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Diel variation in Prochlorococcus optical properties - Station ...

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1640 Claustre et al.Table 1. Biochemical and <strong>optical</strong> characteristics <strong>in</strong> <strong>Prochlorococcus</strong> PCC 9511 grown <strong>in</strong> turbidostatunder a daily light cycle.Dv-Chl a (fg cell 1 )0.390.06Zeaxanth<strong>in</strong> (fg cell 1 )0.510.09C (fg cell 1 )276C : Dv–Chl a (g : g)7013Zeaxanth<strong>in</strong> : Dv–Chl a (g : g) 1.310.20a* (440) (m 2 mg Dv–Chl a 1 ) 0.08580.0119a* (676) (m 2 mg Dv–Chl a 1 ) 0.01860.0023b* (555) (m 2 mg Dv–Chl a 1 ) 0.11390.0345 b (555) (10 14 m 2 cell 1 ) 4.481.53 c (660) (10 14 m 2 cell 1 ) 3.181.08c* (660) (m 2 gC 1 c )1.040.15Average 1 SD M<strong>in</strong>–Max† Dawn value 1 SD‡0.24–0.500.28–0.6917–3854–940.94–1.740.0686–0.11310.0143–0.02560.0590–0.20391.94–7.691.40–5.380.77–1.32—0.370.08 (73%)194 (101%)562 (93%)1.040.07 (52%)0.07350.0058 (43%)—0.06820.0081 (154%)2.400.46 (182%)1.700.26 (184%)0.810.05 (58%)† Range of observed values.‡ The number <strong>in</strong> parentheses refers to the percentage of <strong>in</strong>crease <strong>in</strong> this parameter from dawn to dusk.Fig. 2. Temporal <strong>variation</strong>s of biochemical characteristics of<strong>Prochlorococcus</strong> PCC 9511 cells grown <strong>in</strong> a cyclostat. (A) Carboncellular concentration. (B) (C) Dv–Chl a ratios.(Nalgene). The replicate spectra were subsequently averaged.The scatter<strong>in</strong>g coefficient, b(), was calculated as thedifference between c() and a(). The absorption [ a ()],scatter<strong>in</strong>g [ b ()], and attenuation [ c ()] cross sections werecalculated by normaliz<strong>in</strong>g a(), b(), and c() by cell density.These coefficients have dimension of m 2 cell 1 . TheDv–Chl a specific coefficients for absorption [a*()] andscatter<strong>in</strong>g [b*()] were calculated by normaliz<strong>in</strong>g a() andb() to the Dv–Chl a concentration and have dimension ofm 2 (mg Dv–Chl a) 1 . The carbon-specific attenuation coefficient,c*() (<strong>in</strong> m 2 gC 1 ), is obta<strong>in</strong>ed by normaliz<strong>in</strong>g c()by the carbon concentration.Results and DiscussionBiomass and biochemical ratios—The pigmentation ofPCC 9511 grown at such high irradiance was very simple:besides the ma<strong>in</strong> pigments zeaxanth<strong>in</strong> and Dv–Chl a, only carotene was detected. Conversely to previous studies onMED4 (Moore et al. 1995; Cailliau et al. 1996; Bricaud etal. 1999), a closely related but not identical clone, the Dv–Chl b/Dv–Chl a ratio was, on average, below 0.06, whichis an <strong>in</strong>dication of adaptation to very high irradiances(Moore et al. 1995).The average cellular Dv–Chl a content (0.39 0.06 fgcell 1 ) (Table 1) was significantly lower than the values reportedby Cailliau et al. (1996) (1.5 fg Dv–Chl a cell 1 ) forMED4 grown at a constant PAR (photosynthetically availableradiation) of 56.7 mol quanta m 2 s 1 , but this valueagrees with the large range of values reported by Morel etal. (1993) (0.15–1.7 fg cell 1 ) or Gibb et al. (2001) (0.22–1.83 fg cell 1 ) for natural <strong>Prochlorococcus</strong> populations. Thecellular zeaxanth<strong>in</strong> (0.51 0.09 fg cell 1 ) was nearly twotimes lower than that reported by Cailliau et al. (1996) andMoore et al. (1995) (1 fg zeaxanth<strong>in</strong> cell 1 ). The same observationapplies for carbon content (27 6 fg C cell 1 ,Table 1) when compared to the results of Cailliau et al.(1996) (49 fg C cell 1 ). These low cellular contents likelyresult from adaptation to high irradiances (e.g., reduction ofthe pigment content) and also possibly of a reduction of cellsize affect<strong>in</strong>g the overall pool of organic material. IndeedBricaud et al. (1999) have shown that cell size of MED4grown under high light is smaller than size of cells grownunder low light.Over the first two cycles <strong>in</strong>vestigated, the daily fluctuations<strong>in</strong> cellular carbon were regular. The C content <strong>in</strong>creaseddur<strong>in</strong>g the day and decreased dur<strong>in</strong>g the night (Fig.2) by a factor of 2 (Table 1). Similarly, the C to Dv–Chla ratio <strong>in</strong>creased dur<strong>in</strong>g the day and decreased just after thelight-to-dark transition. These values are with<strong>in</strong> the rangereported for eukaryotic algae (e.g., Geider 1987).The cellular content of zeaxanth<strong>in</strong> (Fig. 3) always de-

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