Acclimation of photosynthetic apparatus in the mesophilic red alga Dixoniella giordanoi

Abstract

Eukaryotic algae are photosynthetic organisms capable of exploiting sunlight to fix carbon dioxide into biomass with highly variable genetic and metabolic features. Information on algae metabolism from different species is inhomogeneous and, while green algae are, in general, more characterized, information on red algae is relatively scarce despite their relevant position in eukaryotic algae diversity. Within red algae, the best-known species are extremophiles or multicellular, while information on mesophilic unicellular organisms is still lacunose. Here, we investigate the photosynthetic properties of a recently isolated seawater unicellular mesophilic red alga, Dixoniella giordanoi. Upon exposure to different illuminations, D. giordanoi shows the ability to acclimate, modulate chlorophyll content, and re-organize thylakoid membranes. Phycobilisome content is also largely regulated, leading to almost complete disassembly of this antenna system in cells grown under intense illumination. Despite the absence of a light-induced xanthophyll cycle, cells accumulate zeaxanthin upon prolonged exposure to strong light, likely contributing to photoprotection. D. giordanoi cells show the ability to perform cyclic electron transport that is enhanced under strong illumination, likely contributing to the protection of Photosystem I from over-reduction and enabling cells to survive PSII photoinhibition without negative impact on growth.

FIGURE 1

Transmission electron microscopy of D. giordanoi cells. (A) Complete view of a cell. Specific features and organelles are marked as n, nucleus; g, Golgi apparatus; ch, chloroplast; st, starch. (B,C) show details of thylakoids and dictyosomes of the Golgi apparatus, respectively. The white arrows indicate phycobilisomes associated with the thylakoids

FIGURE 2

Composition of D. giordanoi photosynthetic apparatus. (A) Western blotting targeting different components of the photosynthetic apparatus from PSII (D2), PSI (PsaA), RuBisCO, and antenna complexes (LHCX1, VCP, and LHCII). Different dilutions of total cell extracts were loaded. The ×1 corresponds to 1 μg of Chl for each D. giordanoi sample, except for the case of PsaA, where 2 μg were loaded. As positive control (C+) total proteins extracted from the moss Physcomitrella patens were loaded for the targeting of D2, PsaA, RuBisCO, and LHCII. Extracts from Nannochloropsis gaditana were used as a positive control for LHCX1 and VCP. The dashed line indicates the removal of a lane not essential for the picture. (B) Absorption spectrum of isolated PBS fraction. Three absorption peaks are identified as phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (APC) from their absorption maximum

FIGURE 3

D. giordanoi growth under different light regimes. (A) Growth curve for D. giordanoi cells exposed for 4 days to LL, ML, and HL (20, 100, and 400 μmol of photons m⁻² s⁻¹), shown as black squares, red circles, and blue triangles, respectively. The fitting of experimental data is shown as dashed lines. Average ± sd are reported (n > 4). (B) Photosystem II quantum yield quantified from Fv/Fm, of D. giordanoi after 4 days of growth in LL, ML, and HL (black, red, and blue, respectively). a, b, and c indicate differences statistically significant from ML, HL, and LL, respectively (one‐way ANOVA, p < 0.05, n > 4, ±sd). Fv/Fm values after 24 h of dark recovery are represented with grey bars (n = 3, ±sd)

FIGURE 4

Acclimation of D. giordanoi cells to different light illumination. (A) Average PSII antenna size for all samples. Fluorescence measurements were taken with 10 million DCMU‐treated cells in the presence of 80 or 150 μmol of photons m⁻² s⁻¹ of actinic light at 630 nm. (B) Oxygen evolution activity of acclimated cells exposed to increasing light intensity. Measurements were taken with 1 million cells. For both the pictures, LL, ML, and HL cells are represented in black, red, and blue, respectively. Data are reported as the average of three biological replica ±sd

FIGURE 5

Ultrastructure of D. giordanoi cells acclimated to LL, ML, and HL. Panel (A) shows images of the whole cells. Specific features and organelles are marked as p, pyrenoid; ch, chloroplast lobes; st, starch. In (B) details of thylakoid membranes are shown. The white arrows indicate phycobilisomes associated with the thylakoids

FIGURE 6

Photosynthetic regulation of D. giordanoi to different light intensities. Chl fluorescent kinetics were used to calculate PSII quantum yield expressed as Y(II) of acclimated cells and illuminated with a progressively stronger light. LL, ML, and HL cells are represented in black, red, and blue, respectively. During the measurements, the far‐red light was switched on. Data are expressed as the average of three biological replica ±sd

FIGURE 7

Non photochemical quenching in D. giordanoi. (A) NPQ was measured exposing acclimated cells to increasing light intensity. During the measurements, the far‐red light was switched on. (B) NPQ of acclimated cells exposed to saturating pulses every minute with actinic light off (far‐red off). For both figures, LL, ML, and HL cells are represented in black, red, and blue, respectively. Data are shown as the average of three biological replica ±sd. (C) Fluorescence traces of LL cells illuminated by a first saturating pulse followed by a second one after 1, 5 10, 20, 40, 60, or 130 min (in dark yellow, magenta, cyan, blue, green, red, and black, respectively). (D) The graph reports the kinetics of NPQ relaxation after one saturating pulse. The fitting of experimental data is shown as a dotted line

FIGURE 8

Regulation of photosynthetic electron transport in D. giordanoi. (A) Representative P₇₀₀ redox kinetics from cells grown in ML. Cells acclimated to 100 μmol of photons m⁻² s⁻¹ (ML) were exposed to saturating light intensity (2050 μmol of photons m⁻² s⁻¹) for 15 s before the dark recovery kinetics. Reduction of P₇₀₀ ⁺ after switching light off is followed by monitoring differential absorption at 705 nm. Untreated cells, cells treated with DCMU, and cells treated with DCMU+DBMIB are shown as black squares, red circles, and blue triangles, respectively. (B) Rates of cyclic electron flow (CEF, white) and total electron flow (TEF, gray) were evaluated from P₇₀₀ ⁺ reduction kinetics after illumination in LL, ML, and HL acclimated cells. a, b, and c indicate differences statistically significant from ML, HL, and LL, respectively (one‐way ANOVA, p < 0.05, n = 3, ±sd)