Effects Of Herbicide Glyphosate On Phytoplankton And The Mechanisms

Glyphosate is a worldwide used herbicide and is often discharged into the sea.On the one hand,it can be a potential P source for phytoplankton to support growth considering the marine environment is always P-limited;on the other hand,glyphosate can inhibit the growth of phytoplankton as an herbicide.Firstly we investigated the physiological effects of glyphosate on fifteen species representing five major coastal phytoplankton phyla(Haptophyta,Bacillariophyta,Dinoflagellata,Raphidophyta,and Chlorophyta).Results indicated that phytoplankton can be classified into five groups according to their differential growth responses to different concentrations of glyphosate under contrasting DIP conditions.Group I(Emiliania huxleyi,Skeletonema costatum,Phaeodactylum tricornutum)could utilize glyphosate as sole P-source to support growth in axenic culture,but in the presence of DIP,they were inhibited by both 36-pM and 360-?M glyphosate.Group II(Karenia mikimotoi,Prorocentrum minimum,Dumaliella tertiolecta,Symbiodinium sp.,Heterosigma akashiwo and Alexandrium catenella)could not utilize glyphosate as sole P-source to support growth,and in the presence of DIP growth was not affected by 36-?M but inhibited by 360-?M glyphosate.Glyphosate consistently enhanced growth of Group III(Isochrysis galbana)and inhibited Group IV(Thalassiosira weissflogii,Thalassiosira pseudonana and Chattonella marina)regardless of DIP condition.Group V(Amphidinium carterae and Prorocentum donghaiense)exhibited no measurable response to glyphosate regardless of DIP condition.Then we focused on one representative dinoflagellate species Prorocentrum donghaiense,which causes harmful blooms in the East China Sea.It belongs to Group V described above,which could neither utilze glyphosate as sole P source nor be inhibited by glyphosate with the presence of DIP.But we observed P.donghaiense could utilize glypshaote as P source to promote growth after a long delay(>60 days),while P.donghaiense in the control cultures(normal growth medium)had entered the declining and dying phase.This suggested that glyphosate might have induced growth of bacteria in the cultures that were able to metabolize glyphosate(possessing C-P bond breaking genes)and release phosphate to the medium.After analyzing the bacterial community structure by sequencing the 16S rRNA gene(V4-V5 region)using Illumina high throughput sequencing method,we concluded that glyphosate could shape the bacterial community structure co-occuring in P.donghaiense culture,and selectively promote the growth of particular groups of bacteria which could in turn degrade glyphosate and release phosphate to promote the growth of P.donghaiense.Our study is the first report to illustrate that glyphosate stimulates eukaryotic phytoplankton as sole P-source and demonstrate differential responses of phytoplankton to glyphosate based on a systematic survey.The findings lay a foundation for further exploration in the future of the molecular mechanisms of these differential reponses.Meanwhile,we found the bacteria-mediated mechanism of P.donghaiense's unique response to glyphosate,which provides baseline information for further exploration concerning the other phytoplankters' mechanism of glyphosate utilization and tolerance.