De novo transcriptome assembly and molecular response mechanism analysis of a diatom Cyclotella meneghiniana Kützing exposed to cadmium

Cadmium is a persistent heavy metal commonly found in aquatic ecosystems and has a strong toxic effect on organisms. The sensitivity of phytoplankton to environmental changes and its role as an indicator of aquatic ecosystem health have been well-established. However, the mechanisms by which phytoplankton respond to cadmium remain incompletely understood. In this study, we chose the typical planktonic diatom Cyclotella meneghiniana Kützing, by integrating physiological-biochemical data and transcriptome analysis, to reveal the molecular mechanisms of C. meneghiniana responing to cadmium. Under cadmium stress, the cell density and chlorophyll-a content of C. meneghiniana significantly decreased, while MDA content and SOD activity gradually increased. At 72 h of cadmium stress, we found that at this time point, cell abundance and physiological variation were very significant, therefore we selected 72 h for subsequent analysis. To better understand the cadmium stress response mechanisms of C. meneghiniana, a de novo transcriptome method was used to analyse C. meneghiniana under cadmium stress for 72 h, and 1704 (M vs. CK) and 4788 (H vs. CK) differentially expressed genes were found. Our results showed that the changes in gene expression were closely correlated to the physiological-biochemical changes. Although cadmium stress could promote the nitrogen metabolism pathway, ROS scavenging system, and photosynthesis. While, C. meneghiniana under medium and high concentrations of cadmium can also limit various intracellular metabolic pathways, such as the MAPK pathway and phosphatidylinositol metabolic pathway, and the degree of inhibition increases with the increase of stress concentration. In present study, the complete molecular mechanism of the planktonic diatom response to cadmium has been established, which provided important information for further studies on heavy metal pollutants and the multiple functional genes responsible for cadmium sensitivity and tolerance in planktonic diatoms.