# Linomycin-Induced Transcriptional Alterations in the Green Alga _Raphidocelis subcapitata_
## 1 Introduction  
Nowadays, the potential impact of antibiotics on the environment is an important topic of discussion and research, owing to their widespread use and slow degradation rate [1]. Linomycin (LIN) is a commonly used antibiotic by pig and chicken farms. In 2013, the annual consumption was estimated to be 7000 tons in China [2]. LIN is not fully metabolized by animals and the majority is excreted through urine and feces, and therefore pass into the environment. Moreover, LIN is stable inthe aquatic system, and has a half-life time up to 15 days and even 2033 days in certain conditions [3,4]. In line with this, a substantial amount of LIN has been measured in surface waters within the level of 175 to 1413 ng L\({}^{-1}\) and was deemed a priority compound based on its ecological risk to algae [5; 6; 7]. In most aquatic systems, algae form the bases of food webs and are directly involved in nutrient cycling in the aquatic environment. To conserve their ecosystem services, certain algae have been served as model experimental organisms in toxicological risk assessments [7,8]. In particular, the freshwater green alga _Raphidocelis subcapitata_ (_R. subcapitata_) is commonly employed to assess the chemical risk, owing to its global distribution, easy cultivation, and high sensitivity towards chemical exposure, establishing an excellent ecotoxicological dataset [9]. However, the majority of the ecotoxicity data are confined to EC50 levels on account of algal growth. Besides, the realistic exposure level of chemicals was far below the acute toxicity test concentrations, approximately 1-6 orders of magnitude lower [10]. Therefore, it is crucial to move to chronic testing using environmentally relevant concentrations, focusing on alteration at the genomic level [11]. LIN can disrupt the protein synthesis in bacteria by combining with the 50 S subunits of ribosomes and restrain the peptide linkage formation in transcription [12]. However, the chloroplast of plants represented a high homology with bacteria [13]. On this basis, it is inferred that LIN may inhibit the photosynthesis of photosynthetic organisms by disturbing the process of transcription and translation in their chloroplasts [14]. The responses of bacteria to LIN or macrolides with similar modes of action were demonstrated by estimating the physicochemical parameters. For example, the total photosynthesis rate of _Desmodesmus subspicatus_ was detected to reduce 0.12 units after exposure to LIN for 4 days at 0.4 mg L\({}^{-1}\)[15]. Moreover, rovithromycin (ROX) can inhibit fatty acid synthesis to refrain the algae growth by Fourier transform infrared spectroscopy [16]. Extra ROS is likely to produce in algal cells at antibiotics exposure (e.g., rovithromycin and clarithromycin) [17; 18; 19]. However, the toxic molecular mechanism in green algae _R. subcapitata_ remains largely unknown. The transcriptomic analysis by next-generation sequence (NGS) can facilitate the identification of molecular-level changes that underlie responses to chemical stressors [20]. This technique has been used to detected algal responses to environmental pollutants (e.g., nonylphenol and erythromycinycin) [10,21]. For instance, transcriptome analysis demonstrated that carbon fixation in photosynthesis, photosynthesis, fatty acid synthesis, and other pathways were changed at 0.09 mg L\({}^{-1}\) ROX dosing groups in _R. subcapitata_[9]. However, there is still a lack of transcriptome studies on the antibiotic limomycin (LIN) to algae. In this study, a 7-day growth inhibition test was performed to examine the effects of LIN on alga _R. subcapitata_, consisting of the three concentrations of 0 \(\upmu\)g L\({}^{-1}\) (control), 0.5 \(\upmu\)g L\({}^{-1}\) (low), 5 \(\upmu\)g L\({}^{-1}\) (medium), and 400 \(\upmu\)g L\({}^{-1}\) (high), and followed by performing the transcriptomic analysis. It was hypothesized that high LIN exposure may disturb the pathways of the carbon fixation process in photosynthetic organisms, such as photosynthesis and the fatty acid pathway.