Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Abstract

Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14 C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section ( PSII ) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/ PSII ), the chlorophyll-specific maximum rate of carbon fixation (P ), and the light-saturation intensity (E k ), all decreased from the surface to the subsurface chlorophyll * max maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance.Photoacclimation describes the phenotypic response of algae to changes in irradiance at the organism level (Falkowski and LaRoche 1991) and can be assessed by measuring dif-1 Present address: University of Essex, Colchester C04 3SQ, United Kingdom (cmmoore@essex.ac.uk). AcknowledgmentsWe thank E. Le Floch, G. Harris, M. Lucas, H. Thomas, and G. Tilstone for assistance with data collection at sea and M. Zubkov for assistance with the flow cytometry analysis. S. Laney kindly provided software and contributed to many useful discussions on the analysis of raw FRR fluorometer data. We also thank the officers and crew of the RRS James Clark Ross for their assistance during cruise JR98. Insightful comments from J. Cullen and an anonymous reviewer considerably improved an earlier version of this manuscript.

Southern Ocean phytoplankton physiology in a changing climate

“…Differences in photoacclimation strategy are due to the heterogeneity in photosynthetic structure and accessory pigment composition between species, influencing the rate and quantity of photons received by the photosystem (Wilhelm, 1990). Some species acclimate by changing the size of the light harvesting antenna of the individual reaction centre, while others increase the total number of reaction centres, keeping antenna size constant (Falkowski and LaRoche, 1991;Moore et al, 2006). As with temperate diatoms (Ruban et al, 2004;Dimier et al, 2007;Lavaud et al, 2007), SO-and sea ice diatoms are highly plastic to fluctuations in light with a strong dependence on rapid and reversible xanthophyll cycling (Moisan et al, 1998;Moisan and Mitchell, 1999;Kropuenske et al, 2009;Petrou et al, 2011a,c), however, not all species utilise the same strategies, nor have the same level of physiological plasticity (Kropuenske et al, 2009;Mills et al, 2010;Petrou et al, 2011a).…”

Photosynthetic electron turnover in the tropical and subtropical Atlantic Ocean

“…Dark-adapted measurements of the effective absorption cross section, s PSII , are affected by both physiological (phenotypic) and taxonomic (genotypic) signatures of the phytoplankton community (Suggett et al, 2004;Moore et al 2005Moore et al , 2006. Specifically, smaller s PSII values are indicative of large eukaryotes or cyanobacteria but may also indicate a down-regulation in light harvesting for photochemistry; higher s PSII values are indicative of smaller eukaryotes but also an up-regulation of light harvesting for photochemistry (Suggett et al, 2004).…”

“…However, these approximations of non-photochemical quenching assume that the dark-adapted effective absorption cross-section remains constant throughout the water column. Recent evidence from both the laboratory (Suggett et al, 2004) and field (Moore et al, 2005(Moore et al, , 2006) has demonstrated that s PSII,478 can be highly variable both between taxa and with photoacclimatory state. This would imply that the dark-adapted effective absorption cross-section is not constant with depth where distinct phytoplankton populations occur at different depths within the water column, as frequently occurs within tropical and subtropical Atlantic provinces (Zubkov et al, 2000;Barlow et al, 2002).…”

“…This latter phenomenon is equivalent to the maximum achievable turnover rate of electrons, ETR max , that is, in turn, determined by acclimative and adaptive constraints on both light harvesting and processing electrons 'downstream' of PSII (Moore et al, 2006). Least-squares non-linear regression was used to fit Eq.…”

“…Direct in situ FRR measurements of s PSII 0 are weighted to the LED spectrum of the FRR fluorometer (termed s PSII 0 ,478 , Moore et al, 2006, and references therein). Accurate application of Eq.…”