Effects of Tetracycline on Scenedesmus obliquus Microalgae Photosynthetic Processes

Abstract

Tetracycline (TC) antibiotics can be detected worldwide in the aquatic environment due to their extensive use and low utilization efficiency, and they may affect the physiological processes of non-target organisms. In this study, the acute and sub-acute toxicities of TC on the freshwater microalga Scenedesmus obliquus were investigated with an emphasis on algal photosynthesis and transcription alterations during an 8 d TC exposure. The results showed that the IC₁₀, IC₃₀ and IC₅₀ values were 1.8, 4.1 and 6.9 mg/L, respectively. During sub-acute exposure, the microalgae of the IC₁₀ treatment was able to recover comparable growth to that of the control by day 7, while significantly lower cell densities were observed in the IC₃₀ and IC₅₀ treatments at the end of the exposure. The photosynthetic efficiency F_v/F_M of S. obliquus first decreased as the TC concentration increased and then returned to a level close to that of the control on day 8, accompanied by an increase in photosynthetic activities, including light harvesting, electron transport and energy dissipation. Transcriptomic analysis of the IC₁₀ treatment (1.8 mg/L TC) revealed that 2157 differentially expressed genes were up-regulated and 1629 were down-regulated compared with the control. KEGG and GO enrichments demonstrated that 28 photosynthesis-related genes involving light-harvesting chlorophyll protein complex, photosystem I, photosystem II, photosynthetic electron transport and enzymes were up-regulated, which may be the factor responsible for the enhanced photosynthesis and recovery of the microalgae. Our work may be helpful not only for gaining a better understanding of the environmental risk of TC at concentrations close to the real levels in natural waters, but also for explaining photosynthesis and related gene transcription induced by antibiotics.

Keywords: hormesis; photosynthesis; photosynthesis-related genes; tetracycline; transcriptomics.

Figure 1

Growth curves of Scenedesmus obliquus exposed to different concentrations of tetracycline for 8 days.

Figure 2

OJIP curves of Scenedesmus obliquus after exposure to different concentrations of tetracycline for 4 days (A) and 8 days (B).

Figure 2

OJIP curves of Scenedesmus obliquus after exposure to different concentrations of tetracycline for 4 days (A) and 8 days (B).

Figure 3

Transcriptomic profiling of Scenedesmus obliquus in the control (C) and tetracycline treatment (T) after 8 d exposure. (A) Volcano plot based on the differentially expressed genes (DEGs) in T vs. C groups; (B) principal components analysis based on the expression quantity; (C) correlation analysis of patterns of gene expression; (D) a heatmap of the centered and scaled FPKM values of DEGs.

Figure 4

Scatter plots of enrichment results for the KEGG pathways (A) and GO function enrichment (B) for the IC₁₀ treatment of Scenedesmus obliquus compared with the control (C). The x axis shows the rich factor (the ratio of differentially expressed genes and total gene numbers of the pathway) for each KEGG or GO pathway. The color of the dot indicates the FDR value; the size of the dot denotes the number of genes involved in each pathway.

Figure 4

Scatter plots of enrichment results for the KEGG pathways (A) and GO function enrichment (B) for the IC₁₀ treatment of Scenedesmus obliquus compared with the control (C). The x axis shows the rich factor (the ratio of differentially expressed genes and total gene numbers of the pathway) for each KEGG or GO pathway. The color of the dot indicates the FDR value; the size of the dot denotes the number of genes involved in each pathway.

Figure 5

Significantly enriched Gene Ontology (GO) biological process terms and up-regulated differentially expressed genes (DEGs) in Scenedesmus obliquus exposed to tetracycline, compared to the control. The colors of the nodes denote the p-value (decreasing with the deepening of color from light yellow to deep red), which is also shown after each item. The numbers in each term represent DEGs and the total genes involved in this term. The top 10 GO terms are displayed in the rectangular frames.

Figure 6

Schematic of the light-harvesting chlorophyll protein complex (A) and photosystem (B). Proteins and subunits are distinguished by different colors. The up-regulated proteins are marked in light red at the bottom of figure.