An in situ study of photosynthetic oxygen exchange and electron transport rate in the marine macroalga Ulva lactuca (Chlorophyta) |
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Authors: | Longstaff Ben J. Kildea Tim Runcie John W. Cheshire Anthony Dennison William C. Hurd Catriona Kana Todd Raven John A. Larkum Anthony W.D. |
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Affiliation: | (1) Department of Botany, The University of Queensland, Brisbane, Qld, 4072, Australia;(2) Department of Botany, The University of Adelaide, Adelaide, 5005, SA, Australia;(3) School of Biological Sciences, The University of Sydney, NSW, 2006, Australia;(4) Present address: South Australia Research and Development Institute, P.O. Box 120, Henley Beach, SA, 5022, Australia;(5) Department of Botany, University of Otago, P.O. Box 56, Dunedin, New Zealand;(6) Horn Point Laboratory, P.O. Box 220, Cambridge, Maryland, USA;(7) Division of Environmental and Applied Biology, School of Life Sciences, University of Dundee, Dundee, DD1 4HN, UK |
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Abstract: | Direct comparisons between photosynthetic O2 evolution rate and electron transport rate (ETR) were made in situ over 24 h using the benthic macroalga Ulva lactuca (Chlorophyta), growing and measured at a depth of 1.8 m, where the midday irradiance rose to 400–600 μmol photons m−2 s−1. O2 exchange was measured with a 5-chamber data-logging apparatus and ETR with a submersible pulse amplitude modulated (PAM) fluorometer (Diving-PAM). Steady-state quantum yield ((Fm′−Ft)/Fm′) decreased from 0.7 during the morning to 0.45 at midday, followed by some recovery in the late afternoon. At low to medium irradiances (0–300 μmol photons m−2 s−1), there was a significant correlation between O2 evolution and ETR, but at higher irradiances, ETR continued to increase steadily, while O2 evolution tended towards an asymptote. However at high irradiance levels (600–1200 μmol photons m−2 s−1) ETR was significantly lowered. Two methods of measuring ETR, based on either diel ambient light levels and fluorescence yields or rapid light curves, gave similar results at low to moderate irradiance levels. Nutrient enrichment (increases in [NO3 −], [NH4 +] and [HPO4 2-] of 5- to 15-fold over ambient concentrations) resulted in an increase, within hours, in photosynthetic rates measured by both ETR and O2 evolution techniques. At low irradiances, approximately 6.5 to 8.2 electrons passed through PS II during the evolution of one molecule of O2, i.e., up to twice the theoretical minimum number of four. However, in nutrient-enriched treatments this ratio dropped to 5.1. The results indicate that PAM fluorescence can be used as a good indication of the photosynthetic rate only at low to medium irradiances. This revised version was published online in June 2006 with corrections to the Cover Date. |
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Keywords: | benthic algae bioenergetics electron transport fluorescence nutrient uptake oxygen evolution PAM photosynthesis primary production pulse amplitude modulation |
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