Explore how blue light impacts cyanobacteria by reducing their photosynthetic efficiency and what this means for aquatic ecosystems.
Almost 140 years ago, Professor Theodor Engelmann demonstrated that spectral quality is a critical determinant of photosynthetic efficiency (Engelmann 1882). In his landmark experiment, Engelmann mounted a filamentous green alga of the genus Cladophora on a microscope slide and projected dispersed sunlight—via a prism—across the filament, thereby generating a spatially resolved spectrum. By adding aerotactic bacteria and mapping their accumulations, he visualized localized photosynthetic O2 evolution and produced the first “living” action spectrum, revealing maximal chlorophyll activity in red and blue light.
Subsequent investigations with cyanobacteria of the genus Oscillatoria extended these findings, showing elevated O2 evolution under red, blue, and, notably, orange excitation (Engelmann 1883, 1884). Although initially contested, Engelmann’s conclusions were vindicated six decades later when Emerson and Lewis identified phycobiliproteins as the accessory pigments that enable cyanobacteria and red algae to harvest green-to-orange photons efficiently (Emerson and Lewis 1942). These water-soluble chromoproteins assemble into supramolecular antennae termed phycobilisomes (PBSs), comprising an allophycocyanin core and peripheral rods of phycocyanin, often terminated with phycoerythrin. Each phycobiliprotein carries covalently attached linear tetrapyrrole chromophores (bilins): phycocyanobilin (λmax ≈ 620 nm), phycoerythrobilin (λmax ≈ 545 nm), and phycourobilin (λmax ≈ 495 nm) (Grossman et al. 1993; Tandeau de Marsac 2003; Six et al. 2007). Contemporary structural and functional analyses of PBSs are comprehensively reviewed by Tamary et al. (2012), Watanabe and Ikeuchi (2013), and Stadnichuk and Tropin (2017).
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Light energy captured by phycobilisomes (PBSs) is efficiently funneled through allophycocyanin to the chlorophyll a (Chl a) pigments embedded in the photosystems (Arnold and Oppenheimer 1950; Duysens 1951; Lemasson et al. 1973). Although PBSs were long thought to deliver the bulk of their excitation energy to photosystem II (PSII), it is now firmly established that cyanobacteria dynamically redistribute PBSs between PSII and photosystem I (PSI) via state transitions, thereby re-equilibrating photochemical excitation (van Thor et al. 1998; Mullineaux 2008). These reversible associations—completed within seconds to minutes—enable PBSs to couple with PSII (state 1) or PSI (state 2) and to transfer absorbed photons to the reaction center with which they are physically docked (Kirilovsky 2015). Over longer time scales, cyanobacteria further optimize photon utilization by remodeling the stoichiometric ratio of PSI to PSII in response to spectral and intensity cues (Fujita 1997). Consequently, the PSI:PSII ratio in cyanobacteria fluctuates between 5:1 and 2:1—substantially higher than the approximately 1:1 ratio typical of eukaryotic phototrophs—thereby maximizing photosynthetic efficiency under fluctuating environmental conditions (Shen et al. 1993; Murakami et al. 1997; Singh et al. 2009; Allahverdiyeva et al. 2014; Kirilovsky 2015).
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Multiple independent studies have reported that cyanobacteria harvest blue photons less efficiently for photosynthesis than most eukaryotic phototrophs, yet a mechanistic, wavelength-resolved analysis has been missing. Here we quantify the impact of blue (450 nm), orange (625 nm), and red (660 nm) irradiation on growth and photochemistry of the model cyanobacterium Synechocystis sp. PCC 6803, the chlorophyte Chlorella sorokiniana, and additional phycocyanin- or phycoerythrin-rich cyanobacteria. Cyanobacterial specific growth rates were statistically indistinguishable under orange and red light but declined markedly under blue excitation. Conversely, C. sorokiniana grew equally well under blue and red light yet more slowly under orange wavelengths. In Synechocystis sp. PCC 6803, oxygen evolution at low photon flux was five-fold lower with blue than with orange or red light; at saturating intensities, however, evolution rates converged across all three spectral bands. Low-temperature (77 K) chlorophyll fluorescence revealed a reduced photosystem I/photosystem II stoichiometry (PSI:PSII) and a higher proportion of phycobilisomes energetically coupled to PSII (state 1) under blue light relative to orange or red excitation. These spectroscopic signatures support a model in which blue photons—poorly absorbed by phycobilisomes—generate an energetic imbalance that over-energizes PSI while under-energizing PSII, thereby limiting linear electron flow. Our findings clarify why phycobilisome-containing cyanobacteria are spectrally disadvantaged in blue-enriched environments compared with organisms that rely on chlorophyll-based antenna systems such as Prochlorococcus, green algae, and terrestrial plants.
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Although chlorophyll a absorbs blue and red photons with comparable efficiency, and phycobiliproteins harvest the intermediate wavelengths, photochemical performance is not identical across the spectrum. Consistent evidence shows that blue light drives significantly lower oxygen-evolution rates than red light in cyanobacteria (Lemasson et al. 1973; Pulich and van Baalen 1974; Jørgensen et al. 1987; Tyystjärvi et al. 2002), in cyanolichens (Solhaug et al. 2014), and in red algae that retain phycobilisomes (Ley and Butler 1980; Figueroa et al. 1995). Parallel work demonstrates that blue-enriched illumination suppresses growth in diverse cyanobacterial strains (Wyman and Fay 1986), including Synechocystis sp. PCC 6803 (Wilde et al. 1997; Singh et al. 2009; Bland and Angenent 2016), Synechococcus sp. (Choi et al. 2013), and Spirulina platensis (Wang et al. 2007; Chen et al. 2010).
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A plausible explanation for the diminished performance of cyanobacteria under blue light is that the bulk of their chlorophyll is housed within photosystem I (Myers et al. 1980; Fujita 1997; Solhaug et al. 2014; Kirilovsky 2015). Consequently, blue light preferentially excites PSI while leaving PSII under-excited, skewing the photosystem balance. This imbalance is corroborated by chlorophyll-fluorescence studies in which blue excitation light distorts the signal, complicating interpretation of cyanobacterial photophysiology (Campbell et al. 1998; Ogawa et al. 2017). Although prior work has documented growth rates or pigment profiles under monochromatic light, critical parameters such as O₂ evolution, PSI:PSII stoichiometry, and state transitions remain sparsely reported. Conversely, investigations quantifying O₂ production or photosystem ratios seldom integrate growth or pigment data. Thus, integrative studies delineating the full photophysiological response of cyanobacteria to blue light are still scarce, and a consensus on why blue light constrains their photosynthetic efficiency has yet to emerge.
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