Abstract
Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PETλ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PETλ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PETλ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PETλ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter Ek (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PETλ are discussed in the context of turbulent coastal ecosystems.
Highlights
In nature, phytoplankton must respond to multiple variations in the quantity and quality of light at different temporal and spatial scales (MacIntyre, Kana & Geider, 2000; Dubinsky & Schofield, 2010)
We focused on wavelength-dependent parameters: a, ETRmax, Ek, non-photochemical quenching (NPQ) and high light absorption capacity from 440–625 nm for different natural phytoplankton communities sampled across environmental gradients of a coastal sea
Our results indicate that natural phytoplankton communities can photoacclimate to light quality dynamically under contrasting environmental conditions in temperate coastal seas in response to the available energy balance between red and blue wavelengths
Summary
Phytoplankton must respond to multiple variations in the quantity and quality of light (i.e. light climate) at different temporal (from day to year) and spatial (from environmental coastal gradients to large hydrological structures) scales (MacIntyre, Kana & Geider, 2000; Dubinsky & Schofield, 2010). Light-use efficiency (initial slope, a, see Table 1 for symbols, abbreviations and definitions related to photosynthetic parameters and variable fluorescence measures) and maximum photosynthetic rate (ρmax) parameters of PE curves are the two main parameters traditionally used to investigate biophysical, biochemical and metabolic processes that influence photosynthesis (MacIntyre et al, 2002; Falkowski & Raven, 2007) in response to variations in the light climate. Understanding better the response of cells to potential light stress in surface water requires studying the distribution of light energy between the photochemical and non-photochemical pathways, which includes thermal dissipation of excess absorbed light energy (Lavaud, 2007). These processes are well documented for diatoms (Brunet & Lavaud, 2010) and can be studied by quantifying the light response (E from light Energy) of non-photochemical quenching (NPQ, Serôdio & Lavaud, 2011)
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