Impact of long-wavelength chlorophyll forms in PSII antennae of Chromera velia and Pheodactylum tricornutum on the photochemical quantum efficiency

Grown under limiting light regimes (~20 ?E m-2 sec-1) and shading conditions leading to a light spectrum enriched in near-infrared (NIR), the red algae C. velia and P. tricornutum show an intriguing adaptive strategy associated to the synthesis of specific antenna isoforms. These harbour moderately red-shifted Chlorophyll forms having maximal emission at ~710-715 nm at room temperature (RT), and are clearly discernible from the principal emission form of cells grown under non-shaded conditions that is maximal at ~684 nm [1; 2]. The 684/710 nm peaks ratio decreases with increasing cell density and the subsequent NIR enrichment in growth lights, due to self-shading [3]. In very dense cultures, or when grown in far-red lights, the 715 nm emission might become dominant [3]. At RT, the yield of emission of the 715 nm emission form and its observed variable fluorescence yield upon PSII trap closure, indicates that, contrary to the extensively studied red emission forms of green algae, plants and cyanobacteria located in PSI, those of C. velia and P. tricornutum grown under shading conditions are, in part or primarily, associated to PSII [3, 4]. In order to acquire insight into the physiological role of PSIIassociated red 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 cells with 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 (?av) of the cells increases progressively towards the long wavelength emission edge. The extent of this variation (which ranges from 100-200 ps in the 660-690 nm range to 300-400 ps in the 700-750 nm one) depends on the extent of red-form accumulation. This process resembles previous findings in the PSI, interpreted as a partial kinetic bottleneck for energy diffusion due to unfavourable energy transfer from these forms to the photochemical trap (i.e. transfer-to-the-trap limitation). In P. tricornutum, however, a similar ?av 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 bulk might therefore be relatively slow.