Molecular and Photosynthetic Responses to Prolonged Darkness and Subsequent Acclimation to Re-Illumination in the Diatom Phaeodactylum tricornutum

Abstract

Photosynthetic diatoms that live suspended throughout the water column will constantly be swept up and down by vertical mixing. When returned to the photic zone after experiencing longer periods in darkness, mechanisms exist that enable the diatoms both to survive sudden light exposure and immediately utilize the available energy in photosynthesis and growth. We have investigated both the response to prolonged darkness and the re-acclimation to moderate intensity white irradiance (E = 100 µmol m−2 s−1) in the diatom Phaeodactylum tricornutum, using an integrated approach involving global transcriptional profiling, pigment analyses, imaging and photo-physiological measurements. The responses were studied during continuous white light, after 48 h of dark treatment and after 0.5 h, 6 h, and 24 h of re-exposure to the initial irradiance. The analyses resulted in several intriguing findings. Dark treatment of the cells led to 1) significantly decreased nuclear transcriptional activity, 2) distinct intracellular changes, 3) fixed ratios of the light-harvesting pigments despite a decrease in the total cell pigment pool, and 4) only a minor drop in photosynthetic efficiency (ΦPSII_max). Re-introduction of the cells to the initial light conditions revealed 5) distinct expression profiles for nuclear genes involved in photosynthesis and those involved in photoprotection, 6) rapid rise in photosynthetic parameters (α and rETRmax) within 0.5 h of re-exposure to light despite a very modest de novo synthesis of photosynthetic compounds, and 7) increasingly efficient resonance energy transfer from fucoxanthin chlorophyll a/c-binding protein complexes to photosystem II reaction centers during the first 0.5 h, supporting the observations stated in 6). In summary, the results show that despite extensive transcriptional, metabolic and intracellular changes, the ability of cells to perform photosynthesis was kept intact during the length of the experiment. We conclude that P. tricornutum maintains a functional photosynthetic apparatus during dark periods that enables prompt recovery upon re-illumination.

A novel type of light-harvesting antenna protein of red algal origin in algae with secondary plastids

“…The transcript levels of RedCAP, LHCF2, OHP1-like 2, OHP2 and SEPX were significantly down-regulated after the onset of HL illumination (Figure 4B, and Table S3, see Additional file 9). The observed expression patterns of LHCF2 and RedCAP during a LL to HL shift were similar to the ones reported by Nymark et al [37,40]. The transcript level change of LHCF2 during a shift from LL to HL also seems to be reflected by a change of its protein amount [45].…”

“…Following a similar pattern, also transcript levels of RedCAP and OHP1-like 1 were significantly up-regulated during the light period and down-regulated (no significant difference compared to the transcript level at the onset of darkness) during darkness. A similar expression pattern of RedCAP upon a shift from D to LL was recently reported [37]. In the D condition (no illumination) the amounts of LHCF2 and RedCAP transcripts were significantly down-regulated, although a transient up-regulation of the transcript level was measured at the time when the light was previously switched on, this effect was also observed (however, not statistically significant) for LHCF2 (Figure 4 and Table S3, see Additional file 9).…”

Transcriptional Orchestration of the Global Cellular Response of a Model Pennate Diatom to Diel Light Cycling under Iron Limitation

“…LHC transcripts were also downregulated in P . tricornutum when illuminated following a 48hr dark period [75]. This suggests that cellular signals generated upon illumination are involved in repressing LHC transcription.…”

Transcript level coordination of carbon pathways during silicon starvation‐induced lipid accumulation in the diatom Thalassiosira pseudonana

“…With the increasing availability of time‐course‐resolved transcriptomes, it is becoming more apparent that different organisms follow highly choreographed transcriptional programs by which large suites of genes are coordinated (Zinser et al ., 2009; Monnier et al ., 2010; Singh et al ., 2010; Ashworth et al ., 2013; Chauton et al ., 2012; Dyhrman et al ., 2012; Kanesaki et al ., 2012; Nymark et al ., 2013; Hennon et al ., 2015; Poliner et al ., 2015). In these cases, transcript level fluctuations are often correlated with light.…”