The Effect of Photoinhibition on Photosynthetic Oxygen Production in the Brown AlgaDictyota dichotoma

Synergistic effects of temperature and light affect the relationship between Taonia atomaria and its epibacterial community: a controlled conditions study

In the context of global warming, this study aimed to assess the effect of temperature and irradiance on the macroalgal Taonia atomaria holobiont dynamics. We developed an experimental set-up using aquaria supplied by natural seawater with three temperatures combined with three irradiances. The holobiont response was monitored over 14 days using a multi-omics approach coupling algal surface metabolomics and metabarcoding. Both temperature and irradiance appeared to shape the microbiota and the surface metabolome, but with a distinct temporality. Epibacterial community first changed according to temperature, and later in relation to irradiance, while the opposite occurred for the surface metabolome. An increased temperature revealed a decreasing richness of the epiphytic community together with an increase of several bacterial taxa. Irradiance changes appeared to quickly impact surface metabolites production linked with the algal host photosynthesis (e.g. mannitol, fucoxanthin, dimethylsulfoniopropionate), which was hypothesized to explain modifications of the structure of the epiphytic community. Algal host may also directly adapt its surface metabolome to changing temperature with time (e.g. lipids content) and also in response to changing microbiota (e.g. chemical defences). Finally, this study brought new insights highlighting complex direct and indirect responses of seaweeds and their associated microbiota under changing environments.

Antarctic intertidal macroalgae under predicted increased temperatures mediated by global climate change: Would they cope?

The Antarctic Peninsula is one of the regions to be most affected by increase in sea surface temperatures (SSTs) mediated by Global Climate Change; indeed, most negative predictions imply an up to 6 °C increment by the end of the XXI century. Temperature is one of the most important factors mediating diversity and distribution of macroalgae, although there is still no consensus as to the likely effects of higher SSTs, especially for polar seaweeds. Some available information suggests that potential strategies to withstand future increases in SSTs will be founded upon the glutathione-ascorbate cycle and the induction of chaperone-functioning heat shock proteins (HSPs); however, their eventual role, even for general stress responses, is unclear. The intertidal green, brown and red macroalgae species Monostroma hariotii, Adenocystis utricularis and Pyropia endiviifolia, respectively, from King George Island, Antarctic Peninsula, were exposed to 2 °C (control) and 8 °C (climate change scenario) for up to 5 days (d). Photosynthetic activity (α_ETR and ETR_max, and Ek_ETR), photoinhibition (F_v/F_m) and photoprotection processes (α_NPQ, NPQ_max, and Ek_NPQ) provided no evidence of negative ecophysiological effects. There were moderate increases in H₂O₂ production and levels of lipid peroxidation with temperature, results supported by stable levels of total glutathione and ascorbate pools, with mostly higher levels of reduced ascorbate and glutathione than oxidized forms in all species. Transcripts of P. endiviifolia indicated a general upregulation of all antioxidant enzymes and HSPs genes studied under warmer temperature, although with different levels of activation with time. This pioneering investigation exploring different levels of biological organization, suggested that Antarctic intertidal macroalgae may be able to withstand future rise in SSTs, probably slightly altering their latitudinal distribution and/or range of thermal tolerance, by exhibiting robust glutathione-ascorbate production and recycling, as well as the induction of associated antioxidant enzymatic machinery and the syntheses of HSPs.

Interactions between seagrasses and seaweeds during surge nitrogen acquisition determine interspecific competition

Seagrasses dominate shallow coastal environments where nitrogen (N) availability in the water column is often sporadic and mainly in the form of pulses. We investigated the N uptake competition between seagrasses and seaweeds through a series of 15N surge uptake experiments combining single-species and mixed incubations across ammonium concentrations. N surge uptake rates of seagrasses were 2 to 14-fold higher than those of seaweeds in the majority of combinations, showing that seagrasses are generally in a competitive advantage over seaweeds in N-poor environments with N-pulses. No threshold concentration of ammonium was found beyond which seaweeds performed better than seagrasses. Mixed incubations revealed interspecific interactions that affected rates positively and negatively. Uptake rates obtained in single-species incubations, therefore, cannot always be used to predict the outcome of uptake competition. Only two (Zostera marina vs. Ulva rotundata and Zostera marina vs. Codium decorticatum) of the nine combinations tested (Z. marina, Z. noltei and Cymodocea nodosa vs. U. rotundata, C. decorticatum and Dictyota dichotoma) were found to enhance macroalgal uptake. Our results showed that the surge uptake capacity of seagrasses represents an important mechanism in their N acquisition strategy that justifies their dominance in shallow oligotrophic environments.

Sensitivity of photosynthesis to UV radiation in several Cosmarium strains (Zygnematophyceae, Streptophyta) is related to their geographical distribution

The application of UV radiation in vitro on desmid strains collected from various climatic areas and long-term grown under identical laboratory conditions revealed their preference for specific climatic niches, as judged from their different photosynthetic behaviours.

Protection strategies of Cosmarium strains (Zygnematophyceae, Streptophyta) isolated from various geographic regions against excessive photosynthetically active radiation

Numerous in vitro investigations have suggested that macroalgae exhibit regular geographic and depth distribution patterns in accordance with the light and temperature predominance at their habitats; however, there have been only a few similar studies concerning microalgae. We examined the potential influence of irradiance on patterns of distribution of four Cosmarium strains isolated from various climatic zones and cultured long term (>15 years) under a constant temperature-light regime. All the Cosmarium strains demonstrated physiological responses that were consistent with the light intensity prevailing at their source location, confirming that these responses are genetically preserved, as concluded from chlorophyll fluorescence and oxygen evolution rates measurements. Addition of inhibitors of chloroplast-encoded protein synthesis (chloramphenicol and streptomycin) and violaxanthin de-epoxidase (dithiothreitol) indicated that the Cosmarium strains developed "sun- or shade-plant" protection strategies, in accordance with the climate at their sampling sites. The polar Cosmarium strains exhibited a "shade-plant strategy"-to suffer some photoinhibition, but acquire increasing protection from photoinhibited PSII centers, whereas the tropical strains displayed a "sun-plant strategy"-to counteract photoinhibition of PSII by a high rate of repair of photoinhibited PSII reaction centers and a high xanthophyll cycle turnover.

Growth and photosynthetic responses of Picea asperata seedlings to enhanced ultraviolet-B and to nitrogen supply

Ultraviolet-B (UV-B) radiation and nitrogen are expected to increase simultaneously with future changes in global climate. In this study, growth and photosynthetic responses of Picea asperata seedlings to enhanced UV-B and to nitrogen supply were studied. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 kJ m-2 d-1; enhanced UV-B, 14.33 kJ m-2 d-1) and two nitrogen levels (0; 20 g N m-2) to determine whether nitrogen can alleviate the negative impacts of enhanced UV-B on seedling growth and photosynthesis. Enhanced UV-B significantly inhibited plant growth and impaired net photosynthetic rate, stomatal conductance, transpiration rate, the light-saturated assimilation rate, assimilation capacity, light compensation point, dark respiration rate, apparent quantum yield, photosynthetic pigments and maximum quantum yield of photosynthesis of P. asperata seedlings, whereas minimal fluorescence and intercellular CO2 concentration increased by enhanced UV-B. On the other hand, nitrogen supply improved the photosynthetic performance and plant growth, but only under ambient UV-B. In fact, nitrogen supply could not alleviate the photosynthetic impairments in P. asperata seedlings exposed to enhanced UV-B radiation.

Reduction of UV-B radiation causes an enhancement of photoinhibition in high light stressed aquatic plants from New Zealand lakes

Anthropogenic stratospheric ozone depletion causes an increase of UV-B radiation impinging on the earth surface, which is a threat to plants not adapted to higher UV-B irradiances. Investigations were undertaken with aquatic plants from New Zealand, where UV-irradiances are naturally higher due to the southern latitude, to compare with former results of polar species. The experiments reported in this study were undertaken with plants collected from different lakes of the South Island, with different UV transparencies. Photoinhibition was induced under controlled conditions using a sun simulator, which mimicked the natural underwater radiation spectrum. Photosynthetic activity during high light stress, and during recovery in dim light, was determined in vivo by measuring fluorescence changes, using a PAM fluorometer device. A comparison of different species showed that the extent to which UV causes an additional decrease of photosynthetic performance during high light stress varies according to the depth of growth and UV transparency of the water body. This observation fits with previous studies. However, a new finding was that some species were even more strongly inhibited when UV-B was filtered out of the simulated sun spectrum, indicating a supporting effect of the short UVR wavelength range against photoinhibition. These results were also confirmed by field experiments under natural radiation conditions. Thus, UV-B does not solely cause negative effects on photosynthesis, but it may even support recovery processes in aquatic plants adapted to a high UV-radiation environment. The latter is in contrast to earlier studies, in which UV-B radiation was considered causing only harmful effects on photosynthesis of aquatic plants.