Seagrass Halophila ovalis is affected by light quality across different life history stages

Abstract

Seagrass meadows provide crucial ecosystem services to the coastal zone but are threatened globally. Seagrass loss to date has mainly been attributed to anthropogenic activities that reduce light quantity (amount of photosynthetic photon flux density), such as dredging, flooding and eutrophication. However, light quality (wavelengths of light within the visible spectrum) is also altered by these anthropogenic stressors. This study addressed the effect of light quality changes on seagrasses. Aquarium-based experiments were conducted to determine whether the seagrass Halophila ovalis (R.Br.) Hook f. responds to different light quality treatments. Separate experiments were performed in which adults, seeds or seedlings were subjected to monochromatic light treatments in the blue (peak λ = 451 nm), green (peak λ = 522 nm), yellow (peak λ = 596 nm) and red (peak λ = 673 nm) wavelengths with a control of full-spectrum light (λ = 400 − 700 nm, at 200 µmol photons m −2 s −1). This study is unique in that it measured seagrass responses to light across several plant scales (physiology, productivity, morphology and biomass) as well as across life-history stages (seeds, seedlings, adults and flowering). Adult plants responded differently to seeds and seedlings but were generally consistent with terrestrial angiosperms: blue light decreased below-ground productivity; green light influenced morphology (through increased rhizome internode length); red light enhanced seed germination and survival. The findings indicate that both natural and human-induced changes in light quality could significantly affect seagrass growth and reproduction. As a range of anthropogenic activities are currently contributing to the global losses of seagrasses, this research provides timely information on how light quality influences different seagrass life history stages.

Short-term Responses of Posidonia australis to Changes in Light Quality

“…Light treatments were provided through aquarium Light Emitting Diode (LED) Grow8 TM modules (MarinTech Pty Ltd., ACT, Australia) customized to a spectrum similar to sunlight set on a 12 h light/dark cycle at 200 μmol photons m -2 s -1 consistent across all treatments (as measured at the sediment surface using a MicroPAR quantum sensor from In-situ Marine Optics Pty Ltd., Bibra Lake, WA, Australia). This experiment was carried out using the same aquaria set-up (light filters, aquarium lights, sediment type, display tanks, sump tanks and foam block filters) as described in ( Strydom et al, 2017 ). Full-spectrum tanks received light directly from the LED modules and light quality treatments were imposed by placing yellow, red and blue color filters underneath light modules (Rosco heat resistant gel filter sheets, Figure 2 ).…”

“…The setting of circadian rhythms, flower induction, seed germination, photosynthesis, adult and seedling growth are all influenced by specific wavelengths of light ( Fankhauser and Chory, 1997 ; Chen et al, 2004 ; Whitelam and Halliday, 2008 ). In the marine environment, light quality has also been found to be important for marine angiosperm (seagrass) life history processes ( Strydom et al, 2017 ), but information on the influence of specific wavelengths of light for most species is limited ( York et al, 2016 ).…”

“…>10 m ( Kirk, 1994 ). Additionally, wavelengths present within the water column can vary among locations of the same depth depending on particulates and dissolved components of water ( Figure 1 , Strydom et al, 2017 ). For example, chromophoric dissolved organic matter (CDOM) expelled from rivers and estuaries strongly absorbs short-wave radiation, thus leading to a yellow and red-shifted light field in shallow coastal and estuarine waters ( Kirk, 1994 ).…”

“…The effect of light quality (specific wavelengths of light) on growth of adult plants has recently been reported for H. ovalis , where monochromatic blue, yellow and green light negatively impacted below-ground productivity ( Strydom et al, 2017 ). For seagrass seeds and seedlings, contrasting responses to different wavelengths of light have been reported for different species.…”

“…For seagrass seeds and seedlings, contrasting responses to different wavelengths of light have been reported for different species. In Thalassia hemprichii , blue light stimulated seedling growth (at 50 μmol photons m -2 s -1 ) ( Soong et al, 2013 ), whereas in H. ovalis , red light enhanced seed germination and seedling survival (at 200 μmol photons m -2 s -1 ) ( Strydom et al, 2017 ). These differing responses to light quality may reflect differences in photoreceptor composition among species.…”