Coupling between Photosystem II electron transport and carbon fixation in microphytobenthos

Abstract

Photosynthetic parameters of a microphytobenthic (MPB) biofilm grown in a tidal mesocosm were measured on undisturbed sediment using variable fluorescence-based measurements of electron transport rate (ETR), as well as by ETR and 14 C assimilation measurements in optically thin suspensions of algal cells. Absorption cross-sections of the MPB suspensions were quantified using the quantitative filter technique and by reconstruction using HPLC-derived pigment concentrations. Photosynthetic parameters derived by the 3 methods were compared on 3 days, representing different biofilm growth/[chl a ] conditions, at the start, middle and end of the daytime tidal emersion. Comparisons of ETR and radioisotope-derived photosynthetic parameters measured on optically thin suspensions were not significantly different, confirming that with an appropriate estimation of the irradiance absorbed by Photosystem II (PSII), under optically well-defined conditions, variable fluorescence is a reliable measure of MPB photosynthetic rates. In contrast, significant differences of up to 60% were observed between the maximum photosynthetic capacity (P B max ) measured on undisturbed sediment and in suspensions. These differences were observed at high [chl a] (coinciding with low growth rates) towards the end of emersion periods. Comparison of the effective quantum efficiency (ΔF/F m ') at the highest light steps of photosynthetic-irradiance (P-E) curves suggested that the overestimation was due to the poor definition of the complex sediment optics, which interacted presumably with photo-taxis and/or single species migrations. Definition of the optics within undisturbed sediments, particularly considering the complex effects of migration, is a serious challenge, limiting the application of variable fluorescence techniques in situ on undisturbed sediments.KEY WORDS: Variable fluorescence · 14 C uptake · Microphytobenthos · Maximum photosynthetic capacity · Maximum quantum yield of photosynthesis · Sediment · Absorption cross-section · PAM fluorescence Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 50: [301][302][303][304][305][306][307][308][309][310][311] 2008 with their own specific advantages and disadvantages (reviewed by Underwood & Kromkamp 1999). In situ tracer incubations measure the fixation of 13 C or 14 C from bicarbonate solution applied to the sediment surface (Middelburg et al. 2000); however, calculations of fixation rates can be difficult due to the unknown specific activity of the tracer in the sediment pore water (Vadeboncoeur & Lodge 1998). This problem may be serious in cohesive sediments (Jonsson 1991), or in dense biofilms where drawdown of the porewater DIC pool is extremely fast and atmospheric CO 2 is the main inorganic carbon source. In contrast, photosynthetic measurements using suspensions of MPB cells in a medium (typically filtered seawater) can give useful information about photosynthetic characteristics under controlled temperature and irradiance cond...

Photosynthetic activity and community shifts of microphytobenthos covered by green macroalgae

“…These densities were roughly equivalent to a sediment surface PPF of 10 and 50 µmol photons m −2 s −1 respectively. While these light levels are not particularly low considering the wide range of photo‐acclimation observed in benthic diatoms (Sundback and Granéli, 1988; Mercado et al ., 2004), the available light was mostly green, a region of the spectrum where diatoms have relatively low absorption cross‐sections (Mercado et al ., 2004; Morris et al ., 2008).…”

One pulse, one light curve: Fast characterization of the light response of microphytobenthos biofilms using chlorophyll fluorescence

“…This enabled to characterize in detail the shape of the light‐response curve, which was used to design a light mask consisting of 3 × 8 light levels, considered as a good compromise between detailing the light response and replication. Additionally, the use of eight light levels allowed to better compare the results of the presented method and of standard protocols, as it is the number of light levels used in many studies (e.g., White and Critchley ; Ralph and Gademann ; Serôdio et al ; Perkins et al ; Morris et al ; Mouget et al ; Lefebvre et al ).…”

Littoral‐benthic primary production estimates: Sensitivity to simplifications with respect to periphyton productivity and basin morphometry

“…Direct production measurements are particularly important for ecosystem‐scale research because the chlorophyll‐based monitoring techniques developed for phytoplankton translate poorly to periphyton (Baulch et al ; Vadeboncoeur et al ). PAM measurements cannot be directly related to carbon fixation without detailed information about the thickness of the photosynthetically active algal biofilm, which cannot be assessed in situ (Morris et al ; Perkins et al ). However, unlike oxygen exchange methods, PAM fluorometry is an excellent tool for collecting spatially extensive data that characterize photoacclimation of periphyton growing at different depths.…”