Abstract
The effects of light-limitation stress were investigated in natural stands of the seagrasses Zostera marina and Cymodocea nodosa in Ria Formosa coastal lagoon, southern Portugal. Three levels of light attenuation were imposed for 3 weeks in two adjacent meadows (2–3 m depth), each dominated by one species. The response of photosynthesis to light was determined with oxygen electrodes. Chlorophylls and carotenoids were determined by high-pressure liquid chromatography (HPLC). Soluble protein, carbohydrates, malondialdehyde and phenol contents were also analysed. Both species showed evident signs of photoacclimation. Their maximum photosynthetic rates were significantly reduced with shading. Ratios between specific light harvesting carotenoids and the epoxidation state of xanthophyll cycle carotenoids revealed significantly higher light harvesting efficiency of C. nodosa, a competitive advantage in a low light environment. The contents of both soluble sugars and starch were considerably lower in Z. marina plants, particularly in the rhizomes, decreasing even further with shading. The different carbohydrate energy storage strategies found between the two species clearly favour C. nodosa's resilience to light deprivation, a condition enhanced by its intrinsic arrangement of the pigment pool. On the other hand, Z. marina revealed a lower tolerance to light reduction, mostly due to a less plastic arrangement of the pigment pool and lower carbohydrate storage. Our findings indicate that Z. marina is close to a light-mediated ecophysiological threshold in Ria Formosa.
Highlights
IntroductionThese are generally attributed to man-induced disturbances that lead to eutrophication and siltation, which deteriorate the light environment for these plants [1], [2], [3]
Light response curves The maximum photosynthetic rates of both Zostera marina and Cymodocea nodosa were significantly reduced with the shading treatment (Fig. 1 and Table 1)
The saturation irradiance (Ik) of Z. marina was not affected by shading, whereas in C. nodosa it decreased at least four-fold from the control to all shading levels, with no significant differences among these (Table 1)
Summary
These are generally attributed to man-induced disturbances that lead to eutrophication and siltation, which deteriorate the light environment for these plants [1], [2], [3]. Published literature addressing the effects of light on seagrass photosynthesis has been recently reviewed [4]. The authors concluded that, despite the several published reports on the effects of light reduction on seagrass photosynthesis, morphology, growth and survival, essential knowledge on the underlying physiological mechanisms of light harvesting and resource allocation is still missing. Added constraints preventing generic assumptions for seagrasses are the interspecific variety of ecological strategies, growth rates, morphological and photosynthetic plasticity, photoacclimation potential and energy management strategies
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