Research On Biosynthesis Mechanism And Ecological Function Of Thraustochytrids Based On Omics Analysis

Thraustochytrids are unicellular fungal-like marine protists with ubiquitous existence in marine environments and have long been thought to play significant role in marine microbial ecology.However,there is still a lack of understanding of the molecular mechanisms of their ecological functions.This study focuses on biosynthetic capabilities and biodegradability of thraustochytrids using bioinformatics methods to do genomic and transcriptomic analysis,and phylogenetic analysis.Our results illustrate a variety of potential role of different thraustochytrids genera in marine ecology and suggest the existence of two types of saprobic thraustochytrids.Firstly,the whole genome of Aurantiochytrium sp.PKU#Mn4 and Aurantiochytrium sp.PKU#SW8 were sequenced on the Illumina HiSeq 2500 HT System and PacBio RS System(single-molecular sequencing technology)and performed a hybrid assembly of the HiSeq data and PacBio data for the more elaborate genomic profiles.Secondly,the genomic and transcriptomic data of thraustochytrids indicate that Aplanochytrium sp.prefer to synthesize saturated fatty acids and polysaccharides whereas Aurantiochytrium sp.and Schizochytrium sp.were more inclined to synthesize polyunsaturated fatty acids(PUFAs).Contrast to traditional views of synthesis mechanism of PUFAs,thraustochytrids may have an atypical third synthesis mechanism.Some expressed enzymes that may be involved in the FAS system or the PKS system were detected,which points out the direction for the follow-up study.Thirdly,different genera of thraustochytrids have different abilities to break down organic matter in marine.The distribution of CAZymes shows that Aurantiochytrium sp.and Schizochytrium sp.have no ability to degrade environmental organic matter whereas Aplanochytrium sp.has a vary of glycoside hydrolases that can participate in a variety of organic matter degradation.Based on the qualitative and quantitative assay of cellulase activity,it's sure that the genus Aurantiochytrium,represented by PKU#Mn4,can't secrete extracellular enzymes to degrade polysaccharides of plant cell wall and other sources in marine,especially cellulase.Fourthly,different biochemical characteristics were formed in the evolutionary development of different thraustochytrids genera,e.g.the synthesis of PUFAs,polysaccharides and extracellular enzymes.The same genus may produce different ecotypes due to habitat isolation,e.g.Aurantiochytrium sp.was divided into two groups according to mangrove leave and seawater.In summary,results of this study indicate that there are two ecotypes of thraustochytrids in the ocean.The first group,which has a high-yield of PUFAs and does not produce cellulase,serves as a dietary source for the plankton of higher trophic levels;the other group has the ability to degrade marine organic matter and plays an indispensable decomposer in marine ecosystems.This study provides a theoretical basis for comprehensive evaluation of the role of thraustochytrids in marine ecosystems,and a mechanism guidance for the large-scale fermentation of the active products of thraustochytrids such as PUFAs,terpenoids and polysaccharides.