Photoacclimation and Light Thresholds for Cold Temperate Seagrasses

Abstract

Water quality deterioration is expected to worsen the light conditions in shallow coastal waters with increasing human activities. Temperate seagrasses are known to tolerate a highly fluctuating light environment. However, depending on their ability to adjust to some decline in light conditions, decreases in daily light quantity and quality could affect seagrass physiology, productivity, and, eventually, survival if the Minimum Quantum Requirements (MQR) are not reached. To better understand if, how, and to what extent photosynthetic adjustments contribute to light acclimation, eelgrass (Zostera marina L.) shoots from the cold temperate St. Lawrence marine estuary (Rimouski, QC, Canada) were exposed to seven light intensity treatments (6, 36, 74, 133, 355, 503, and 860 μmol photons m–2 s–1, 14:10 light:dark photoperiod). Photosynthetic capacity and efficiency were quantified after five and 25 days of light exposure by Pulse Amplitude Modulated (PAM) fluorometry to assess the rapid response of the photosynthetic apparatus and its acclimation potential. Photoacclimation was also studied through physiological responses of leaves and shoots (gross and net primary production, pigment content, and light absorption). Shoots showed proof of photosynthetic adjustments at irradiances below 200 μmol photons m–2 s–1, which was identified as the threshold between limiting and saturating irradiances. Rapid Light Curves (RLC) and net primary production (NPP) rates revealed sustained maximal photosynthetic rates from the highest light treatments down to 74 μmol photons m–2 s–1, while a compensation point (NPP = 0) of 13.7 μmol photons m–2 s–1 was identified. In addition, an important package effect was observed, since an almost three-fold increase in chlorophyll content in the lowest compared to the highest light treatment did not change the leaves’ light absorption. These results shed new light on photosynthetic and physiological processes, triggering light acclimation in cold temperate eelgrass. Our study documents an MQR value for eelgrass in the St. Lawrence estuary, which is highly pertinent in the context of conservation and restoration of eelgrass meadows.

Limited recovery following a massive seagrass decline in subarctic eastern Canada

“…Eelgrass ( Zostera marina ) is the most widely distributed seagrass species in the Northern Hemisphere and is found in the arctic and subarctic coastal regions (Assis et al, 2022; Short et al, 2007). This species has high light requirements (Léger‐Daigle et al, 2022; Wong et al, 2021), grows better in oligotrophic oxygenated waters (Zharova et al, 2001), and can adapt to a variety of conditions, with meadows being annual, perennial, or a combination of both (Murphy et al, 2021; O'Brien et al, 2018). The recovery capacity of eelgrass is higher than persistent seagrass species (e.g., the genera Posidonia and Ehhalus ) but lower than fast‐growing colonizing species (e.g., the genera Ruppia and Halophilia ) (O'Brien et al, 2018) and varies along a latitude gradient, with northern cold‐water populations having slower leaf turnover than warm‐water populations (Clausen et al, 2014; Olesen et al, 2015).…”

The Use of Photo-Biological Parameters to Assess the Establishment Success of Posidonia oceanica Cuttings after Transplantation

“…The photosynthetic parameters measured here indicate limited physiological differences among the explanatory variables Origin and Bathymetry. Many studies across different species highlight the utility of variable chlorophyll a fluorescence in understanding the photo-physiological dynamics of seagrasses thriving in different light environment, for example, due to different depths [15,18,20,47,50,51]. Previous studies on seagrasses revealed acclimation responses from the plant along a decreasing light gradient, including an increasing α for enhanced photosynthetic efficiency under low light intensities, a decreasing ETRmax, a reduction in the saturating irradiance (Ek), and an increasing Fv/Fm [10,50,[52][53][54].…”

“…In new light environments after transplantation to different environmental conditions, photo-acclimation can induce changes in the concentration and distribution of photosynthetic pigments [18]. With the exception of the Chl-a/Chl-b ratio, the individual pigments and their sum show a similar behaviour as a function of bathymetry, with significantly higher values for shallow sites in September 2022 and higher values for deep sites in May 2023.…”

“…The analysis of fluorometric and derived photosynthetic parameters provides valuable insights into the photo-physiological response of a plant [15][16][17]. In a new light environment, such as may occur after transplantation to a different depth or density of meadow, photo-acclimation can lead to changes in the content and ratio of different photosynthetic pigments [18]. In fact, light-harvesting systems consist not only of both chlorophyll a and b but also of other pigments such as xanthophyll and carotenoid.…”

Establishment and optimization of a new model organism to study early land plant evolution: Germination, cultivation and oospore variation of Chara braunii Gmelin, 1826

“…The adaptation to light regimes is regulated by photoreceptors, including the three classes, phytochroms, cryptochroms/ phototropins and UV-B receptors. Phytochroms, classified in light-labile and light-stabile ones, are one of the major photoreceptors families and regulate on biochemical and physiological level processes such as germination, growth and photosynthesis (Inoue et al, 2016;Leǵer-Daigle et al, 2022). Structural, phytochroms exhibit both termini, one of cyanobacterial and one of proteobacterial origin (Buchberger and Lamparter, 2015).…”