Abstract
ABSTRACTThis study characterizes the photoacclimation and photoregulation mechanisms that allow calcified macroalgae of the genus Corallina (Corallinales, Rhodophyta) to dominate rock pool habitats across the NE Atlantic despite the highly variable irradiance regimes experienced. Rapid light curves (RLCs) were performed with pulse amplitude modulation (PAM) fluorometry in situ across a full seasonal cycle in the UK intertidal with C. officinalis and C. caespitosa. Latitudinal comparisons were performed across the full extent of C. officinalis’ range in the NE Atlantic (Iceland–northern Spain), and for C. caespitosa in northern Spain. Ex situ RLCs with dark recovery were further employed to assess the optimal, as compared with actual, photophysiology across seasons and latitudes. Corallina species were shown to photoacclimate at seasonal timescales to changing irradiance, increasing light-harvesting during low-light autumn/winter periods and protecting photosystems during high-light summer conditions. Seasonal photoacclimation was achieved through alteration in the number of photosystem (PS) units (PSII and light harvesting antennae) over time. Non-photochemical quenching (NPQ) served as an important photoregulation mechanism utilized by Corallina to prevent or minimize photoinhibition over shorter time scales (seconds–hours), though the efficiency of NPQ was dependent on the seasonal-acclimated state. With increasing latitude the efficiency of photoregulation decreased, representing potential differential photoadaptation of Corallina across species ranges in the NE Atlantic. In contrast, highly conserved inter-specific patterns in photophysiological responses to irradiance were apparent. This study demonstrates the photophysiological mechanisms allowing Corallina to optimize use of the variable irradiance conditions apparent in rock pool environments, when and how they are employed, and their limitations.
Highlights
Irradiance is an essential, yet highly variable, resource for macroalgal growth and survival (Henley & Ramus, 1989)
This study addresses the photophysiology of two intertidal macroalgae of the genus Corallina (Corallinales, Rhodophyta) across the NE Atlantic, namely Corallina officinalis Linnaeus and Corallina caespitosa Walker, Brodie & Irvine
The aim of this study was to build on the initial work of Williamson et al (2014b) in order to identify the suite of photoacclimation and photoregulation mechanisms that allow Corallina to optimize light use in the variable intertidal environment, contributing to their dominance of NE Atlantic rock pools, and to characterize the use of these mechanisms in space and time
Summary
Irradiance is an essential, yet highly variable, resource for macroalgal growth and survival (Henley & Ramus, 1989). For a benthic macroalga in a fixed position in the intertidal zone, the challenge is to optimize the use of the variable irradiance regime experienced (Henley & Ramus, 1989). To complicate this further, the quantity of photosynthetically active radiation The photochemical apparatus operates to optimize photosynthesis at low light levels associated with immersion, with the result that emersed plants are exposed to a large excess of light energy (Davison & Pearson, 1996). Intertidal macroalgae must respond to changes in irradiance in a manner that both optimizes photosynthesis and growth, whilst controlling for potential stress (Müller et al, 2001)
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