Documenting the sex and sexual reproduction of the microalgae is very difficult, as most of the results are based on the microscopic observation that can be heavily influenced by genetic, physiological and environmental conditions. Understanding the reproduction strategy of some microalgae is required to breed them in large scale culture industry. Instead of direct observation of sex and sexual reproduction under microscope, the whole set or the majority of core meiosis genes may evidence the sex and sexual reproduction in the unicellular algae, as the meiosis is necessary for maintaining the genomic stability and the advantages of genetic recombination. So far, the available genome sequences and bioinformatic tools(in this study, homolog searching and phylogenetic analysis) allow us to propose that at least 20 core meiosis genes(among them ≥6 must be meiosis specific) are enough for an alga to maintain its sexual reproduction. According to this assumption and the genome sequences, it is possible that sexual reproduction was carried out by Micromonas pusilla and Cyanidioschyzon merolae, while asexual reproduction was adopted by Bigelowiella natans, Guillardia theta, Nannochloropsis gaditana, N. oceanica, Chlorella variablis, Phaeodactylum tricornutum and Thalassiosira pseudonana. This understanding will facilitate the breeding trials of some economic microalgae(e.g., N. gaditana, N. oceanica, C. variablis and P. tricornutum). However, the reproduction strategies of these microalgae need to be proved by further biological experiments.
Phaeodactylum tricornutum is a diatom that is rich in lipids.Recently,it has received much attention as a feedstock for biodiesel production.Nitrogen defi ciency is widely known to increase the content of neutral lipids(mainly triacylglycerides,or TAGs)of microalgae,including P.tricornutum,but the mechanism is unclear.In this study,we deciphered the correlations between TAG content and nine key enzymatic genes involved in lipid synthesis in P.tricornutum.After being cultured under nitrogen-free conditions for 0,4,24,48,72,120,and 168 h,the TAG contents of P.tricornutum cells were assayed and the transcript abundances of the target genes were monitored by quantitative real-time PCR.The results show that the abundances of four target gene transcripts(LACS3,G3PDH2,G3PDH3,and G3PDH5)were positively correlated with TAG content,indicating that these genes may be involved in TAG synthesis in P.tricornutum.The fi ndings improve our understanding of the metabolic network and regulation of lipid synthesis and will guide the future genetic improvement of the TAG content of P.tricornutum.
Zeocin can cause double strand breaks of DNA and thus may be employed as a mutagen.In this study,two strains of Nannochloropsis oceanica,the wild and the Zeocin-tolerant strains,were re-sequenced to verify such function of Zeocin.The results showed that Zeocin can mutate the N.oceanica genome and cause the structural variation.Zeocin either swept away or selected the alleles of genes functioning in ubiquitin-mediated proteolysis,alpha-linolenic acid metabolism,ascorbate and aldarate metabolism,ribosome biogenesis,and circadian rhythm,indicating that N.oceanica may have adjusted its metabolic performances for protein,carbohydrate,and lipid,and changed its ribosome biosynthesis and living rhythm to survive in Zeocin containing medium.In addition,Zeocin caused mutation may have influenced the expression of a set of transcription factors.It was concluded that Zeocin effectively caused the structural variation of the genome of N.oceanica,and forced the microalgae to select out the alleles of a set of genes around these variations in order to adapt to Zeocin containing medium.Further studies on the genetic basis of the phenotypic adaptation of this haploid and asexual microalga and the application of Zeocin to its genetic improvement are very important.
LIN GenmeiZHANG ZhongyiGUO LiDING HaiyanYANG Guanpin
In this study, we compared the transcriptomes of Nannochloropsis oceanica cultured in f/2 medium prepared with seawater and freshwater, respectively, aiming to understand the acclimation mechanism of this alga to freshwater. Differentially expressed genes were mainly assigned to the degradation of cell components, ion transportation, and ribosomal biogenesis. These findings indicate that the algal cells degrade its components(mainly amino acids and fatty acids) to yield excessive energy(ATP) to maintain cellular ion(mainly K+ and Ca2+) homeostasis, while the depletion of amino acids and ATP, and the reduction of ribosomes attenuate the protein translation and finally slow down the cell growth.
Nannochloropsis oceanica promises to be an industrial-level producer of polyunsaturated fatty acids.In this study,the fastest and slowest growing N.oceanica mutants were selected through N-methyl-N'-nitro-N-nitrosoguanidine mutation,and two mutant strains and the wild type(WT) subjected to transcriptome profiling.It was found that the OD_(680) reads at stationary growth phase of both WT and its mutants were proportional to their cell density,thus indicating their division rate and growth speed during culture.This chemical mutation was effective for improving growth performance,and the fast strain divided faster by upregulating the expression of genes functioning in the cell cycle and downregulating genes involved in synthesis of amino acids,fatty acids,and sugars as well as the construction of ribosome and photosynthetic machinery.However,the relationship among the effected genes responsible for cell cycle,metabolism of fatty and amino acids,and construction of ribosome and photosynthetic machinery remained unclear.Further genetic studies are required for clarifying the genetic/metabolic networks underpinning the growth performance of N.oceanica.These findings demonstrated that this mutation strategy was effective for improving the growth performance of this species and explored a means of microalgal genetic improvement,particularly in species possessing a monoploid nucleus and asexual reproduction.
Zeocin can cause double strand breaks of DNA and thus is frequently used as a selective antibiotic of eukaryotic Sh ble transformants. In non-transformation system, Zeocin may function as a mutagen if not totally lethal. To verify such function of Zeocin, we mutated Nannochloropsis oceanica by increasing the concentration of Zeocin in medium gradually, and isolated a N. oceanica strain(single cell culture) which survived Zeocin up to 10.0μg mL^(-1). The Zeocin-tolerant strain entered the exponential growth phase later and grew slower than the wild strain. Transcriptome profiling showed that the Zeocin-tolerant N. oceanica strain survived Zeocin mainly by adapting(heritable), rather than acclimating(plastic) to Zeocin. Hence mutating N. oceanica with Zeocin was approved effective. Meanwhile, the physiological characteristics of this Zeocin-tolerant strain were demonstrated. As we proposed, N. oceanica tolerated Zeocin by strengthening its protein degradation and antioxidation. The genes controlling cell division and cellular response to stimuli may also have played important roles in the reduction of growth and the tolerance to Zeocin. Our findings evidenced that Zeocin can serve as an appropriate mutagen of microalgae. Creating variations through mutation with Zeocin may help to study the genetic basis of the traits of this monoploidy and asexual microalga, as well as improve its production.
LIN GenmeiWANG YameiGUO LiDING HaiyanHU YongmeiLIANG SijieZHANG ZhongyiYANG Guanpin
