Intercellular Effects Induced By Phosphates

Microalgae are a green bridge between energy and environment,which has attracted growing attention from researchers in wastewater treatment and energy conversion.Phosphorus is not only the basic element of microalgae cells,but also an indispensable nutrient element for cell synthesis and energy metabolism during their growth.Essentially,the diversity of metabolites during the growth of microalgae is mediated by the cell-cell communication via signaling molecules.Under ambient pressure,intercellular communication can be carried out between microorganisms.At present,most of them only exist in the intercellular communication between algae,bacteria and bacteria,but there are few reports between microalgae and microalgae.Aiming at this problem,the effects of phosphate on the growth of microalgae and the intercellular effects induced by phosphate were discussed,and the related mechanisms were also elucidated.Firstly,the effects and mechanisms of phosphate stress on microalgae were investigated.The results showed that the growth of Chlorella regularis was inhibited with the increase of phosphate concentration under normal nitrogen culture conditions.Excessive phosphorus(≥150 mg-P L^-1)has a"toxic effect"on Chlorella regularis.The main manifestations were as follows:cell density and viability decreased by～40%and 70%,respectively,besides,cells showed obvious poisoning status,such as enlarged size,the deformation of cell walls,and the disorganization of organelles.These toxic effects were associated with the over-accumulation of polyphosphates within cells,which may further cause binding of phosphate to intracellular protein.Under nitrogen-limited culture conditions,different concentrations of phosphate presented a hormesis effects for microalgae cultivation,i.e.,slightly excessive phosphate concentration(≤45 mg-P L^-1)stimulated the growth and mitochondrial activity of Chlorella regularis through the stored polyphosphate energy pool in cells and the lipid productivity reached 354.38 mg·（L·d）^-1.However,high phosphorus concentration(≥150 mg-P L^-1)inhibited the growth and lipid production of Chlorella regularis and caused cell toxicity.The toxic mechanism was related to the damage of amide-N and plasma membrane in protonated proteins.The above results illustrated that although phosphorus is an essential nutrient for microalgae,excessive phosphorus could cause toxicity to microalgae.Secondly,the role and mechanism of signaling molecules in microalgae under phosphate stress were discussed.The signaling molecules of microalgae were extracted,identified and backfilled under normal and excessive phosphorus concentration,respectively.It was found that the extracts at high phosphorus concentration promoted microalgae growth.Such as the cell density increased by 77%,carbon absorption rate increased by 43%,speed of cell division accelerated,and DNA replication and TCA cycle were also promoted compared to control group（without extracts）.After identification of extracts,benzoic acid and salicylic acid were found to be the main components of crude signaling molecules under excessive phosphorus stress,and their secretion concentration had a good linear relationship with cell growth.The results showed that intercellular communication between Chlorella vulgaris could be mediated by endogenous signal molecules benzoic acid and salicylic acid under phosphate stress.