Summary In Arabidopsis leaves, high light stress induces rapid expression of a gene encoding a cytosolic ascorbate peroxidase ( APX2 ), whose expression is restricted to bundle sheath cells of the vascular tissue. Imaging of chlorophyll fluorescence and the production of reactive oxygen species (ROS) indicated that APX2 expression followed a localised increase in hydrogen peroxide (H 2 O 2 ) resulting from photosynthetic electron transport in the bundle sheath cells. Furthermore, leaf transpiration rate also increased prior to APX2 expression, suggesting that water status may also be involved in the signalling pathway. Abscisic acid stimulated APX2 expression. Exposure of ABA‐insensitive mutants ( abi1‐1 , abi2‐1 ) to excess light resulted in reduced levels of APX2 expression and confirmed a role for ABA in the signalling pathway. ABA appears to augment the role of H 2 O 2 in initiating APX2 expression. This regulation of APX2 may reflect a functional organisation of the leaf to resolve two conflicting physiological requirements of protecting the sites of primary photosynthesis from ROS and, at the same time, stimulating ROS accumulation to signal responses to changes in the light environment.
Summary Mechanistic understanding of the costs and benefits of photoacclimation requires knowledge of how photophysiology is affected by changes in the molecular structure of the chloroplast. We tested the hypothesis that changes in the light dependencies of photosynthesis, nonphotochemical quenching and PSII photoinactivation arises from changes in the abundances of chloroplast proteins in Emiliania huxleyi strain CCMP 1516 grown at 30 (Low Light; LL ) and 1000 (High Light; HL ) μmol photons m −2 s −1 photon flux densities. Carbon‐specific light‐saturated gross photosynthesis rates were not significantly different between cells acclimated to LL and HL . Acclimation to LL benefited cells by increasing biomass‐specific light absorption and gross photosynthesis rates under low light, whereas acclimation to HL benefited cells by reducing the rate of photoinactivation of PSII under high light. Differences in the relative abundances of proteins assigned to light‐harvesting (Lhcf), photoprotection ( LI 818‐like), and the photosystem II ( PSII ) core complex accompanied differences in photophysiology: specifically, Lhcf: PSII was greater under LL , whereas LI 818: PSII was greater in HL . Thus, photoacclimation in E. huxleyi involved a trade‐off amongst the characteristics of light absorption and photoprotection, which could be attributed to changes in the abundance and composition of proteins in the light‐harvesting antenna of PSII .
The importance of temporal changes in the vertical distribution of microphytobenthic algae on the overall functioning of intertidal biofilms were investigated with low‐temperature scanning electron microscopy and high‐resolution single‐cell fluorescence imaging of photosystem II efficiency (estimated by the fluorescence parameter F’ q / F’ m ) in intact cores maintained in tidal mesocosms. Early morning biofilms consisted of smaller naviculoid and nitzschioid taxa or euglenoid species. By midday, Gyrosigma balticum and Pleurosigma angulatum were dominant. Some taxa (e.g., Plagiotropis vitrea ) disappeared from surface layers after midday. Species composition continued to change toward the end of the photoperiod, with G. balticum dominating in diatom‐rich biofilms. In Euglena ‐rich biofilms, initial dense surface films of euglenids became progressively dominated by smaller diatoms. F’ q / F’ m (measured at a photosynthetically active photon flux density (PPFD) of 220 µmol m −2 s −1 ) of individual cells of all taxa declined significantly after midday, but increased toward dusk. There were significant differences in F’ q / F’ m between species, particularly after midday. F’ q / F’ m versus irradiance curves and relative electron transport rate (rETR max ) showed higher efficiencies and rETR max for euglenids, whereas G. balticum , Nitzschia dubia , and small Nitzschia sp. were shade‐adapted with low values of F’ q / F’ m , rETR max , and E sat . G. balticum , P. vitrea , and N. dubia showed rapid vertical migration away from the surface with increasing irradiance. Euglenids, P. angulatum , and N. dubia exhibited their highest rETR max values at midday. E sat for algal cells was between 500 and 600 µmol m −2 s −1 , except for N. dubia and small Nitzschia sp., which had an E sat of 300 µmol m −2 s −1 . Differences in behavioral and photophysiological traits between microphytobenthic taxa could be a form of niche separation and need to be incorporated into conceptual models of daily patterns of production in intertidal biofilms.
Rates of primary production by intertidal microphytobenthos within biofilms have been shown to be very high. An essential step toward assessing the contribution of individual species to this level of production is the in vivo measurement of photosynthetic efficiency from individual cells. A strong relationship between photosystem II photochemical efficiency and the fluorescence parameter F q '/F m ′ (where F q ′ = F m ′ − F′) has been established within higher plants and unicellular algae. Calculation of F q ′/F m ′ requires measurement under constant light (at the F′ level of fluorescence) and during a pulse of saturating light (at the F m ′ level of fluorescence). High‐resolution imaging of chlorophyll fluorescence at the F′ and F m ′ levels has allowed the construction of F q ′/F m ′ images from individual cells of several species of diatom and Euglena sp. within intact biofilms. No species differences in the values of F q ′/F m ′ were observed at low levels of incident light. However, Euglena sp. showed significantly higher F q ′/F m ′ values at moderate to high incident light levels than all of the diatom species. Endogenous rhythms of vertical migration during tidal exposure and peaks in photosystem II photochemical efficiency at low tide could also be followed using this technique. Clear differences were observed in the migration of individual taxa to the surface of the biofilm. Images of F q ′/F m ′ were also used to assess the scale of heterogeneity for this parameter. Overall, these data demonstrate that high‐resolution imaging of chlorophyll fluorescence is a valuable technique that allows for determination of the photosystem II photochemical efficiency from different microphytobenthic taxa within biofilms.
Abstract Singe‐turnover active chlorophyll a fluorometry (STAF) can be used to assess phytoplankton photosynthetic rates in terms of the photosystem II photochemical flux (JV PII , μ mol e − m −3 s −1 ) instantaneously, autonomously, and at high resolution. While JV PII provides an upper limit to rates of phytoplankton primary productivity in units of carbon fixation, the conversion between these two rates is variable, limiting our ability to utilize high‐resolution JV PII data to monitor phytoplankton primary productivity. Simultaneous measurements of JV PII and 14 C‐fixation help in understanding the factors controlling the variable ratio between the two rates. However, to date, methodological inconsistencies, including differences in incubation lengths and light quality, have greatly inhibited practical assessment of such electron to carbon ratios (Φ e,C , mol e − mol C −1 ). We here present data from a range of dual‐incubation experiments in northeast Atlantic waters during which JV PII and 14 C‐fixation were measured simultaneously on the same sample. Time‐course experiments show how Φ e,C increases with incubation length, likely reflecting the transition from gross to net 14 C‐fixation. Dual‐incubation experiments conducted under different light levels show a tendency for increased Φ e,C under (super‐)saturating light. Finally, data from a diurnal experiment demonstrate how Φ e,C increases over the course of a day, due to downregulation of 14 C‐fixation. We provide a detailed description of our methodological approach, including a critical discussion of improvements to the calculation of JV PII implemented in the LabSTAF instrument used for active fluorescence measurements and the limitations of the well‐established 14 C‐fixation approach.
Abstract Photosystem II (PSII) photochemistry is the ultimate source of reducing power for phytoplankton primary productivity (PhytoPP). Single turnover active chlorophyll fluorometry (STAF) provides a non-intrusive method that has the potential to measure PhytoPP on much wider spatiotemporal scales than is possible with more direct methods such as 14 C fixation and O 2 evolved through water oxidation. Application of a STAF-derived absorption coefficient for PSII light-harvesting (a LHII ) provides a method for estimating PSII photochemical flux on a unit volume basis (JV PII ). Within this study, we assess potential errors in the calculation of JV PII arising from sources other than photochemically active PSII complexes (baseline fluorescence) and the package effect. Although our data show that such errors can be significant, we identify fluorescence-based correction procedures that can be used to minimize their impact. For baseline fluorescence, the correction incorporates an assumed consensus PSII photochemical efficiency for dark-adapted material. The error generated by the package effect can be minimized through the ratio of variable fluorescence measured within narrow wavebands centered at 730 nm, where the re-absorption of PSII fluorescence emission is minimal, and at 680 nm, where re-absorption of PSII fluorescence emission is maximal. We conclude that, with incorporation of these corrective steps, STAF can provide a reliable estimate of JV PII and, if used in conjunction with simultaneous satellite measurements of ocean color, could take us significantly closer to achieving the objective of obtaining reliable autonomous estimates of PhytoPP.
ABSTRACT An instrument capable of imaging chlorophyll a fluorescence, from intact leaves, and generating images of widely used fluorescence parameters is described. This instrument, which is based around a fluorescence microscope and a Peltier‐cooled charge‐coupled device (CCD) camera, differs from those described previously in two important ways. First, the instrument has a large dynamic range and is capable of generating images of chlorophyll a fluorescence at levels of incident irradiance as low as 0.1 μmol m −2 s −1 . Secondly, chlorophyll fluorescence, and consequently photosynthetic performance, can be resolved down to the level of individual cells and chloroplasts. Control of the instrument, as well as image capture, manipulation, analysis and presentation, are executed through an integrated computer application, developed specifically for the task. Possible applications for this instrument include detection of early and differential responses to environmental stimuli, including various types of stress. Images illustrating the instrument's capabilities are presented.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
