Copper-induced concomitant increases in photosynthesis, respiration, and C, N and S assimilation revealed by transcriptomic analyses in Ulva compressa (Chlorophyta)

Abstract

Background: The marine alga Ulva compressa is the dominant species in copper-polluted coastal areas in northern Chile. It has been shown that the alga tolerates micromolar concentrations of copper and accumulates copper at the intracellular level. Transcriptomic analyses were performed using total RNA of the alga cultivated with 10 μ M copper for 0, 1, 3 and 5 days using RNA-seq in order to identify processes involved in copper tolerance.

Results: The levels of transcripts encoding proteins belonging to Light Harvesting Complex II (LHCII), photosystem II (PSII), cytochrome b6f, PSI, LHCI, ATP synthase and proteins involved in repair of PSII and protection of PSI were increased in the alga cultivated with copper. In addition, the level of transcripts encoding proteins of mitochondrial electron transport chain, ATP synthase, and enzymes involved in C, N and S assimilation were also enhanced. The higher percentages of increase in the level of transcripts were mainly observed at days 3 and 5. In contrast, transcripts involved protein synthesis and degradation, signal transduction, and replication and DNA repair, were decreased. In addition, net photosynthesis and respiration increased in the alga cultivated with copper, mainly at days 1 to 3. Furthermore, the activities of enzymes involved in C, N and S assimilation, rubisco, glutamine synthase and cysteine synthase, respectively, were also increased, mainly at days 1 and 3.

Conclusions: The marine alga U. compressa tolerates copper excess through a concomitant increase in expression of proteins involved in photosynthesis, respiration, and C, N and S assimilation, which represents an exceptional mechanism of copper tolerance.

Keywords: Copper; Marine alga; Photosynthesis; Respiration; Transcriptomic analyses; Ulva compressa.

Fig. 1

Level of the highest increased transcripts encoding proteins involved in photosynthesis (a) and respiration (b) in U. compressa cultivated with 10 μM copper for 0, 1, 3 and 5 days. Transcripts encoding proteins involved in photosynthesis corresponding to subunit PetC of cytb6f complex (PetC), subunit PsbA of PSII (PsbA), the protease FstH1 (FstH1), the protease Deg1 (Deg1), subunit 4 of LHCII (Lhcb4), the kinase ABC1K1, subunit 5 of LHCII (Lhcb1) and subunit PetJ of cytb6f complex (PetJ) are indicated with an arrow (a). Transcripts encoding proteins of the mitochondrial electron transport chain (respiration) corresponding to subunit IV of bc1 complex (complex III), subunit γ of ATP synthase (ATP-γ), subunit 1 of NADH dehydrogenase (complex I), subunit V of bc1 complex (complex III) and subunit 2 of NADH dehydrogenase (complex I) are indicated with an arrow (b). The level of transcripts is expressed as the number of normalized reads and time in days

Fig. 2

Level of the highest increased transcripts encoding enzymes involved in C, N and S assimilation in U. compressa cultivated with 10 μM copper for 0, 1, 3 and 5 days. Transcripts encoding enzymes involved in C assimuilation corresponding to glyceraldehyde 3-P dehydrogenase (G3PDH), small subunit of rubisco (RbcL), phosphorribulo kinase (PRK), large subunit of rubisco (RbcL) and fructose biphosphate aldolase (FBPA) are indicated with an arrow (a). Transcripts encoding enzymes involved in N assimilation corresponding to nitrate reductase (NitrateR), fumarase (Fum), glutamine synthase (GlnS) and arginine-succinate lyase (ArgSL) are indicate with an arrow (b). Transcripts encoding enzymes involved in S assimilation corresponding to ATP sulfurylase (ATPS), APS reductase (APSR) glutamine cysteine ligase (GCL), sulfite reductase (SulfiteR) and cysteine synthase are indicated with an arrow (c). The level of transcripts is expressed as the number of normalized reads and time in days

Fig. 3

Level of oxygen production under light (a) and oxygen consumption in the dark (b) in the marine alga U. compressa cultivated in control condition (open circles) and with 10 μM copper (black circles) for 5 days. The level of oxygen is expressed in nanomoles per microliter per minute, and time in days. Symbols represent the mean value of three independent experiments ± SD. Different letters indicate significant differences (P < 0.05)

Fig. 4

Activity of key regulatory enzymes involved in C assimilation, ribulose 1.5 biposphate carboxylase /oxygenase (rubisco, a), N assimilation, glutamine synthase (GlnS, b) and in S assimilation, cysteine synthase (CysS, c). The activity rubisco and GlnS is expressed in nanomoles per minute per milligram of proteins and the activity of CysS is expressed in micromoles per minute per milligram of proteins, and time in days. Symbols represent mean values of three independent experiments ± SD. Letters indicate significant differences (P < 0.05)