title: Effective light absorption and absolute electron transport rates in the coral Pocillopora damicornis.
abstract: Pulse Amplitude Modulation (PAM) fluorometry has been widely used to estimate the relative photosynthetic efficiency of corals. However, both the optical properties of intact corals as well as past technical constrains to PAM fluorometers have prevented calculations of the electron turnover rate of PSII. We used a new Multi-colour PAM (MC-PAM) in parallel with light microsensors to determine for the first time the wavelength-specific effective absorption cross-section of PSII photochemistry, σII(λ), and thus PAM-based absolute electron transport rates of the coral photosymbiont Symbiodinium both in culture and in hospite in the coral Pocillopora damicornis. In both cases, σII of Symbiodinium was highest in the blue spectral region and showed a progressive decrease towards red wavelengths. Absolute values for σII at 440 nm were up to 1.5-times higher in culture than in hospite. Scalar irradiance within the living coral tissue was reduced by 20% in the blue when compared to the incident downwelling irradiance. Absolute electron transport rates of P. damicornis at 440 nm revealed a maximum PSII turnover rate of ca. 250 electrons PSII(-1) s(-1), consistent with one PSII turnover for every 4 photons absorbed by PSII; this likely reflects the limiting steps in electron transfer between PSII and PSI. Our results show that optical properties of the coral host strongly affect light use efficiency of Symbiodinium. Therefore, relative electron transport rates do not reflect the productivity rates (or indeed how the photosynthesis-light response is parameterised). Here we provide a non-invasive approach to estimate absolute electron transport rates in corals.
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia. Electronic address: milan.szabo@uts.edu.au.
authors.forename: Milán
authors.initials: M
authors.lastname: Szabó
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia.
authors.forename: Daniel
authors.initials: D
authors.lastname: Wangpraseurt
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia.
authors.forename: Bojan
authors.initials: B
authors.lastname: Tamburic
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia.
authors.forename: Anthony W D
authors.initials: AW
authors.lastname: Larkum
authors.affiliation: Julius-von-Sachs Institut für Biowissenschaften, Lehrstuhl Botanik I, Universität Würzburg, Germany.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Schreiber
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia.
authors.forename: David J
authors.initials: DJ
authors.lastname: Suggett
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia; Marine Biological Section, Department of Biology, University of Copenhagen, DK-3000 Helsingør, Denmark.
authors.forename: Michael
authors.initials: M
authors.lastname: Kühl
authors.affiliation: Plant Functional Biology and Climate Change Cluster, University of Technology, Sydney, Broadway 2007, NSW, Australia.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Ralph
mesh_terms: D000818:Animals; D037421:Anthozoa / Q000502:physiology*; D004579:Electron Transport; D008027:Light
keywords: Chlorophyll fluorescence; Coral; Electron transport rate; Light absorption; Spectral attenuation