Photoacclimation of the seagrass Halophila stipulacea to the dim irradiance at its 48‐meter depth limit

Abstract

The seagrass Halophila stipulacea grows in the northern Red Sea from the intertidal to depths of , 50 m. Along that gradient, there is a . 1 order of magnitude difference in irradiance and the spectrum narrows from that of full sunlight to dim blue-green light. Based on these differences, we set out to estimate the molar ratios and potential contributions of photosystem II (PSII) and photosystem I (PSI) to light absorption, and photosynthetic electron transport rates (ETR), in plants growing at 1-m and 48-m depths. The amount of PsaC (a proxy for PSI) was three times higher in the deep-growing plants. On the other hand, the amount of PsbA (a proxy for PSII) did not differ significantly between the two depths. Thus, the PSII : (PSII + PSI) ratio (FII) was 0.62 in the shallowand 0.41 in the deep-growing plants. Similar results were obtained by 77K emission fluorescence. Because ETR is linearly dependent on FII, it follows that the ETR vs. irradiance curves differed significantly if calculated based on the commonly used FII value of 0.5 or the FII values we found. As a result, the photosynthetic parameters ETR max and a also differed when using the different FII values. Correct(ed) FII values should, therefore, be used in the calculation of photosynthetic ETRs. The ability of H. stipulacea to alter its amount of PSI relative to PSII according to the ambient irradiance and spectrum may be one reason why this organism can grow down to its exceptional depth limit in clear tropical waters.In aquatic systems, irradiance is attenuated with depth, and the light spectrum in clear waters shifts from that of full sunlight in the intertidal to a narrower band of bluish light at depth. Accordingly, plants growing along a depth gradient have to acclimate to these differences in the optical properties of the water that surrounds them. In addition to changes in plant morphology (Schwarz and Hellblom 2002) and meadow architecture (Dalla Via et al. 1998;Peralta et al. 2002), marine angiosperms, or seagrasses, may also enhance chlorophyll (Chl) concentrations and photosynthetic efficiencies at low irradiances (reviewed in Ralph et al. 2007). Also spectral changes can affect light absorption by altering the composition and ratio of the two photosystems (PSs) and, thus, photosynthetic rates and plant growth (Fujita 1997; Falkowski and Raven 2007). Among marine macrophytes, such spectral changes were shown to influence the functionality of the two PSs as found for the green macroalgae Ulva pertusa and Bryopsis maxima (Yamazaki et al. 2005). Regarding seagrasses, only two studies have reported on changes in the PS composition: one in shallow-growing plants during various seasons (Major and Dunton 2000) and another along a narrow depth gradient down to 1.6 m (Major and Dunton 2002). To our knowledge there are no studies on photosystems' acclimatory mechanism in seagrasses growing at greater depths where both the irradiance and light spectrum change drastically.In the Gulf of Aqaba (northern Red Sea), the seagrass Halophila stipul...

Comparative Extraction of Chlorophylls in Selected Forest and Savanna Mosses Using Dimethylsulphoxide and Acetone

“…This also agreed with what was reported for seagrass (Sharon et al, 2011). The occurrence of lower chlorophyll a/b ratios in shade plants compared with sun plants was obtained as a consequence of chlorophyll b associated only with light harvesting chlorophyll protein (LHCP) complex of the photosynthetic unit, whereas chlorophyll a is found in LHCP and in other complexes and does not change in response to environmental conditions (Alberte et al, 1976).…”

Response of the seagrass Posidonia oceanica to different light environments: Insights from a combined molecular and photo-physiological study

“…In the seagrass Halophila stipulacea, the PSI:P-SII ratio was found to increase in the deep-growing plants in relation to a higher PsaC concentration (Sharon et al, 2011). Changes in the protein composition of PSI and PSII were also observed in two marine green algae (Bryopsis maxima and Ulva pertusa, Yamazaki et al, 2005) and in other seagrass species (Major and Dunton, 2002;Sharon et al, 2011). Further studies are needed in P. oceanica to clarify regulation of PSI/PSII stoichiometry in response to changes in light intensity and spectral quality.…”

“…Relative changes in gene regulation, stoichiometry, and antenna size between PSII and PSI, do have a role in the adaptation to different light regimes (Ruban, 2009), and these mechanisms are known to increase the efficiency in the utilization of absorbed light, improving the overall efficiency of photosynthesis (Walters and Horton, 1995). In the seagrass Halophila stipulacea, the PSI:P-SII ratio was found to increase in the deep-growing plants in relation to a higher PsaC concentration (Sharon et al, 2011). Changes in the protein composition of PSI and PSII were also observed in two marine green algae (Bryopsis maxima and Ulva pertusa, Yamazaki et al, 2005) and in other seagrass species (Major and Dunton, 2002;Sharon et al, 2011).…”

Estimating primary productivity of marine macroalgae in East Antarctica usingin situfluorometry

“…Any deviation from this (widely assumed) value will necessarily modify ETR, and we suggest this issue should continue to be the focus of further study (e.g. Sharon et al, 2011).…”