-light-adapted phycobilisomes.2.More studies were conducted on one of the Chl f-producing cyanobacteria,Chroococcidiopsis sp.CCNUC1.The separation conditions of phycobiliprotein were explored,and the phycobiliprotein bands separated by suitable conditions were detected by mass spectrometry to analyse the changes of phycobiliprotein components or content of strains cultured under two kinds of light conditions.It is found that the difference between far-red-light-adapted phycobilisome and white-light-type phycobilisome is mainly whether it contains the phycobiliprotein encoded by the far-red light photoacclimation(FaRLiP)core gene cluster.At the same time,we found that for the far-red light cultured strain,the complete separation of the phycobiliprotein and the thylakoid membrane requires a higher concentration of Triton X-100 treatment,which indicates that the interaction of the phycobiliproteins and the thylakoid membrane has changed under far-red light conditions.Treated with only a lower concentration of Triton X-100,the phycobiliprotein separation of the far-red light cultured strain will appear a green band in the 1 M sucrose gradient,thus further studies were made on chlorophyll composition and photosystem-related proteins in each band.It is found that there are chlorophyll a(Chl a)and Chl f in the green band and the band contains more photosystem I related proteins.The relative content of some phycobiliproteins has increased as well.According to the results of mass spectrometry identification,it is speculated that the interaction between the far-red-light-adapted phycobilisome and the thylakoid membrane is related to the substitution of subunits such as ApcD and PsaL.