When grown under concomitant limiting light regimes (~20 ?mol phot m-2 sec-1) and shading conditions leading to an enrichment in near-infrared radiation in the incident light spectrum, the red algae Cromera velia and Pheodactylum tricornutum show an intriguing adaptive strategy associated to the synthesis of specific antenna isoforms. These harbour moderately red-shifted Chlorophyll emission forms that have a maximal emission centred at about 710-715 nm at room temperature, which is clearly discernible from the principal emission form of cultures grown under standard (non-shaded) conditions that display a maximal emission at about 684 nm. The 684 nm/710 nm emission peaks ratio decreases with increasing culture density and the subsequent near-infrared enrichment in the culture growth light, due to self-shading. In very dense cultures, or in cultures grown under enriched far-red lights, the 715 nm emission might become the dominant emission form. At room temperature, the yield of emission of the 715 nm emission form and the observation that it displays a sizable variable fluorescence yield upon PSII trap closure, indicates that, contrary to the most extensively studied long-wavelength emission forms of green algae, plants and cyanobacteria that are known to be located in PSI, those observed in C. velia and P. tricornutum grown under shading conditions are, in part or primarily, associated to PSII. In order to acquire further insight into the physiological role of PSII-associated red chlorophyll forms in these organisms, comparative studies of the fluorescence emission characteristics in the steady state, and dynamics in the picosecond time domain, have been undertaken on cultures accumulating different levels of red-form-to-bulk emission. It is shown that, under conditions approaching PSII open centres (F0'), when red-forms are present in PSII antenna, the average fluorescence lifetime of the cells increases progressively towards the long wavelength emission edge. The extent of this variation (which can range from 100-200 ps in the 660-690 nm window to 300-400 ps in the 700-750 nm window) depends on the extent of red-form accumulation. This process resembles previous findings in the PSI, interpreted in terms of a partial kinetic bottleneck for energy diffusion, due to unfavourable energy transfer from these antenna forms, to the photochemical trap (i.e. a transfer-to-the-trap limitation). In the case of P. tricornutum however, a similar average lifetime increase is also observed under PSII closed trap conditions (FM), indicating that some energy diffusion limitation might occur even in the absence of photochemical quenching, and that energy transfer from the 715 nm form to the bulk antenna might therefore be relatively slow.
Impact of long-wavelength chlorophyll forms in PSII antennae of Cromera velia and Pheodactylum tricornutum on the photochemical quantum efficiency
Anna Paola Casazza;
2019
Abstract
When grown under concomitant limiting light regimes (~20 ?mol phot m-2 sec-1) and shading conditions leading to an enrichment in near-infrared radiation in the incident light spectrum, the red algae Cromera velia and Pheodactylum tricornutum show an intriguing adaptive strategy associated to the synthesis of specific antenna isoforms. These harbour moderately red-shifted Chlorophyll emission forms that have a maximal emission centred at about 710-715 nm at room temperature, which is clearly discernible from the principal emission form of cultures grown under standard (non-shaded) conditions that display a maximal emission at about 684 nm. The 684 nm/710 nm emission peaks ratio decreases with increasing culture density and the subsequent near-infrared enrichment in the culture growth light, due to self-shading. In very dense cultures, or in cultures grown under enriched far-red lights, the 715 nm emission might become the dominant emission form. At room temperature, the yield of emission of the 715 nm emission form and the observation that it displays a sizable variable fluorescence yield upon PSII trap closure, indicates that, contrary to the most extensively studied long-wavelength emission forms of green algae, plants and cyanobacteria that are known to be located in PSI, those observed in C. velia and P. tricornutum grown under shading conditions are, in part or primarily, associated to PSII. In order to acquire further insight into the physiological role of PSII-associated red chlorophyll forms in these organisms, comparative studies of the fluorescence emission characteristics in the steady state, and dynamics in the picosecond time domain, have been undertaken on cultures accumulating different levels of red-form-to-bulk emission. It is shown that, under conditions approaching PSII open centres (F0'), when red-forms are present in PSII antenna, the average fluorescence lifetime of the cells increases progressively towards the long wavelength emission edge. The extent of this variation (which can range from 100-200 ps in the 660-690 nm window to 300-400 ps in the 700-750 nm window) depends on the extent of red-form accumulation. This process resembles previous findings in the PSI, interpreted in terms of a partial kinetic bottleneck for energy diffusion, due to unfavourable energy transfer from these antenna forms, to the photochemical trap (i.e. a transfer-to-the-trap limitation). In the case of P. tricornutum however, a similar average lifetime increase is also observed under PSII closed trap conditions (FM), indicating that some energy diffusion limitation might occur even in the absence of photochemical quenching, and that energy transfer from the 715 nm form to the bulk antenna might therefore be relatively slow.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
