Abstract
Phytoplankton, such as diatoms, experience great variations of photon flux density (PFD) and light spectrum along the marine water column. Diatoms have developed some rapidly-regulated photoprotective mechanisms, such as the xanthophyll cycle activation (XC) and the non-photochemical chlorophyll fluorescence quenching (NPQ), to protect themselves from photooxidative damages caused by excess PFD. In this study, we investigate the role of blue fluence rate in combination with red radiation in shaping photoacclimative and protective responses in the coastal diatom Pseudo-nitzschia multistriata. This diatom was acclimated to four spectral light conditions (blue, red, blue-red, blue-red-green), each of them provided with low and high PFD. Our results reveal that the increase in the XC pool size and the amplitude of NPQ is determined by the blue fluence rate experienced by cells, while cells require sensing red radiation to allow the development of these processes. Variations in the light spectrum and in the blue versus red radiation modulate either the photoprotective capacity, such as the activation of the diadinoxanthin-diatoxanthin xanthophyll cycle, the diadinoxanthin de-epoxidation rate and the capacity of non-photochemical quenching, or the pigment composition of this diatom. We propose that spectral composition of light has a key role on the ability of diatoms to finely balance light harvesting and photoprotective capacity.
Highlights
Originating some 2.3222.45 Gyr ago, oxygenic photosynthesis spread across the Earth, allowing the great diversification of life and globally altering the community structure and ecological function of terrestrial and aquatic habitats [1,2]
The unpredictable passing of clouds, motion of waves and turbulent mixing, superimposed to long-term diel and seasonal periodicity, create very complex patterns of short-term fluctuations in the instantaneously available light that controls phytoplankton photosynthesis [6]. In response to such a heterogeneous light environment, phytoplankton have evolved protective mechanisms to harvest light in conditions of excess detrimental photon flux density (PFD), and minimize photo-oxidative damage caused by the formation of reactive oxygen species in the photosystems [729]
The low synthesis of both Dd and Dt under blue high light condition (B-H) (Fig. 4B,C) is not related to a variation in light absorption, since the absorption coefficient (a*, Table 1) and photosynthetically usable radiation (PUR, Table 1) in B-H were similar to the values found in BRG-H, in which Dd and Dt were significantly produced
Summary
Originating some 2.3222.45 Gyr ago, oxygenic photosynthesis spread across the Earth, allowing the great diversification of life and globally altering the community structure and ecological function of terrestrial and aquatic habitats [1,2]. Phytoplankton, small floating photosynthetic microorganisms that populate the aquatic realms, thrive in a light environment naturally variable over spatial and temporal extremes [3,4]. The unpredictable passing of clouds, motion of waves and turbulent mixing, superimposed to long-term diel and seasonal periodicity, create very complex patterns of short-term fluctuations in the instantaneously available light that controls phytoplankton photosynthesis [6]. In response to such a heterogeneous light environment, phytoplankton have evolved protective mechanisms to harvest light in conditions of excess detrimental PFD, and minimize photo-oxidative damage caused by the formation of reactive oxygen species in the photosystems [729]. The xanthophyll cycle (XC) and non-photochemical quenching (NPQ) are crucial photoprotective processes that are rapidly activated (seconds to minutes) to dissipate excess absorbed light energy and ensure efficient light harvesting in the photosynthetic membrane [10212]
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