Marine Biology

, Volume 148, Issue 2, pp 241–250 | Cite as

Irradiance-induced changes in the photobiology of Halophila johnsonii

  • J. I. Kunzelman
  • M. J. DurakoEmail author
  • W. J. Kenworthy
  • A. Stapleton
  • J. L. C. Wright
Research Article


The endangered seagrass Halophila johnsonii Eiseman, exhibits high-light adapted photophysiology consistent with its distribution in intertidal and shallow subtidal (0–3 m) coastal-lagoon habitats along 200 km of southeastern Florida. To examine the short-term responses of this seagrass to three controlled-irradiance treatments (PAR + UVA + UVB [full spectrum], PAR + UVA, and PAR only), greenhouse-acclimated plants were transferred to outdoor mesocosms during July–August 2002. Chlorophyll fluorescence, UV fluorescence, and samples for pigment extraction were collected in the greenhouse, prior to moving the plants outside and on days 1, 2, 3, 4, 6, 10, and 21 of the 24-day experiment. Typical of sun-adapted plants, effective quantum yields measured by pulse-amplitude modulated (PAM) fluorometry were relatively low in all treatments, ranging from 0.46 ± 0.09 (PAR only) to 0.58 ± 0.08 (PAR + UVA + UVB). In the PAR only treatments, there were strong effects on days 1 and 4, presumably because the irradiance in the greenhouse not only lacked all λ<400 nm, but also had low irradiance maxima (∼700 μmol photons m−2 s−1, compared with ∼1,500 μmol photons m−2 s−1 outside at midday). There were few treatment differences between PAR only and PAR + UVA treatments indicating little effect of UVA radiation on this species. Differences in effective quantum yields and relative electron transport rates between the PAR only and PAR + UVA + UVB treatments on day 4 indicated rapid acclimation to UVB radiation. Tissues of H. johnsonii contained compounds that absorbed strongly in the UV, with a λmax at ∼345 nm (depending on the extraction solvent). Absorption peak maxima and minima changed over the course of the experiment but there were no significant light-treatment differences in any pigment parameters. Percent UV shield values, measured using a newly developed UVA PAM fluorometer, were highest the day after plants were transferred from the greenhouse to the outdoor mesocosms and declined significantly to pretreatment levels in all treatments by day 21. Percent UV shield exhibited a significant positive relationship with UV-absorbing pigment (UVP) absorbance, however, the absence of treatment effects suggests that the wavelengths inducing pigment synthesis must lie between 400 and 700 nm (PAR). The results indicate that H. johnsonii rapidly acclimates to high UVB and PAR which may largely explain its distribution in intertidal and shallow subtidal areas.


Carotenoid Photosynthetic Photon Flux Density Irradiance Level Leaf Pair Relative Electron Transport Rate 
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This project was supported by the National Oceanic and Atmospheric Administration, Recover Protected Species Program, and Center for Coastal Fisheries and Habitat Research, National Ocean Service, Beaufort, NC. Additional support was provided by the University of North Carolina’s Department of Biological Sciences and the Center for Marine Science. The authors thank Drs. Rolf Gademann and Peter Ralph for the loans of their UVA PAM fluorometers.


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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • J. I. Kunzelman
    • 1
    • 3
  • M. J. Durako
    • 1
    • 3
    Email author
  • W. J. Kenworthy
    • 2
  • A. Stapleton
    • 3
  • J. L. C. Wright
    • 1
  1. 1.The University of North Carolina at Wilmington Center for Marine ScienceWilmingtonUSA
  2. 2.Center for Coastal Fisheries and Habitat ResearchNational Ocean Service, NOAABeaufortUSA
  3. 3.Department of Biological SciencesThe University of North Carolina at WilmingtonWilmingtonUSA

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