Fast Repetition Rate Fluorometry Research Articles

Comparing electron transport with gas exchange: parameterising exchange rates between alternative photosynthetic currencies for eukaryotic phytoplankton

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 56:147-162 (2009) - DOI: https://doi.org/10.3354/ame01303 AME Special: 'Progress and perspectives in aquatic primary productivity: highlights of the GAP VIII workshop, Eilat, Israel, 2008' Comparing electron transport with gas exchange: parameterising exchange rates between alternative photosynthetic currencies for eukaryotic phytoplankton David J. Suggett1,*, Hugh L. MacIntyre2, Todd M. Kana3, Richard J. Geider1 1Department of Biological Sciences, University of Essex, Colchester CO4 3SQ, UK 2Dauphin Island Sea Lab, 101B Bienville Blvd, Dauphin Island, Alabama 36528, USA 3Horn Point Laboratory, 2020 Horns Point Rd, Cambridge, Maryland 21613, USA *Email: dsuggett@essex.ac.uk ABSTRACT: Estimates of aquatic primary productivity derived from in situ active chl a fluorescence have rapidly gained popularity over the past 2 decades. This trend has been driven primarily by the need to improve upon ‘conventional’ carbon (C) uptake- or oxygen (O2) evolution-based productivity estimates that require water samples to be incubated ex situ. Unlike the conventional approaches to measuring productivity, chlorophyll fluorescence measurements inherently describe only the activity of photosystem II (PSII) in the light reactions; thus, the photosynthetic ‘currency’ of the fluorescence-based approach is an electron turnover rate for PSII (ETRPSII). A photosynthetic currency of electrons has limited ecological relevance but can be converted to a currency of carbon if an ‘exchange rate’, i.e. a value or factor of equivalence for any single time point, is applied. We used fast repetition rate fluorometry (FRRf), mass inlet membrane spectrometry (MIMS) and 14C uptake to determine ETRPSII, gross and net O2 evolution and C fixation measured simultaneously for 6 microalgal species and for different steady-state growth conditions. Quantifying the PSII reaction centre (RCII) concentration and the spectral dependency of the effective absorption cross section yielded an FRRf approach that provided a robust estimate of the ETRPSII and gross O2 evolution for all species and conditions tested; however, the ETRPSII exceeded carbon dioxide (CO2) uptake by a factor of ~5.4 to 11.6. At least 3 species exhibited substantial light-dependent O2 cycling to account for ~40 to 60% of the difference between the ETRPSII and CO2 uptake. The highly variable nature of the ETRPSII:CO2 uptake ‘exchange rate’ observed here highlights the need for future studies that rely on active fluorescence to examine aquatic productivity to focus towards a systematic description of how electrons are coupled to C fixation in nature. KEY WORDS: Fast repetition rate fluorescence · Electron transport · Photosynthesis · Oxygen evolution · Carbon uptake · Phytoplankton Full text in pdf format PreviousNextCite this article as: Suggett DJ, MacIntyre HL, Kana TM, Geider RJ (2009) Comparing electron transport with gas exchange: parameterising exchange rates between alternative photosynthetic currencies for eukaryotic phytoplankton. Aquat Microb Ecol 56:147-162. https://doi.org/10.3354/ame01303 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 56, No. 2-3. Online publication date: September 03, 2009 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2009 Inter-Research.

Interpretation of fast repetition rate (FRR) fluorescence: signatures of phytoplankton community structure versus physiological state

Introduction of active chlorophyll a fluo- rescence protocols, in particular fast repetition rate (FRR) fluorometry, to oceanography and limnology 15 yr ago has enabled rapid assessment of photosyn- thetic physiology in situ. The FRR protocol generates simultaneous measurements of Photosystem II (PSII) effective absorption cross sections (termed σPSII) and photochemical efficiency (termed Fv/Fm). Both Fv/Fm and σPSII measurements have been utilised to examine the effects of physiological stress on the photosynthetic apparatus of phytoplankton in an ever growing num- ber of fluorescence-based studies. However, it is now becoming clearer that in situ values of Fv/Fm and σPSII also contain taxonomic information. Here, we present

Effect of water column light gradient on phytoplankton fluorescence transients

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 376:85-101 (2009) - DOI: https://doi.org/10.3354/meps07759 Effect of water column light gradient on phytoplankton fluorescence transients M. Raateoja1,4,*, B. G. Mitchell2, H. Wang2, E. Olivo3 1Finnish Institute of Marine Research, 00560 Helsinki, Finland 2Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0218, USA 3Université Pierre et Marie Curie, 75005 Paris, France 4Present address: SYKE Marine Centre, 00251 Helsinki, Finland *Email: mika.raateoja@fimr.fi ABSTRACT: The light management of phytoplankton can be assessed in situ on time scales shorter than those of the non-photochemical fluorescence quenching (qn) mechanisms. We adopted fast repetition rate fluorometry to water column studies in the northwest Pacific Ocean and its adjacent shelf seas. Near-surface depression of the photochemical energy conversion efficiency (PECE) (effective Fq’/Fm’ and maximum Fv’/Fm’; Fq’ = Fm’ – F’, Fv’ = Fm’ – Fo’; Fq’ and Fv’ are variable fluorescence yields in the light, Fv’ is that with maximum photochemical quenching; fluorescence yields in the light: Fm’ maximum, F’ steady-state, and Fo’ minimum) defined specific zones according to whether PECE was depressed by photochemistry (relaxation of photochemical quenching, qp) or photoprotection/photoinhibition (increase of qn). qp seemed to be the main factor depressing PECE, and the depressing effect of qp on PECE also extended to considerably deeper depths than that of qn. When moving towards the surface, the qn effect overrode the qp effect on the PECE decrease at depths of 8 to 13 m, depending on the station. The vertical trends of Fq’/Fm’ and Fv’/Fm’ were modelled according to a typical P–E (photosynthesis–irradiance) dependence to supplement the 14C-based P–E data. E(Fq’/Fm’) and E(Fv’/Fm’) were the light levels at which the vertical trends of Fq’/Fm’ and Fv’/Fm’, respectively, started to decrease. Although the nutrient regime is the main controller of primary photochemistry in general, the ambient light also becomes the controlling factor on PECE whenever the light level rises above E(Fq’/Fm’). At E(Fv’/Fm’), light ultimately overrides the effect of the nutrient status on PECE. E(Fv’/Fm’) also marks the light level at which the photoprotective measures first become necessary. This level is close to the onset of the plateau phase producing the 14C-based light-saturated photosynthetic rate Pmax. KEY WORDS: Pacific Ocean · Phytoplankton · FRR · Fluorescence · Quenching · Photochemical Full text in pdf format PreviousNextCite this article as: Raateoja M, Mitchell BG, Wang H, Olivo E (2009) Effect of water column light gradient on phytoplankton fluorescence transients. Mar Ecol Prog Ser 376:85-101. https://doi.org/10.3354/meps07759Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 376. Online publication date: February 11, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Comparison of in vitro and in situ plankton production determinations

Plankton production was measured using 8 techniques at 4 stations in the Celtic Sea, North Atlantic Ocean, in April 2002. Primary production (PP) was derived from 14 C incorporation into particulate carbon after 24 h simulated in situ, PP( 14 CSIS), and 2 h photosynthesis-irradiance incuba- tions, PP( 14 CPUR), and from 2 published satellite algorithms, PP(VGPM) and PP(M91). Gross production (GP) was calculated from O2 evolution, GP(O2), and 18 O enrichment of dissolved O2, GP( 18 O), after 24 h simulated in situ incubations, and from in situ active fluorescence measured by fast repetition rate fluorometry (FRRF). Net community production (NCP) was determined from changes in in situ dissolved oxygen, NCP(ΔO2), and from changes in oxygen during 24 h simulated in situ incubations, NCP(O2). Dark community respiration (DCR) was derived from changes in oxygen during a 24 h dark incubation, DCR(O2), and daily oxygen uptake, DOU( 18 O, O2), was calculated from the difference between GP( 18 O) and NCP(O2). Three stations were dominated by picoautotrophs and the fourth station was dominated by diatoms. While most of the comparisons between techniques fell within previously published ranges, 2 anomalies occurred only at the diatom-dominated station. Rates of PP( 14 CPUR) were oxygen uptake in the dark. The low rates of PP( 14 CPUR) in relation to PP( 14 CSIS) may have resulted from the heterogeneous nature of the bloom and differences in sampling time. However, it is also possible that dissolved organic material (DOM) released by the stressed diatom population restricted the diffusion of 14 C into the cells, thereby causing a greater underestimate of PP by techniques using short incuba- tions. The significantly higher rates of oxygen uptake in the light are difficult to reconcile, and we do not know whether the light enhanced oxygen uptake was directly linked to carbon fixation. How- ever, the release of DOM may also have provided substrate for enhanced respiration in the light. These anomalies were only revealed through the concurrent measurement of plankton production by this wide range of techniques. Further investigation of DOM excretion and light-enhanced respira- tion during diatom blooms is warranted.

Toxicity of Benzalkonium Chloride on Monoalgal Cultures and Natural Assemblages of Marine Phytoplankton

The current widespread use of benzalkonium chloride (BAC) in different cleaning and disinfecting products led us to investigate its potential toxic effect on phytoplankton. To this aim, different physiological variables were monitored to detect toxicity of four ecologically relevant concentrations of BAC (15–200 μg l−1) on batch cultures of the microalgae Isochrysis galbana and Chaetoceros gracilis, and on a coastal phytoplankton assemblage. Fluorescence variables measured by fast repetition rate fluorometry, specifically variable fluorescence (Fv), were the most suitable to be used as endpoint among all the variables recorded (biomass, growth rate, size structure, and species composition). Calculated effective concentrations from Fv data provided evidences of higher sensitivity in the natural assemblage at short exposure times (natural assemblage—EC50 [24 h] = 36.4 μg l−1 vs. monoalgae—EC50 [24 h] > 120 μg l−1). Despite these differences, standard monoalgal laboratory toxicity tests provided comparable estimates of safe concentrations for microalgae in the environment. Assessed EC10 fall within the range of BAC concentrations reported in the literature for different effluents.

In situ observation of phytoplankton productivity by an underwater profiling buoy system: use of fast repetition rate fluorometry

To gain a better understanding of variability in phytoplankton productivity in the oceans, we developed an underwater profiling buoy system that uses a fast repetition rate (FRR) fluorometer. We used the profiling buoy system to observe phytoplankton productivity in Sagami Bay, Japan, from 15 April to 10 May 2005. During the observation period, chlorophyll a (chl a) con- centration increased gradually, peaking at > 5 mg chl a m -3 at a depth of 10 m, indicating the occur- rence of phytoplankton blooms. Gross oxygen production (PO2), as determined by FRR fluorometry, increased with the development of phytoplankton blooms and peaked at 49.4 mmol O2 m -2 h -1 on 2 May 2005, when both the chl a concentration and surface irradiance were high. To validate FRR- based PO2, the light-utilization efficiency of a water column (ψO2) was calculated and compared with previous values determined for this location by the oxygen evolution method. The ψO2 derived from FRR fluorometry showed good agreement with those by the oxygen evolution method, suggesting that FRR fluorometry was able to roughly estimate in situ daily PO2. Our profiling buoy system has potential benefits for the measurement of oceanic phytoplankton productivity at a high resolution.

Artifacts in measurements of chlorophyll fluorescence transients, with specific application to fast repetition rate fluorometry

A theoretical framework was developed to describe how instrument and sample artifacts affect measurements of variable fluorescence transients in phytoplankton. This framework identified the proper procedure for correcting for common artifacts in the transients measured with a widely used instrument, the Fasttracka fast repetition rate fluorometer. The impulse response of this fluorometer can be substantial, requiring correction for a dynamic instrument artifact in addition to the static artifacts that can be assessed using traditional “blanks.” In a low‐biomass region of the North Pacific, approximately one‐third of the fluorescence transient measured with this instrument represented such artifacts. Correcting for these using filtered seawater blanks only, and not accounting for the instrument's own response, failed to remove errors as high as 22% in estimates in the photochemical yield and as much as −16% to +22% in estimates of the functional cross section of Photosystem II. This analytical framework and the corrective procedure are generalized and can be used to determine how a wide range of artifacts affect measured variable fluorescence transients, including those characteristic of fluorometers other than the Fasttracka or those more relevant in meso‐ or eutrophic regions of the ocean.

Eutrophication‐induced phosphorus limitation in the Mississippi River plume: Evidence from fast repetition rate fluorometry

We assessed nutrient limitation in the Mississippi River plume and Louisiana continental shelf during the summer of 2002 (04‐08 July). We measured nutrient concentrations, alkaline phosphatase (AP) activities, chlorophyll a (Chl a) concentrations, and four fast repetition rate fluorescence (FRRF) parameters: the maximum quantum yield of photochemistry in photosystem II (PSII), Fv:Fm; the functional absorption cross section for PSII, σPSII; the time for photosynthetic electron transport on the acceptor side of PSII, τQa; and the connectivity factor, p, in 24‐h‐long nutrient addition bioassays near the Mississippi River delta. Low phosphorus (P) concentrations, elevated inorganic nitrogen‐to‐phosphorus ratios, high AP activities, and Chl a increases in response to P additions in the bioassays all indicated phosphorus limitation that was confirmed by the response of FRRF parameters. This is the first study to use FRRF to confirm results from basic oceanographic methods to demonstrate phosphorus limitation in a marine setting. Fv:Fm and p responded positively to phosphorus addition, while σPSII and τQa decreased in the same treatments. When nitrate alone was added, none of the measured parameters differed significantly from the control. We therefore suggest that FRRF can be used to rapidly detect phosphorus limitation in marine ecosystems.

Iron–light interactions during the CROZet natural iron bloom and EXport experiment (CROZEX) I: Phytoplankton growth and photophysiology

The CROZet natural iron bloom and EXport experiment (CROZEX) investigated phytoplankton blooms in the vicinity of the Crozet Plateau in the polar frontal zone (PFZ) of the Southern Ocean. Peak chlorophyll concentrations reached during an intense bloom within naturally iron (Fe)-fertilised regions north of the plateau were an order of magnitude higher than those observed in deeper mixed layers and low-Fe waters to the south. To establish the factors influencing phytoplankton dynamics, a suite of in situ phytoplankton physiological measurements and shipboard Fe–light perturbation experiments was performed. Addition of Fe in experiments performed during bloom decline north of the plateau resulted in increased accumulation of phytoplankton biomass and changes in a number of phytoplankton physiological characteristics. In particular photosystem II (PSII) photochemical efficiencies (Fv/Fm) measured by fast repetition rate fluorometry increased above in situ values within 24 h of Fe amendment, suggesting that Fe stress had contributed to bloom termination. In contrast, responses to Fe amendment were minor within an experiment initiated in low-silicic acid, post-bloom waters south of the Plateau. Within the intense bloom in the north, light limitation due to self-shading may have constrained the peak phytoplankton standing stock. However, in the absence of Fe amendment, incubation at higher than in situ irradiance levels had little influence on phytoplankton biomass accumulation for declining bloom populations. Instead reduced Fv/Fm, reflecting increased photoinhibitory damage to PSII, was observed in high-light incubations and was also apparent in situ. Interactions between Fe and light availability thus influenced phytoplankton physiology and growth and potentially contributed to bloom longevity during CROZEX.

Photosynthetic efficiency as a function of thermal stratification and phytoplankton size structure in an oligotrophic alpine lake

Allometric relationships of phytoplankton communities were studied on the basis of a five-year data-set in a deep oligotrophic alpine lake in Austria. The seasonal phytoplankton succession in Mondsee is characterised by diatoms during winter mixing and a distinct metalimnetic population of Planktothrix rubescens during stratification in summer. The variation of phytoplankton photosynthetic efficiency between seasons was assessed using in situ carbon-uptake rates (5 years data) and Fast Repetition Rate Fluorometry (FRRF) (2 years data). The light-saturated, chlorophyll-specific rate of photosynthesis (P*max), irradiance at the onset of saturation (Ek) and maximum light-utilisation efficiency (α*) were determined for winter mixing and summer stratification. Fluorescence-based parameters as the functional absorption cross section of Photosystem II (σPSII) and the photochemical quantum yield (Fv/Fm) were additionally analysed in 2003 and 2004 to study the underlying physiological mechanisms for the variability in photosynthetic performance. Beyond their sensitivity to changing environmental conditions like thermal stratification, phytoplankton populations differ in their photosynthetic behaviour according to their size structure. Therefore Photosynthesis vs. Irradiance (P/E)-relationships were analysed in detail within a 1-year period from size fractionated cell counts, chlorophyll-a and carbon-uptake.

Photosynthetic electron transport in Dunaliella tertiolecta (Chlorophyceae) measured by fast repetition rate fluorometry: relation to carbon assimilation

A comparison of photosynthesis-irradiance response curves (P-E response curves) obtained through fast repetition rate (FRR) fluorometry and radiocarbon ( 14 C) tracer method was made in the chlorophyte, Dunaliella tertiolecta, grown under different irradiance conditions. In FRR-based P-E response curve experiments, actinic light provided by white light-emitting diodes (LEDs) was increased gradually from 0 to 1500 μmol quanta m −2 s −1 and the rate of photosynthetic electron transport was determined at each light level. Short-term experiments (20 min) of 14 C-based P-E response curve were carried out with an improved photosynthetron, which contains white LEDs as the light source. Irrespective of growth irradiance, the ratios of FRR to 14 C-based initial slopes were almost uniform. The ratios of FRR- to 14 C-based maximum rates were 25-36% higher than those of FRR- to 14 C-based initial slopes. The relationship between electron transport and carbon assimilation was non-linear with increasing discrepancy towards high actinic light. This non-linear relationship between FRR- and 14 C-based estimates is primarily due to the effect of physiological processes stimulated at high levels of light, such as cyclic electron flow and the Mehler reaction. The results of this study indicate that the FRR fluorometry can be used as a good indicator of photosynthetic rates from low to middle light levels, but becomes increasingly questionable as the maximum photosynthetic rate is approached. The degree to which this relationship is further affected by nutrient-status warrants investigation.

Functional relationships and bio-optical properties derived from phytoplankton pigments, optical and photosynthetic parameters; a case study of the Benguela ecosystem

The relationships between phytoplankton pigments, optical properties and photosynthetic parameters for different phytoplankton functional types (derived by diagnostic pigment indices, DPI) were determined from data acquired in the Benguela ecosystem and the offshore region in October 2002. We observed robust inter-pigment relationships: total chlorophyll-a (TChla) was highly correlated with total pigment (TP) and accessory pigment (AP). However, the regression equations for stations dominated by flagellates differed from the equations for stations dominated by diatoms and dinoflagellates. The pigment ratio TChla/TP and the optical ratio a676/a440 were not constant but increased non-linearly with increasing TChla or TP; complimentarily the AP/TP and a490/a676 ratios decreased. There were significant non linear relationships between the photosynthetic parameters Fv/Fm or σPSII measured by Fast Repetition Rate Fluorometry and TChla or TP. Pigment ratios, optical ratios, Fv/Fm and σPSII were all inter-correlated with high significance. We determined the distinctive bio-optical properties associated with dominant phytoplankton functional types (derived by DPI) that conformed to the classical partitioning: flagellates (nano-plankton, comprising several taxa) had low biomass,low TChla/TP fraction and low Fv/Fm and high σPSII; diatoms and dinoflagellates (micro-plankton) had high biomass, pigment ratios, Fv/Fm and low σPSII.

Phytoplankton photosynthetic efficiency and primary production rates estimated from fast repetition rate fluorometry at coastal embayments affected by upwelling (Rias Baixas, NW of Spain)

might be related to environmental control of the coupling between photosynthetic electron transport through photosystem II (PSII) and carbon uptake in phytoplankton. In this study, we looked at this relationship under upwelling conditions favourable for phytoplankton growth. The combination of both techniques allowed the calculation of the quantum efficiency of carbon fixation, which averaged 20 0.12 mol C (mol quanta) � 1 indicating that phytoplankton populations were very efficient in converting absorbed photons into photosynthetically produced organic carbon. The tight coupling observed between phytoplankton photosynthetic electron transport through PSII and carbon uptake resulted in a statistically significant linear relationship between FRRF-derived and 14 C-based carbon incorporation rates, with a slope of 1.43. In conclusion, the results of the this study indicate 25 that FRRF can be a useful tool to derive high spatial and temporal resolution primary production estimates under environmental conditions favouring the close coupling between PSII electron transport and carbon uptake, such as those characteristic of this coastal upwelling system.

Gross photosynthesis and lake community metabolism during the spring phytoplankton bloom

Daily productivity determinations of linear photosynthetic electron transfer and of net and gross inorganic CO2 uptake were determined in situ throughout a 6-week sampling period of the spring phytoplankton bloom in Esthwaite Water in the English Lake District. Photosynthetic electron transfer rates, expressed as gross O2 evolution, were determined from fast repetition rate (FRR) fluorometry and discrete laboratory measurements of the photosynthetic unit size. These gross O2 evolution determinations were also made free from contemporaneous dark-adapted FRR measurements and the need for sample blanks. Net and gross CO2 uptake was determined from changes in total inorganic carbon calculated from in situ pH measurements. Two phases to the bloom were observed. An initial bloom was dominated by the diatom Asterionella formosa, while a secondary phase was characterized by an increase in flagellates and cyanobacteria. For both phases, daily FRR-based gross O2 evolution and pH-based gross CO2 uptake were tightly coupled, suggesting that daily gross O2 production was driving daily changes in CO2 assimilation and thus in community metabolism. This is the first investigation to show closely related rates of gross O2 production and CO2 uptake measured in situ.

ASSESSING PRIMARY PRODUCTION VARIABILITY IN THE NORTH PACIFIC SUBTROPICAL GYRE: A COMPARISON OF FAST REPETITION RATE FLUOROMETRY AND 14C MEASUREMENTS1

In situ 14C uptake (dawn to dusk) and fast repetition rate fluorometry (FRRF) measurements at nearly monthly intervals were compared at Station ALOHA (22°45′N, 158°00′W) between August 2002 and September 2003 in order to determine the feasibility of using FRRF profiling as a means for estimating primary production (PP). The FRRF and 14C rates were significantly correlated (r2=0.906, P value <0.05, n=70) with slopes of 2.00 and 1.90 for chl a and light normalized data, respectively. However, the relationship between 14C‐ and FRRF‐derived carbon fixation varied vertically and temporally. The FRRF: 14C ratio was >1.5 in near‐surface water (5–25 m depth) and approached 1.0 deeper in the euphotic zone. Vertical variations probably reflected the effect of different physiological processes (i.e. Mehler reaction, dark respiration, and excretion) on overall photoautotrophic respiration. In particular, the decrease in Mehler reaction rates with increasing water depth may have accounted for the decrease in difference between 14C and FRRF measurements with depth. The influence of in situ light field variability in controlling the absorption cross‐section of photosystem II (PSII) (σPSII′) may also have been responsible for some of this difference. When compared with total community respiration (R), the derived light‐driven photoautotrophic respiration (reported here as the difference between FRRF and 14C measurements) represented approximately 50% of R integrated over the euphotic zone. Our results show that FRRF and 14C measurements were well correlated in oligotrophic waters but the exact relationship between the two processes varies both temporally and vertically, such that a unique relationship between these two techniques could not be derived from first‐order principles.

Application of fast repetition rate fluorometry to phytoplankton photosynthetic parameters in freshwaters

Fast repetition rate fluorometry (FRRF) was successfully applied to various studies in modern oceanography. In this study, for the first time, the seasonality of phytoplankton photosynthetic parameters in a deep alpine lake was observed using FRRF in combination with the traditional (14)C incubation technique. Special attention was given to the differences in photosynthetic behaviour during mixed and stratified conditions, characterised especially during summer by a deep chlorophyll maximum (DCM) dominated by the filamentous cyanobacterial species Planktothrix rubescens. Maximum light-utilisation efficiency (alpha*(14C)) was in the range of 0.01-0.03 mgC (mg Chl-a)(-1) h(-1 )(mumol phot. m(-2) s(-1))(-1), while maximum quantum yields for carbon fixation (Phi(C,max)) varied from 0.01-0.07 molC (mol phot.)(-1). Higher values occurred during thermal stratification indicating acclimation of the phytoplankton assemblage. These findings were supported by FRRF-based estimates, although cyanobacterial blooms could not be characterised by FRRF-excitation due to methodological deficiencies. In general, however, instantaneous photosynthetic rates measured by FRRF-excitation correlate well at sub-saturating light-intensities with conventional (14)C-uptake rates, although they operate on different time-scales.

The evolution and termination of an iron-induced mesoscale bloom in the northeast subarctic Pacific

We initiated and mapped a diatom bloom in the northeast subarctic Pacific by concurrently adding dissolved iron and the tracer sulfur hexafluoride to a mesoscale patch of high-nitrate, low-chlorophyll waters. The bloom was dominated by pennate diatoms and was monitored for 25 d, which was sufficiently long to observe the evolution and termination of the bloom and most of the decline phase. Fast repetition–rate fluorometry indicated that the diatoms were iron-replete until day 12, followed by a 4–5-d transition to iron limitation. This transition period was characterized by relatively high rates of algal growth and nutrient uptake, which pointed to diatoms using intracellularly stored iron. By days 16–17, the bloom was probably limited simultaneously by both iron and silicic acid supply, because low silicic acid concentrations were evident. Modeling simulations, using data from our study, provided an estimate of the critical threshold for algal aggregation. Observed diatom abundances during the bloom exceeded this threshold between days 13 and 17. Mass sedimentation of diatoms and diatom aggregates was recorded in surface-tethered free-drifting sediment traps at 50 m in depth on day 21. Although the termination of the bloom was probably controlled by the availability of both iron and silicic acid, we cannot rule out the role of algal aggregation. The bloom decline was likely triggered by the onset of mass sedimentation. During our study, evidence of both diatom species succession and species-specific aggregation point to important links between algal nutrient stress and the initiation of algal aggregation.

Maximum photosynthetic efficiency of size-fractionated phytoplankton assessed by 14 C uptake and fast repetition rate fluorometry

Under high nutrient concentrations and sufficient light conditions, large phytoplankton may display higher photosynthetic efficiency than smaller cells. This is unexpected since smaller phytoplankton, because of their higher surface to volume ratio, possess a greater ability to take up nutrients and absorb light. In order to investigate the causes of the increased photosynthetic efficiency in larger phytoplankton, we assessed the maximum photosynthetic efficiency of coastal assemblages in three size classes (<5, 5–20, and >20 µm) by concurrently conducting 14C-based photosynthesis-irradiance experiments and fast repetition rate fluorescence measurements. The light-saturated, chlorophyll-specific photosynthesis (Pbmax) and the maximum photosystem II (PSII) photochemical efficiency (Fv/Fm) of each size class were determined during winter mixing (March 2003) and summer stratification (June 2003). During winter mixing, size-fractionated Pbmax and Fv/Fm were similar in all size classes. In contrast, during summer stratification, size-fractionated Pbmax and Fv/Fm were significantly higher in the >20-µm size class. In the entire data set, size-fractionated Pbmax and Fv/Fm were not significantly correlated. However, a significant relationship was found between size-fractionated Pbmaxand Fv/Fm for phytoplankton assemblages acclimated to low light conditions. Under high light, an excess PSII capacity may be responsible for the discrepancy between size-fractionated Pbmax and Fv/Fm measurements, whereas under low light conditions, photosynthetic electron transport chain and components downstream of PSII become more balanced, which results in a tight covariation between both variables. Higher maximum photosynthetic efficiencies of large-sized phytoplankton might be associated with a higher PSII photochemical efficiency characteristic of certain taxonomic groups such as diatoms.

Physiological state of phytoplankton communities in the Southwest Atlantic sector of the Southern Ocean, as measured by fast repetition rate fluorometry

The majority of the Southern Ocean is a high-nutrient low-chlorophyll (HNLC) ecosystem. Localized increases in chlorophyll concentration measured in the wake of bathymetric features near South Georgia demonstrate variations in the factors governing the HNLC condition. We explore the possibility that the contrast between these areas of high-chlorophyll and surrounding HNLC areas is associated with variations in phytoplankton photophysiology. Total dissolvable iron concentrations, phytoplankton photophysiology and community structure were investigated in late April 2003 on a transect along the North Scotia Ridge (53–54°S) between the Falkland Islands and South Georgia (58–33°W). Total dissolvable iron concentrations suggested a benthic source of iron near South Georgia. Bulk community measurements of dark-adapted photochemical quantum efficiency (F v/F m) exhibited a sharp increase to the east of 46°W coincident with a decrease in the functional absorption cross-section (σPSII). Phytoplankton populations east of 46°W thus displayed no physiological symptoms of iron or nitrate stress. Contrasting low F v/F m west of 46°W could not be explained by variations in the macronutrients nitrate and silicic acid and may be the result of taxon specific variability in photophysiology or iron stress. We hypothesize that increased F v/F m resulted from local relief from iron-stress near South Georgia, east of Aurora Bank, an area previously speculated to be a “pulse point” source of iron. Our measurements provide one of the first direct physiological confirmations that iron stress is alleviated in phytoplankton populations near South Georgia.

Comparative assessment of phytoplankton photosynthesis using conventional 14C-determination and Fast Repetition Rate Fluorometry in freshwaters

Click to increase image sizeClick to decrease image sizeKey words: phytoplankton photosynthesislakesFast Repetition Rate Fluorometry