Photosynthetic responses of Halophila stipulacea to a light gradient. I. In situ energy partitioning of non-photochemical quenching

Abstract

The quantum yield of photosystem II (φ II , also termed ΔF /F m ' or F v /F m in light-or darkacclimated plants, respectively) of the tropical seagrass Halophila stipulacea was measured in situ using modulated fluorescence techniques over diel periods at a range of depths. Photosynthetic electron transport rates (ETRs), as derived from φ II values at specific ambient photosynthetically available radiation (PAR) irradiances, increased in direct proportion to increasing irradiance in the morning and, at shallow sites (7 to 10 m), reached saturating rates and then declined in the afternoon with lower PAR-specific ETRs. On the other hand, plants at 32 to 33 m showed no saturation even at midday, and the percentage reduction in PAR-specific afternoon ETRs was less than that of the shallower plants. The use of an automated shutter in the measuring device enabled non-photochemical quenching due to down-regulation and basal intrinsic non-radiative decay to be distinguished. While midday values of down-regulation were lower in deeper water, basal intrinsic non-radioactive decay remained fairly constant at 30 to 40% at all depths, with more variation in shallow waters. The maximal φ II (i.e. F v /F m ) reached similar values at midnight regardless of depth. H. stipulacea acclimates to the widely varying irradiances across this depth gradient by regularly modulating down-regulationbased non-photochemical quenching processes, while dissipating a large proportion of light energy through intrinsic decay regardless of depth.

Establishing Research Strategies, Methodologies and Technologies to Link Genomics and Proteomics to Seagrass Productivity, Community Metabolism, and Ecosystem Carbon Fluxes

“…In addition, every second measurement was followed by 10 s of exposure to far red light (FRL) with ambient light excluded using the shutter; this was followed by another saturating pulse measurement. From the measurement immediately following the FRL we determined Fo’, and used this value to calculate components of non-photochemical quenching (Runcie et al, 2009). Effective quantum yield measurements (excluding those immediately after exposure to FRL, or those obtained during a RLC) were used to calculate ETR, and diel PE curves were constructed by comparing ETR with ambient irradiance measured at the time of measurement.…”

“…While the quantitative accuracy of this method has been verified for several (e.g., Beer et al, 1998), and especially thin-leaved (Beer and Björk, 2000) seagrasses, its main drawback is that it ignores respiration and, thus, only photosynthetic rates per se can be measured. In order to obtain time series of these photosynthetic measurements, modulated fluorometers have been developed that can measure photosynthetic parameters in situ continuously for several days (Runcie et al, 2009; Runcie and Riddle, 2012). …”

Estimating primary productivity of marine macroalgae in East Antarctica usingin situfluorometry

“…While our multichannel fluorometer (incorporating a Junior-PAM fluorometer, Walz, Germany; see Runcie & Riddle, 2004) brought multi-sample fluorescence monitoring to underwater systems, the more recent development and implementation of specialized autonomous submersible fluorometers (e.g. Shutter Fluorometer, Aquation, Australia) has enabled an increase in both the temporal resolution and replication of in situ macroalgal and seagrass fluorescence (Runcie et al, 2009;Runcie & Riddle, 2011).…”

“…Active fluorescence measurements of Iridaea sp. and H. grandifolius were conducted in situ using either the multichannel fluorometer, or with 'sidefire' or 'shutter' custom fluorometers (Runcie et al, 2009;text and fig. 1 in Runcie & Riddle, 2011).…”

Measuring seagrass photosynthesis: methods and applications

“…The coupling of these characteristics opened the way for autonomous in situ measurements of effective quantum yields on plants exposed to natural conditions. Novel, automated, multichannel chlorophyll fluorometers, first described by Runcie & Riddle (2004) and able to withstand prolonged deployments, were used to investigate light acclimation processes of Halophila stipulacea (Runcie et al 2009, this Theme Section) during the recent 8th Group for Aquatic Productivity (GAP) workshop. Those instruments allow autonomous measurements to be conducted for longer periods (> 24 h), opening new and exciting possibilities in seagrass photosynthetic research.…”