Molecular Mechanism Involved In The Colony Formation Of Phaeocystis Globosa

Phaeocystis globosa is one of the most cosmopolitan bloom-forming haptophyte genus.It has a complex polymorphic life cycle alternating free-living solitary cells and colonial cells.P.globosa blooms are usually dominated by very den se colonial morphotype.Thousands of colonial cells embedded in polysaccharide colonial matrix,is one of the reasons why P.globosa became one of the most successful algae in the marine system.Plenty of research focused on how environmental factors influence the colony formation process has been done,however,there is very limitedresearch on the molecular mechanism of colony formation.In recent years,P.globosa blooms in Beibu Gulf occurs more frequently,and are gradually expanding.Consequently,notonly marine ecosystem are negatively affected,but also restricted the local economic development.In this study,transcriptome signal changes in four different stages of colony formation were invested,combined with transcriptome response of temperature on colony formation,the mechanism of colony formation are explained from several perspectives:glycosaminoglycan biosynthesis and degradation,carbon and nitrogen metabolism,protein biosynthesis,cell movement,cell defense and signal transduction.The main results are as follows:?1?Solitary cells of P.globosa?SC₂0?exhibit four distinct stages duringthe process of colony formation:two-cells stage?C2₂?,four-cells stage?C4₄?and colonial cells?CC₂0?and non-colonial cells?NCC₂0?.The solitary cells released from mature colonieswerenon-flagellated,then the cells exhibit motility,followed by loss of flagella,sedimentation to the surface of the substrate,release of the mucus matrix,and the formation of colonies.At 32?,the growth rate of solitary cells was higher than 20?.The flagellar structure was preserved throughout the life history,and flagella development and loss were not observed.?2?In this study,79,824 unigenes were identified by transcriptome.The differentially expressed genes were mainly involved in glycosaminoglycan biosynthesis and degradation,nitrogen metabolism,signal transduction,cell defense and cell movement.Compared with NCC₂0,glycosaminoglycan biosynthesis increased significantly,degradation decreased significantly and led to glycosaminoglycan accumulated in different stages of colony formation.Nitrogen metabolism and glutamine synthesis increased significantly in colonial cells,which provided a large amount of precursor for glycosaminoglycan biosynthesis.The proteins related to signal transduction and plant-pathogen pathway were significantly down-regulated during the colony formation process,indicating that the existence of the colony matrix provided protection for colonial cells.During the process of colony formation,flagella showed a development and loss,the expression of motility-related proteins was significantly down-regulated.?3?Temperature affects glycosaminoglycan accumulation and flagella formation in Phaeocystis globosa.Glycosaminoglycan is an important matrix for the formation of cysts.High temperature accelerates the degradation of glycosaminoglycan,which results in the degradation of glycosaminoglycan matrix and the formation of cysts.The genes related to cell defense were significantly up-regulated at 32 C without cyst protection at high temperature.In order to conserve energy for the biosynthesis of cyst matrix or other life activities,genes associated with cell movement in cyst cells were significantly down-regulated,including intraflagellar transport,kinesin,kinesin and myosin.Studies have shown that temperature changes the accumulation of intracellular glycosaminoglycans and affects the formation of cyst matrix.