A Regulatory Cascade Of Phosphorus-nutrient Strategies In Phaeodactylum Tricornutum

Phosphorus(P)is an essential nutrient for marine phytoplankton,the base of marine ecosystems.However,in various parts of the world’s ocean,P-nutrient,primarily dissolved inorganic phosphorus(DIP),is limited.To maintain intracellular P homeostasis and population growth,phytoplankton have evolved a set of strategies to cope with P limitation,including increase of inorganic phosphate(Pi)transporters,induction of hydrolases for scavenging organophosphates,and reducing P demand by replacing phospholipids with sulfur-or nitrogen-lipids.Wherein,they mainly achieve this via alkaline phosphatases(APs)that hydrolyze phosphomonoesters,which account for～75%of total DOP in the ocean,and other enzyme systems that hydrolyze phosphonates,which account for～25%of total DOP.How phytoplankton such as diatoms regulate the expression and function of these enzymes for scavenging different sources of P,maintaining P homeostasis and responding to low-P stress is poorly understood.Diatoms,which play a crucial role in the biogeochemical cycle,contribute～40%of global primary production.Diatoms are relatively young in evolution and are widespread in various natural waters.Phaeodactylum tricornutum has been widely used as a model diatom species because its genome has been sequenced.Here,we use CRISPR/Cas9 to knock out two AP genes(PhoA and PhoD)and the gene of SPX protein,a P regulator previously known only in plants,and the perform transcriptome profiling to characterize their modes of functions.The main results are as follows:1)Compensatory regulation between different AP genes.PhoA and PhoD gene expression compensates for each other after one is disrupted;the DOP→PhoA→P_uptake and the DOP_uptake→PhoD→P_pathways function interchangeably for different DOP substrates.2)AP also functions,besides P-nutrient scavenging when DIP is limited,to constrain pigment biosynthesis,photosynthesis,fatty acid biosynthesis,and cell division,with implications in balancing metabolic processes and preventing premature cell division.3)SPX is a negative regulator of P uptake and P-stress response.SPX regulation of P uptake and metabolism involves a phosphate starvation response gene(PHR)as an intermediate to combine a SPX-PHR-PSI central regulatory cascade.This study is a primer of using the CRISPR/Cas9 genome editing technology to characterize how phytoplankton coping with phosphorus-nutrition.As the technique only recently began to be accessible for phytoplankton research,there is a wide-open field to apply the technique.These findings have important ecological implications regarding how varying P conditions can shape a phytoplankton assembly and how phytoplankton will respond or evolve to future ocean environments in the context of climate change.The mutants generated here will be a valuable resource for future studies to further dissecting the molecular machinery underlying phytoplankton acclimation and adaptation to P variability.