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Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton, and the relevance for pulse-amplitude-modulated and fast-repetition-rate fluorometry 1

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Biooptical characteristics of PSII and PSI in 33 species (13 pigment groups) of marine phytoplankton, and the relevance for pulse-amplitude-modulated and fast-repetition-rate fluorometry 1
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  BIOOPTICAL CHARACTERISTICS OF PSII AND PSI IN 33 SPECIES (13 PIGMENTGROUPS) OF MARINE PHYTOPLANKTON, AND THE RELEVANCE FOR PULSE- AMPLITUDE-MODULATED AND FAST-REPETITION-RATE FLUOROMETRY  1 Geir Johnsen  2 and Egil Sakshaug  Department of Biology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway   We studied the variability of in vivo absorptioncoefficients and PSII-scaled fluorescence excitation(fl-ex) spectra of high light (HL) and low light (LL)acclimated cultures of 33 phytoplankton species that belonged to 13 different pigment groups (PGs) and10 different phytoplankton classes. By scaling fl-exspectra to the corresponding absorption spectra by matching them in the 540–650 nm range, weobtained estimates for the fraction of total chl  a  that resided in PSII, the absorption of light by PSII,PSI, and photoprotective carotenoids. The in vivored peak absorption maxima ranged from 673 to679 nm, reflecting bonding of chl  a   to different pig-ment proteins. A simple approach is presented for quantifying intracellular self-shading and evaluating the impact of photoacclimation on biooptical char-acteristics of the different PGs examined. In view of these results, parameters used in the calculationof oxygenic photosynthesis based on pulse-ampli-tude-modulated (PAM) and fast-repetition-rate(FRR) fluorometers are discussed, showing that theratio between light available to PSII and totalabsorption, essential for the calculation of the oxy-gen release rate (using the PSII-scaled fluorescencespectrum as a proxy) was dependent on species andphotoacclimation state. Three subgroups of chromo-phytes exhibited 70%–80%, 60%–80%, and 50%–60% chl  a   in PSII-LHCII; the two subgroups of chlo-rophytes, 70% or 80%; and cyanobacteria, only 12%.In contrast, the mean fraction for chromo- and chlo-rophytes of quanta absorbed by PSII was 73% inLL- and 55% in HL-acclimated cells; thus, the corre-sponding ratios 0.55 and 0.73 might be used as cor-rection factors adjusting for quanta absorbed by PSII for PAM and FRR measurements. Key index words:   13 pigment groups of phytoplank-ton; absorbed quanta to PSII; chl  a   Æ  C ) 1 ratio;package effect; photoacclimation; PSII-scaled fl-ex spectra; variable fluorescence  Abbreviations:   ACP, chl  a  –chl  c  –peridinin proteincomplex; chl  a   Æ  C ) 1 ratio, chl  a   to carbon ratio(w:w); DCMU, dichlorophenyl methyl urea; E  , irradiance ( l mol photons  Æ  m ) 2 Æ  s ) 1 ); fl-ex, fluorescence excitation; FRR, fast repetition rate;HL, high light; LHC, light-harvesting complex(bonded to PSII or PSI, denoted LHCII andLHC1, respectively); LL, low light; PAM, pulseamplitude modulated; PCP, peridinin–chl  a  –pro-tein complex; PG, pigment group; POC, particu-late organic C; PON, particulate organic N The diversity of chloroplast pigmentation in pho-tosynthetic microalgae is extremely large (Rowan1989, Jeffrey and Vesk 1997, Rodrı´guez et al. 2006,Zapata et al. 2006), mainly because of the numberof secondary and tertiary endosymbiotic plastidacquisitions from eukaryotic algae during evolution(Delwiche 1999, Falkowski et al. 2004). In fact, thepigments of marine phytoplankton comprise   10major chlorophylls (chl  a  , chl  b   and divinyl  a   and  b  ,and the chl  c   group; Zapata et al. 2006), >30 differ-ent major carotenoids (carotenes and xanthophylls, Jeffrey and Vesk 1997), and three major phycobili-protein groups (allophycocyanins, phycocyanins,and phycoerythrins; Rowan 1989). In living cells,the pigments are bonded to apoproteins, togetherforming a variety of pigment–protein complexesthat are of considerable taxonomic, phylogenetic,and physiological value (Pre´zelin and Boczar 1986,Green et al. 2003). The dinoflagellate-specific carot-enoid peridinin in  Prorocentrum minimum   (Pavill.) J. Schiller and  Heterocapsa pygmaea   (A. R. Loebl.,R. J. Schmidt et Sherley), for example, differs only by its bonding to different apoproteins in the twospecies: only the ACP (chl  a  –chl  c  –peridinin proteincomplex) in  P. minimum   and both ACP and PCP(peridinin–chl  a  –protein complex) in  H. pygmaea  (Johnsen et al. 1994b, 1997, Jovine et al. 1995).These differences indicate a different phylogenetichistory and are also of photobiological significancedue to their impact on the photophysiologicalresponse of the two species (Schofield et al. 1996).Pigment–protein complexes form light-harvestingcomplexes (LHCs) that transfer the energy absorbed by the different pigments to PSII and PSI(Govindjee 2000). Essentially, the pigment and pro-tein diversity of the LHCs is responsible for the highdiversity of absorption and fluorescence excitation(fl-ex) signatures occurring in algae (Haxo andBlinks 1950, Haxo 1985, Pre´zelin and Boczar 1986, 1 Received 7 December 2006. Accepted 6 June 2007. 2  Author for correspondence: e-mail geir.johnsen@bio.ntnu.no.  J. Phycol.  43,  1236–1251 (2007)   2007 Phycological Society of AmericaDOI: 10.1111/j.1529-8817.2007.00422.x 1236  Neori et al. 1988, Johnsen et al. 1994a, 1997, Fal-kowski and Raven 1997, Lutz et al. 2001, Sakshaugand Johnsen 2005).The algae exhibit considerable intraspecific varia-tion in pigment content largely because of photoacc-limation, regulation of the pigment content tomaximize growth rate in a variable light climate (i.e.,irradiance, its spectral composition, and day length)(Sakshaug and Holm-Hansen 1986). For example,there are more light-harvesting pigments in cellsgrown in low light (LL) than in high light (HL).Photoacclimation plasticity, however, varies consider-ably among species, affecting the biooptical signa-ture of a given species (Johnsen and Sakshaug 1993).In vivo absorption and fl-ex largely have the sameshape, typically exhibiting the red and blue peaks of chl  a.  They differ, however, due to several factors,in particular in vivo fl-ex does not respond toabsorption by photoprotective pigments. Arising inPSII, like the photosynthetic oxygen release, theshape of the fl-ex spectrum resembles that of thecorresponding oxygen action spectrum (Haxo 1985,Neori et al. 1988). A major hindrance for widespread use of in vivofl-ex spectra has been the need for scaling andquantum correction (using dyes or photocounters;Kopf and Heinze 1984, Neori et al. 1988, Sakshauget al. 1991, Johnsen and Sakshaug 1993, Culveret al. 1994, Lutz et al. 2001). Quantum correctionaccounts for variation in spectral quality and energy output from the excitation lamp, and the spectralsensitivity of the instrument photomultiplier. Origi-nally, the dye Rhodamine red was used; however,because it covers only the 400–600 nm band, it misses the strong absorption by chl  a   in the redband (Haxo 1985, Neori et al. 1988). Introducingthe dye Basic Blue 3, Kopf and Heinze (1984) wereamong the first to carry out quantum correctionacross the whole PAR band (400–700 nm). Thisquantum correction method was first used on phyto-plankton by Sakshaug et al. (1991) and was not scaled to the oxygen action spectrum but to theabsorption spectrum, giving the units of the latter. After scaling by matching the peak of the red fl-exto the corresponding absorption peak at 675 nm,the spectra have been used as if they were totalabsorption minus the absorption by photoprotectivepigments. This method can lead to an overestima-tion of light absorbed by PSII in some wavelengthbands (overshoot), a problem particularly pro-nounced in certain haptophytes, cyanobacteria, andcryptophytes with a significant amount of chl  a  bonded to PSI (Johnsen et al. 1992, Johnsen andSakshaug 1996). Five factors can induce overestima-tion of PSII-specific fl-ex spectra, inducing over-shoot in some spectral regions relative to thecorresponding absorption spectra (Johnsen andSakshaug 1993, Johnsen et al. 1997): (i) variablefluorescence caused by sinking or swimming of algalcells and incomplete closing of the reaction centersin PSII [the latter solved by adding dichlorophenylmethyl urea (DCMU; Pre´zelin 1981)], (ii) samplesinsufficiently dense so as to cause an erroneous sig-nal, (iii) PSII–PSI transitions, (iv) the proportion of chl  a   in PSI and PSII usually not being known, and(v) the light energy transfer at 672–679 nm (in vivored peak of chl  a  ) being <100% in major LHCIIantenna (e.g., found in PCP complex in dinoflagel-lates, Johnsen et al. 1997).Using dinoflagellates as model organisms to study the distribution of chl  a   in different pigment pro-teins, and comparing the fl-ex and absorption char-acteristics of whole cells, thylakoid micelles, andfunctional pigment proteins (PCP, ACP, PSII, andPSI), Johnsen et al. (1997) noted that the overshoot problem was eliminated if the fl-ex at the red peakcould be matched to the absorption caused by chl  a  residing in PSII and LHCII only. A more convenient (albeit approximate) procedure was to match the fl-ex to the absorption in the 550–700 nm band wherelight-harvesting pigments that absorb and transferenergy to chl  a   only associated with PSII with closeto 100% efficiency (Johnsen and Sakshaug 1996, Johnsen et al. 1997, Scheer 2006; Table 1, Fig. 2).The two approaches yielded very similar results(Johnsen et al. 1997) and have subsequently formedthe basis for the ‘‘no-overshoot’’ procedure, alsoused by Lutz et al. (1998, 2001).In this study, we compiled our data for in vivoabsorption, fl-ex spectra, and the chl  a   Æ  C ) 1 ratio of 33 species (13 different pigment groups [PGs]) of bloom-forming marine phytoplankton cultures accli-mated to HL and LL. Data for four of the species were published previously, namely those for thedinoflagellates  Prorocentrum minimum  ,  Karlodinium veneficum   (synonym  Gymnodinium galatheanum  ) ,Karenia mikimotoi   (see Table 1 for taxonomicauthors) (synonym  Gyrodinium aureolum  , Johnsenand Sakshaug 1993), and the cyanobacterium  Syn- echococcus   sp. (Johnsen and Sakshaug 1996). Datafor the other 29 species, hitherto unpublished, rep-resent a follow-up of an earlier taxonomic   ⁄   statisticalstudy that dealt with relative log-transformed opticaldensity spectra and the corresponding pigment sig-natures (Johnsen et al. 1994a). We also present esti-mates for the total amount of quanta absorbed by the cell, the fraction of light available to PSII, PSI,and photoprotective carotenoids. The impact of spe-cies and acclimation-dependent variation in PSII-specific light absorption, using PSII scaled fl-exspectra, on the calculation of the oxygen releaserate based on pulse-amplitude-modulated (PAM)and fast-repitition-rate (FRR) fluorometry is dis-cussed.  Fluorescence excitation and scaling to PSII: physiological background.  An in vivo fl-ex spectrum from phyto-plankton describes red light emission (chl  a   fluores-cence) as a function of different excitation wavelengths. The emission intensity is related to theintensity of the different wavelengths of excitation BIOOPTICAL CHARACTERISTICS OF PSII  1237  Table 1.  Species [species nomenclature following Throndsen et al. (2003)], growth conditions, and pigment groups (PGs)based on specific pigment markers (Johnsen et al. 1994a, Rodrı´guez et al. 2006, Zapata et al. 2006). PG SpeciesIrradiance (  E  )( l mol photons  Æ  m ) 2 Æ  s ) 1 )Day length(h)Temperature(  C) PG 1, Bacillariophyceae(fucoxanthin, chl  c  1+2 ) Chaetoceros   cf.  gracilis   Bohlin 75 24 15 Phaeodactylum tricornutum   Bohlin 1550024241515 Skeletonema costatum   Grev. 757524121515 Synedropsis hyperborea   (Grunow) Hasle 35 24 0 Thalassiosira nordenskioeldii   Greve 7533012121515 Thalassiosira pseudonana   Hasle et Heimdal 3550024241515PG 2, Dinophyceae I(peridinin, chl  c  2 ) Ceratium lineatum   (Ehrenb.) Cleve 7533012121515 Gonyaulax spinifera   (Clap. et J. Lachm.) Diesing 7533012121515 Heterocapsa rotundata   (Lohmann) Ge. Hansen 7533012121515 Prorocentrum minimum   (Pavill.) J. Schiller (2 strains) 3550024241515PG 3, Dinophyceae II(acyl-oxy-fucoxanthins,gyroxanthin diester, chl  c  3 ) Karlodinium veneficum   (D. Ballant.) J. Larsen( =Gymnodinium galatheanum  )3017012122020 Karenia mikimotoi   (Miyake et Komin. ex Oda)Ge. Hansen et Moestrup (= Gyrodinium aureolum  )3017012122020PG 4, Coccolithophyceaein Haptophyta (previously Prymnesiophyceae);(acyl-oxy-fucoxanthins,chl  c  3 ) Chrysochromulina simplex   Estep, P. G. Davis, Hargraveset Seiburth3550024241515 Chrysochromulina leadbeateri   Estep, P. G. Davis, Hargraveset Seiburth3550024241515 Chrysochromulina polylepis   Manton et Parke 3550024241515 Chrysochromulina   Lackey sp. B 7533012121515  Emiliania huxleyi   (Lohmann) W. H. Hay et H. Mohler 3550024241515 Phaeocystis globosa   Scherff. 3550024241515 Phaeocystis   cf  . pouchetii   (Hari.) Lagerh. 3550024241515 Prymnesium parvum   cf.  patelliferum   (J. C. Green,D. J. Hibberd et Pienaar) A. Larsen3550024241515PG 5, Pavlovophyceae inHaptophyta (previously Prymnesiophyceae);(fucoxanthin, chl  c  1+2 ) Isochrysis galbana   Parke 3550024241515 Pavlova lutheri   (Droop) J. C. Green 3525024241515PG 6, Prasinophyceae I(prasinoxanthin, [3,8]-proto-chlorophyllide,chl  b  ) Bathycoccus prasinos   Eikrem et Throndsen 3525024241515 Micromonas pusilla   (R. W. Butcher) Manton et Parke 3550024241515 Pseudoscourfieldia marina   (Throndsen) Manton 3550024241515PG 7, Prasinophyceae II(lutein, chl  b  ) Pyramimonas   Schmarda sp. 7533012121515PG 8, Euglenophycea(neoxanthin, chl  b  )  Eutreptiella gymnastica   Throndsen 3550024241515PG 9, Chlorophyceae(lutein, chl  b  )  Dunaliella maritima   Massjuk 3550024241515PG 10, Chrysophyceae(fucoxanthin, chl  c  1+2 ) Pseudopedinella pyriformis   N. Carter 3550024241515PG 11, Raphidophyceae(violaxanthin, chl  c  1+2 ) Heterosigma akashiwo   (Hada) Hada ex Y.Hara et Chihara3550024241515 Olisthodiscus luteus   N. Carter 35 24 18PG 12, Cryptophyceae(phycobiliprotein,alloxanthin, chl  c  2 ) Rhodomonas baltica   G. Karst. 3525024241515PG 13, Cyanophyceae(phycobiliproteins,zeaxanthin) Synechococcus   Na¨geli sp. 35 24 15 1238  GEIR JOHNSEN AND EGIL SAKSHAUG  light absorbed by PSII. In phytoplankton, a photo-synthetic unit consists of PSII, PSI, and their respec-tive LHCs (LHCII and LHCI), in which the bulk of pigments reside (Pre´zelin and Boczar 1986, Greenet al. 2003). Studies of isolated LHCs and photosys-tems have shown that they have different in vivored-peak absorption maxima, ranging from 673 to679 nm (LHCII from 673 to 674, PSII at 676, andPSI at 679 nm). PSII and PSI are conservative withrespect to pigment composition and insensitive tophotoacclimation; thus, most of the pigment diver-sity is associated with the LHCs (Pre´zelin 1981,Pre´zelin and Boczar 1986, Jovine et al. 1995, Johnsen et al. 1997, Green et al. 2003). Since thebulk of LHCs transfer energy to PSII (Green et al.2003, Scheer 2006), PSI receives energy mainly fromits core antenna (Green et al. 2003). Consequently,  95% of the in vivo fluorescence srcinates in PSII,leaving <5% from PSI (Butler 1978, Owens 1991,Kiefer and Reynolds 1992, Johnsen et al. 1997).The mechanisms that regulate energy betweenPSII and PSI differ between phytoplankton contain-ing phycobiliprotein, chl  b  , and chl  c  . Such mechan-isms may be mediated by state transitions (e.g.,phosphorylation and dephosphorylation causingcomponents of LHCII to move, transferring energy to PSI) and   ⁄   or by dissipation of heat in HL condi-tions (de-epoxidation) or epoxidation (from HL toLL conditions) by xanthophyll cycling caused by  D pH across the thylakoid membrane (Falkowski andChen 2003, Raven and Geider 2003). The short-term regulation of light distribution between PSIand PSII makes it possible to vary the optical cross-section of PSII and PSI (Larkum 2003) in responseto variation in spectral irradiance (Falkowski andChen 2003, Raven and Geider 2003). State transi-tions are known in phycobiliprotein-containingcyanobacteria and Rhodophyta, in most green algae,and in higher plants. In contrast, for most chromo-phytes, state transitions are absent or small (Larkumand Howe 1997, Finazzi et al. 1999, MacIntyre et al.2000, Raven and Geider 2003).In extreme cases of differences in chl  a   distribu-tion between PSII and PSI, as in some cyanobacte-ria, cryptophytes, and rhodophytes, in which >70%of total chl  a   is associated with PSI, this energy imbalance between PSII and PSI can be particularly large (Ley and Butler 1980, Myers et al. 1980, Ravenand Geider 2003) and can be manifested in many types of measurements. One example is the classicalEmerson enhancement effect, which arises wheninfrared light (absorbed by PSI) and blue light (absorbed by PSII) are given in combination todiatoms, haptophytes, dinoflagellates, and possibly other groups (Emerson 1958, Schofield et al. 1996). Variations between normalized fl-ex and absorptionspectra for a few species of phytoplankton grownunder different irradiances can be attributed to animbalance of the chl  a   distribution between PSI andPSII, as reported elsewhere (Johnsen et al. 1997,Lutz et al. 1998, 2001, Suggett et al. 2004). Theuncoupling of components of LHCII from PSII hasbeen observed in chlorophytes, dinoflagellates, andcyanobacteria (Mullineuax and Allen 1988, Kroonet al. 1993). The effect of state II–I transitions canbe modeled by introducing the activity of a kinaseand a phosphatase, which will dynamically alter thesize of the LHCs, PSII, and PSI as a function of theredox state of the plastoquinone pool or the cyto-chrome b6   ⁄   f complex (Allen 1995, Kroon andThoms 2006). State II–I transitions and photopro-tective carotenoids affect in vivo PSII fluorescencebecause both can reduce the magnitude and spec-tral characteristics of the fl-ex output.The use of variable fluorescence is commonplacein aquatic research, but we still have major knowl-edge gaps in dealing with light absorption in PSIIand PSI. When estimating absorbed quantaabsorbed and utilized by PSII (oxygenic photosyn-thesis) using variable fluorescence fluorometers,such as PAM fluorometers and FRR fluorometers, we need to distinguish between the optical absorp-tion cross-section of PSII and the functional absorp-tion cross-section of PSII (see definitions below).The PAM technique relies upon independent mea-surements of the spectrally integrated optical cross-section of PSII, which we present here as light absorbed by PSII,  a   PSII  (unit m 2 Æ  mg chl  a  ) 1 ) toobtain absolute rates of photosynthetic electrontransfer (Kroon et al. 1993, Falkowski and Chen2003, Kromkamp and Forster 2003, Suggett et al.2004).In contrast to PAM, the FRR technique enables adirect biophysical estimation of both photochemicalefficiency and light absorption by PSII. Both out-comes are expressed per photosynthetic unit; the first is known as the functional absorption cross-section of PSII reaction centers,  r PSII  [units in A ˚ 2 Æ  quantum ) 1 or mol RCII  Æ  (mol chl  a  ) ) 1 ], and the second as thephotosynthetic unit size of PSII,  n  PSII  [mol RCII  Æ (mol chl  a  ) ) 1 ], according to Falkowski and Kolber(1995). Importantly,  r PSII  is different from  a   PSII because  r PSII  is determined from narrow waveband(  20 nm in the blue part centered   478 nm) andtherefore has to be extrapolated to cover the 400–700 nm by using in vivo PSII-specific fl-ex spectra(Falkowski and Chen 2003). It is presently not knownto what extent   r PSII  is dependent on taxonomic andphotophysiological differences in or between differ-ent PGs of phytoplankton (Suggett et al. 2004).Conversion of   n  PSII  measured as mol O 2 :mol chl  a   tomol RCII:mol chl  a   assumes that four electrons alwaysproduce one O 2  molecule. This assumption may beincorrect depending upon the ratio of cyclic to linearelectron flow, and due to photorespiration and Meh-ler activity (Beardall et al. 2003).By assuming that both photosystems obtain 50%of excitation energy from 400 to 700 nm, a relatively close relationship between biophysical ( r PSII ) andmodeled optical ( a   PSII ) absorption cross-sections BIOOPTICAL CHARACTERISTICS OF PSII  1239  have been shown between chromophytes and cyano-bacteria (Suggett et al. 2004). However, we present several approaches indicating that in general light isnot equally received by PSII and PSI in different PGsof phytoplankton. In addition, photoprotective car-otenoids significantly reduce the fraction of absorbed light reaching PSII in HL-acclimated cells.The maximum quantum yield of oxygenic photo-synthesis ( U max ) is a function of the effectiveabsorption cross-section for a photosynthetic unit for O 2  evolution ( n  r PSII ) and the spectrally inte-grated optical absorption cross-section of phyto-plankton (total absorption of cell [eq. 1], Falkowskiand Chen 2003). U max  ¼  n    r PSII   a   u ð 1 Þ  We propose to use the measured fraction of light absorbed by PSII from spectrally averaged PSII-scaled fl-ex spectra  a   PSII  instead of     a   u  (eq. 1) forthe estimation of quanta absorbed by PSII whenusing instruments based on variable fluorescence. MATERIALS AND METHODS Culture conditions, growth rates, and cell chemistry.  Samples were collected from the midexponential phase of batchcultures kept in 2 L vessels (Teflon or polycarbonate), in amedium of sterilized seawater enriched with the f    ⁄   2 medium of Guillard and Ryther (1962), with silicate enrichment for thediatoms, soil-extract enrichment for euglenophytes and cyano-bacteria, and 20 nM selenite enrichment for haptophytes.Species, PGs, growth conditions, definitions, and symbols areshown in Tables 1 and 2, respectively. Light was provided by cool-white fluorescent tubes from opposite sides of the cultureflasks (Philips TLD 18W    ⁄   95 and TL 40W    ⁄   55, Guilford, Surrey,UK; output and spectral composition varied <3%–5% interms of spectrally weighted phytoplankton absorption).Growth irradiance was 170–500  l mol photons  Æ  m ) 2 Æ  s ) 1 (HL-acclimated cells) or 15–75  l mol photons  Æ  m ) 2 Æ  s ) 1 (LL-acclimated cells) and was measured with a scalar PAR sensor(QSL 100; Biospherical Instruments, San Diego, CA, USA)inside the culture flasks filled with distilled water. Temperaturein the cultures was controlled in the surrounding water bathsby Haake P-3 thermostats (Karlsruhe, Germany). Cultures wereregularly diluted to avoid CO 2  (checked by pH electrode, pH  8.0–8.5) and nutrient limitation. At sampling, >1  l M phos-phate (Merry 1995) was present in the medium.The exponential growth rate was calculated on the basis of measurements of in vivo fluorescence with and without addition of 50  l M DCMU (Serva, Heidelberg, Germany), usinga Turner Designs fluorometer (Sunnyvale, CA, USA) accordingto Sakshaug et al. (1984). Cell counts were made in a BurkerTu¨rk haemocytometer (Assistent, Sontheim, Germany;  n   = 6–9;Table 3). Typically, ±CV of mean values was 26% for cellcounts, 9% in vivo fluorescence, and 6% for in vivo fluores-cence with DCMU.Samples for determination of the chl  a   concentration,checked by HPLC using 7:3 MeOH:acetone (v    ⁄    v) as solvent (see below), typically ranged from 30 to 100  l g chl  a   Æ  L ) 1 ,avoiding significant shading between cells in cultures ( n   = 3,±CV of mean values <5%). In vivo chl  a  –specific absorptioncoefficients ( n   = 3, see section below) were collected on Whatman GF   ⁄   F or GF   ⁄   C glass-fiber filters (Whatman Inc.,Florham Park, NJ, USA) at a differential pressure <50 hPa to Table 2.  Symbols and abbreviations. Symbol Definition  E   Irradiance,  l mol photons  Æ  m ) 2 Æ  s ) 1 PG Pigment groupLHC Light-harvesting complex (bonded to PSII or PSI, denoted LHCII and LHCI)HL, LL High light and low light acclimated cells (see Materials and Methods)Chl  a   Æ  C ) 1 Chl  a   to carbon ratio (w:w)DCMU Dichlorophenyl methyl ureaBiooptical coefficients k max a   u ð red Þ  Red peak in vivo absorption wavelength maximum of chl  a  , 674–679 nm a   u ð red Þ  Red peak in vivo chl  a  –specific absorption coefficient, m 2 Æ  mg chl  a  ) 1 a  C u ð red Þ  Red peak in vivo C-specific absorption coefficient, m 2 Æ  mg C ) 1 Q   a   Specific absorption efficiency, dimensionless, indicates the ‘‘package effect’’ Q   a    ½ a   u ð red Þ = a   u ð red Þ max  .  a   u ð red Þ max  = 0.033 m 2 Æ  mg chl  a  ) 1 a   u ð k Þ  In vivo chl  a  –specific absorption coefficient, m 2 Æ  mg chl  a  ) 1 , LHCI + LHCII & PSI + PSII  F   PSII ð k Þ  Chl  a  –specific PSII scaled fl-ex spectra, m 2 Æ  mg chl  a  ) 1 , LHCII + PSII (from no-overshoot scaling procedure, see Table 4)  F   PSII ð red Þ  Red peak of PSII scaled fl-ex spectra, m 2 Æ  mg chl  a  ) 1 Spectrally averaged absorption and PSII-specific fl-ex spectra a   u  Average total pigment absorption, 400–700 nm, m 2 Æ  mg chl  a  ) 1 , indicating light absorbed by the cell (LHCI + LHCII & PSI + PSII) a   PSII  Average PSII scaled fl-ex spectrum, 400–700 nm, m 2 Æ  mg chl  a  ) 1 indicating the fractionof light absorbed by PSII and LHCIIBiooptical ratios  f   II  Theoretical quanta absorbed by PSII  F  II  The fraction of cellular chl  a   in PSII and its associated LHCII, dimensionless;  F  II  =  F   PSII ð red Þ = a   u ð red Þ  f    AQ  PSII  The fraction of absorbed quanta to PSII, 400–700 nm, dimensionless;  f    AQ  PSII  =  a   PSII = a   u 1240  GEIR JOHNSEN AND EGIL SAKSHAUG
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