Abstract
Photosynthesis uses light as a source of energy but its excess can result in production of harmful oxygen radicals. To avoid any resulting damage, phototrophic organisms can employ a process known as non-photochemical quenching (NPQ), where excess light energy is safely dissipated as heat. The mechanism(s) of NPQ vary among different phototrophs. Here, we describe a new type of NPQ in the organism Rhodomonas salina, an alga belonging to the cryptophytes, part of the chromalveolate supergroup. Cryptophytes are exceptional among photosynthetic chromalveolates as they use both chlorophyll a/c proteins and phycobiliproteins for light harvesting. All our data demonstrates that NPQ in cryptophytes differs significantly from other chromalveolates – e.g. diatoms and it is also unique in comparison to NPQ in green algae and in higher plants: (1) there is no light induced xanthophyll cycle; (2) NPQ resembles the fast and flexible energetic quenching (qE) of higher plants, including its fast recovery; (3) a direct antennae protonation is involved in NPQ, similar to that found in higher plants. Further, fluorescence spectroscopy and biochemical characterization of isolated photosynthetic complexes suggest that NPQ in R. salina occurs in the chlorophyll a/c antennae but not in phycobiliproteins. All these results demonstrate that NPQ in cryptophytes represents a novel class of effective and flexible non-photochemical quenching.
Highlights
Photosynthetic organisms are often exposed to varying environmental conditions, such as excessive irradiation
In order to elucidate the basic characteristics of non-photochemical quenching (NPQ) in R. salina we measured chlorophyll a fluorescence quenching analysis by using orange actinic light preferentially absorbed by the chlorophyll a/c antennae (Figure 1)
Xanthophyll cycle is not involved in NPQ in R. salina We have explored possible changes in pigment composition following the long-term light stress to ascertain if R. salina is able to transform xanthophylls under excessive irradiation
Summary
Photosynthetic organisms are often exposed to varying environmental conditions, such as excessive irradiation. Non-photochemical quenching (NPQ), is a feed-back regulatory mechanism in which excessive light irradiation is dissipated as heat (reviewed in [3,4]). The process of energy dissipation in NPQ is triggered by low pH in the thylakoid lumen and modulated by several factors including zeaxanthin [8] and the PsbS protein [9,10]. The fast energetic quenching (qE) is triggered by lumen acidification, that is further stimulated under prolonged irradiation by zeaxanthin [15]. Long-term exposition (hours and days) to excessive irradiation results in photoinhibitory quenching - qI which exact mechanism is still matter of debate [4,16]
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