title: Photosynthetic response of Ulva rotundata to light and temperature during emersion on an intertidal sand flat.
abstract: We have investigated the diurnal response of photosynthesis and variable photosystem II (PSII) chlorophyll fluorescence at 77 K for thalli of the chlorophyte macroalga, Ulva rotundata, grown in outdoor culture and transplanted to an intertidal sand flat in different seasons. The physiological response in summer indicated synergistic effects of high PFD and aerial exposure, the latter probably attributable to temperature, which usually increased by 8 to 10° C during midday emersion. Except at extreme emersed temperatures in summer (38° C), the light-saturated photosynthesis rate (Pm) did not decline at midday. In contrast, light-limited quantum yield of photosynthetic O2exchange (τ) and the ratio of variable to maximum fluorescence yield (Fv/Fm) reversibly declined during midday low tides in all seasons. Shade-grown thalli exhibited a fluorescence response suggestive of greater photodamage to PSII, whereas sun-grown thalli had greater photoprotective capacity. The fluorescence decline was smaller when high tide occurred at midday, and was delayed during morning cloudiness. These results suggest that the diurnal response to PFD in this shallow water species is modified by tidal and meteorological factors. U. rotundata has a great capacity for photoprotection which allows it to tolerate and even thrive in the harsh intertidal environment.
authors.affiliation: Duke University Marine Laboratory, 28516, Beaufort, NC, USA.
authors.forename: W J
authors.initials: WJ
authors.lastname: Henley
authors.affiliation: Duke University Marine Laboratory, 28516, Beaufort, NC, USA.|Department of Botany, Duke University, 27706, Durham, NC, USA.
authors.forename: S T
authors.initials: ST
authors.lastname: Lindley
authors.affiliation: Duke University Marine Laboratory, 28516, Beaufort, NC, USA.|Station Biologique, CNRS, 29680, Roscoff, France.|Université Paris VI, 29680, Roscoff, France.
authors.forename: G
authors.initials: G
authors.lastname: Levavasseur
authors.affiliation: Department of Botany, Duke University, 27706, Durham, NC, USA.
authors.forename: C B
authors.initials: CB
authors.lastname: Osmond
authors.affiliation: Duke University Marine Laboratory, 28516, Beaufort, NC, USA.|Department of Botany, Duke University, 27706, Durham, NC, USA.
authors.forename: J
authors.initials: J
authors.lastname: Ramus
keywords: Chlorophyll fluorescence; Photoinhibition; Photosynthesis; Temperature stress; Ulva