However, the effective light harvesting mechanism adopted by these algae remains undefined and it is not known how phycobilisome energy transfer cascades respond to environmental change, limiting our ability to explain the mesophotic success of red coralline algae. The mesophotic success of red coralline algae has been attributed to their low growth rates coupled with the light harvesting advantages posed by phycobilisomes. These extra light harvesting molecules effectively increase the bandwidth of photosynthetically available light energy, particularly in the mid-range of the visible light spectrum - which becomes increasingly dominant at increasing water depth. Phycobilisomes consist of phycoerythrin (PE), phycocyanin (PC), allophycocyanin (APC), and linker proteins, precisely arranged in an energy absorption cascade towards subsequent energy transfer to chlorophyll- a in photosystems I and II (PSI and PSII). As with other red algae and cyanobacteria, red coralline algae harbour multi-pigment light harvesting complexes in addition to chlorophyll- a. Red coralline algae are the deepest known habitat-forming algae, found even at 270 + m depth and at irradiance levels less than 0.001% of those found at the water surface. Tolerance to a mesophotic light regime therefore poses a significant competitive advantage for increasing an organism’s distributional extent, but the physiological mechanism for how this tolerance is achieved remains unclear. To overcome the limitations of reduced irradiance, algae are known to increase their photosynthetic efficiency and reduce their minimum light requirements (e.g. Light availability is therefore the primary limiting factor in the maximum depth distribution of macroalgae, which in turn negatively impacts the ecological and biogeochemical benefits they can provide. lower growth rates), understanding the bottom-up mechanisms that drive an organism’s chemical energy supply is a first and crucial step in understanding its presence at extreme depth. Although organismal energy requirements may be reduced in the mesophotic (e.g. Mesophotic photosynthetic organisms therefore have to contend with both reduced light intensity and bandwidth compared to their shallow-water counterparts to meet their energy requirements. Light attenuation in the oceans with depth is characterised by both a decline in the intensity of light available for photosynthesis (photosynthetic active radiation, 400–700 nm) and a narrowing of the spectral composition - resulting in a blue dominance in oligotrophic oceanic waters. Despite low light levels, these organisms create complex habitats of significant ecological importance, supporting high biodiversity and efficient biogeochemical cycling. Recent developments in ocean exploration have begun to reveal the extent of mesophotic benthic photosynthetic organisms (such as corals and algae). The mesophotic zone − arbitrarily defined as > 30 m water depth and/or < 1% surface light intensity to the lower limit of light penetration in the oceans − is a critical ‘transition’ zone between the ocean surface and the deep sea. Our results demonstrate that responsive light harvesting by phycobilisomes and photosystem functional acclimation are key to red algal success in the mesophotic zone. The rate of energy transfer remained consistent across experimental treatments, indicating an acclimatory response that maintains energy transfer. Low light intensity, and to a lesser extent a mesophotic spectrum, caused significant acclimatory change in chromophores and biliproteins, including a 10% increase in phycoerythrin light harvesting capacity and a 20% reduction in chlorophyll- a concentration and photon requirements for photosystems I and II. This enabled delivery of 94% of excitations to reaction centres. Resultsįollowing initial absorption by phycourobilin and phycoerythrobilin in phycoerythrin, energy was transferred from the phycobilisome to photosystems I and II within 120 ps. Red coralline algae are the deepest known marine benthic macroalgae - here we investigated the light harvesting mechanism and mesophotic acclimatory response of the red coralline alga Lithothamnion glaciale. Despite a global prevalence of photosynthetic organisms in the ocean’s mesophotic zone (30–200+ m depth), the mechanisms that enable photosynthesis to proceed in this low light environment are poorly defined.
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