Chlorophylls are essential components of the photosynthetic apparati that sustain all of the life forms that ultimately depend on solar energy. However, a drawback of the extraordinary photosensitizing efficiency of certain chlorophyll species is their ability to generate harmful singlet oxygen. Recent studies have clarified the catabolic processes involved in the detoxification of chlorophylls in land plants, but little is understood about these strategies in aquatic ecosystem. Here, we report that a variety of heterotrophic protists accumulate the chlorophyll a catabolite 13 2 ,17 3 -cyclopheophorbide a enol (cPPB-aE) after their ingestion of algae. This chlorophyll derivative is nonfluorescent in solution, and its inability to generate singlet oxygen in vitro qualifies it as a detoxified catabolite of chlorophyll a. Using a modified analytical method, we show that cPPBaE is ubiquitous in aquatic environments, and it is often the major chlorophyll a derivative. Our findings suggest that cPPB-aE metabolism is one of the most important, widely distributed processes in aquatic ecosystems. Therefore, the herbivorous protists that convert chlorophyll a to cPPB-aE are suggested to play more significant roles in the modern oceanic carbon flux than was previously recognized, critically linking microscopic primary producers to the macroscopic food web and carbon sequestration in the ocean. phototoxicity of chlorophylls | microbial herbivory | phagocytosis | biodiversity of eukaryotes | microbial loop C hlorophylls are crucial to sustaining most life forms on Earth, the majority of which ultimately depend on solar energy. Photoexcitation of chlorophylls initiates various photosynthetic reactions that convert the energy of photons into chemical potentials, which in turn, drive the full range of metabolic reactions throughout the global ecosystem. Chlorophylls play a central role in the photosynthetic apparatus by absorbing light and transferring the excitation energy to the reaction centers of photosystems before photosynthetic electron transport. However, without measures to contain the excited energy, chlorophylls can harm organisms because of their high photosensitizing potential.

Chlorosomes are one of the most unique natural light-harvesting antennas and their supramolecular nanostructures are still under debate. Chlorosomes contain bacteriochlorophyll (BChl)-c, d and e molecules and these pigments self-aggregate under a hydrophobic environment inside a chlorosome. The self-aggregates are mainly constructed by the following three interactions: hydrogen bonding, coordination bonding and π-π stacking. Supramolecular nanostructures of self-aggregated BChls have been widely investigated by spectroscopic and microscopic techniques. Model compounds of such chlorosomal BChl molecules have been synthesized and the effects of esterified long alkyl chains at the 17-propionate residue for their self-aggregation have been studied. Structurally simple zinc chlorophyll derivatives possessing an oligomethylene chain as the esterifying group at the 17-propionate residue were prepared as chlorosomal BChl models. The synthetic zinc BChls self-aggregated in nonpolar organic solvents to give precipitates. The resulting insoluble self-aggregated solids were investigated on a variety of substrates, including hydrophobic, neutral and hydrophilic substrates, by visible absorption, circular dichroism and polarized light absorption spectroscopies, as well as atomic force, transmission electron and scanning electron microscopies. The self-aggregates of synthetic Zn-BChls formed rods with an approximately 5 nm diameter and wires with further elongated growth of the rods (aspect ratio >200). The diameter size was consistent with that estimated for natural chlorosomal rods in a filamentous anoxygenic phototroph, Chloroflexus aurantiacus. The supramolecular formation and stability of the rod on the examined substrates depended on the length of an oligomethylene chain at the 17-propionate residue as well as on the surface properties. Especially, the number of the 5 nm rods on the substrates increased with an elongation of the chain.

Various supramolecular nanotubes have recently been built up by lipids, peptides, and other organic molecules. Major light-harvesting (LH) antenna systems in a filamentous anoxygenic phototroph, Chloroflexus (Cfl.) aurantiacus, are called chlorosomes and contain photofunctional single-wall supramolecular nanotubes with approximately 5 nm in their diameter. Chlorosomal supramolecular nanotubes of Cfl. aurantiacus are constructed by a large amount of bacteriochlorophyll(BChl)-c molecules. Such a pigment self-assembles in a chlorosome without any assistance from the peptides, which is in sharp contrast to the other natural photosynthetic LH antennas. To mimic chlorosomal supramolecular nanotubes, synthetic models were prepared by the modification of naturally occurring chlorophyll(Chl)-a molecule. Metal complexes (magnesium, zinc, and cadmium) of the Chl derivative were synthesized as models of natural chlorosomal BChls. These metal Chl derivatives self-assembled in hydrophobic environments, and their supramolecules were analyzed by spectroscopic and microscopic techniques. Cryo-transmission electron microscopic images showed that the zinc and cadmium Chl derivatives could form single-wall supramolecular nanotubes and their outer and inner diameters were approximately 5 and 3 nm, respectively. Atomic force microscopic images suggested that the magnesium Chl derivative formed similar nanotubes to those of the corresponding zinc and cadmium complexes. Three chlorosomal single-wall supramolecular nanotubes of the metal Chl derivatives were prepared in the solid state and would be useful as photofunctional materials.

The stimulation of photosynthesis is a strategy for achieving sustainable plant production. Red light is useful for plant growth because it is absorbed by chlorophyll pigments, which initiate natural photosynthetic processes. Ultraviolet (UV)-to-red wavelength-converting materials are promising candidates for eco-friendly plant cultures that do not require electric power. In this study, transparent films equipped with a UV-to-red wavelength-converting luminophore, the Eu3+ complex, were prepared on commercially available plastic films for plant growth experiments. The present Eu3+-based films absorb UV light and exhibit strong red luminescence under sunlight. Eu3+-painted films provide significant growth acceleration with size increment and biomass production for vegetal crops and trees in a northern region. The plants cultured with Eu3+-painted films had a 1.2-fold height and 1.4-fold total body biomass than those cultures without the Eu3+ luminophores. The present film can promote the plant production in fields of agriculture and forestry.

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