LAUSR

ABSTRACT The soil algal strain Parachlorella kessleri TY possesses unique metabolic capacity. However, systemic information relevant to the molecular regulation of its metabolic processes under stress is unavailable. This study investigated the performance of P. kessleri under nitrogen deficiency (N–) and normal nitrogen (N+) conditions. The results showed that P. kessleri accumulated high amounts of carbohydrates and lipids under N– and concomitantly decreased its biomass yield, chlorophyll content, photochemical efficiency of photosystem II (Fv/Fm), and total protein levels. Transcriptome sequencing to identify the molecular responses of the microalgae under N– resulted in a total of 112 811 assembled transcripts annotated and classified into different biological categories and pathways based on the results of BLAST against diverse databases. An in-depth gene expression profiling analysis showed that many differentially expressed genes in P. kessleri under N– versus N+ conditions are involved in lipid metabolism. The transcript abundance of the genes that encode enzymes involved in fatty acid and unsaturated fatty acid biosynthesis (such as accC, fabF, fabH, FAD2 and SCD) were significantly increased. However, a set of genes that encode enzymes involved in lipid degradation decreased their expression under N–. This difference highlights the coordinated regulation of the lipid metabolism at the transcription level. The de novo transcriptome assembly lays a foundation for the genetic improvement of the lipid content and composition of P. kessleri in the future.