Introduction

1. Lamont-Doherty Earth Observatory of Columbia University Palisades New York 10964 USA pgalvez@ldeo.columbia.edu Short exposure Short exposure Long exposure Long exposure Time (h) Time (h) Thalassiosira pseudonana Isochrysis galbana On the relationship between carbon fixation efficiency and bio-optical characteristics of phytoplankton. Prieto, L., Vaillancourt, R.D., Marra, J. Introduction Results and Discussion The density structure and chlorophyll fluorescence distribution of the shelfbreak front is showed in this SeaSoar profile. The front separated two chlorophyll maxima, both were sited over the nutricline. Discrete biological data from both sides of the front were not significant different. The photosynthetic rate of phytoplankton depends on the irradiance, the ability of the cells' pigments to absorb light, and the quantum efficiency for carbon assimilation. The light dependence of photosynthesis can be observed as an instantaneous phenomenon, or one dependent upon the longer-term light history of the cell. In the latter sense, the light regime to which the cell is exposed can impart a change in pigment composition and photo-physiological characteristics that persists as a 'memory' of the cells' recent light history, which in the pelagic ecosystem is closely dependent on the physical regime of the water column. We propose here that the study of the time dependence of photosynthesis can suggest new pathways to estimate primary production. Agood estimation of photosynthetic efficiency of phytoplankton per biomass (aB) was achieved by the following multiple regression: aB = -0.005 + 1.703 aphslope + 0.344 Rec + 0.023 aphblue:red - 0.077 KPAR (error:0.004) being: aphslope = [aph(488)- aph(532)] [aph(676)*(488-532)] -1 , indicator of the proportion of photoprotective pigments (Eisner et al., 2003). Rec (min-1) = [(Fv/Fm(m) - Fv/Fm(0)) (0.65 - Fv/Fm(0)) -1] t -1,recovery kinetics of the quantum yield of PSII (Fv/Fm) in the dark. Fv/Fm values were corrected of phaeophytin levels (Fuchs et al. 2002). aphblue:red = [aph(435)] [aph(676)] -1 , index of pigment packaging. KPAR , calculated every 5 m of the water column. Material and Methods The parameter that explained more variability of aB (r = 0.61) was Rec. In the laboratory we observed that the recovery was dependent on the photon dose experienced by cells, that is, both the duration of exposure and the light intensity. When the duration of the light treatment was 30 min, both T. Field data: Samples for analyses were taken during a late summer (7-17 August, 2002) cruise aboard the RV Endeavor (EN 374) at the New England Shelfbreak near 40º N and 70º W. The cruise was designed to evaluate the coupling of physical and biological processes at a shelfbreak front. Continuous temperature, salinity, chlorophyll fluorescence, and photosynthetically active radiation (PAR) records were obtained by means of a towed undulating SeaSoar, the “Lamont Pumping SeaSoar” which has a pumping system that permits analyses of water pumped onboard with a lag time of about 300 s. Discrete samples were taken from this flow-stream to measure chlorophyll a, phaeopigments, phytoplankton absorption (aph()) and variable fluorescence parameters using a Pulse Amplitude Modulated (PAM) fluorescence system. Photosynthetic parameters were derived from P vs. E experiments performed concurrently on the same water samples spiked with 14C-bicarbonate in a Photosynthetron. pseudonana and I. galbana recovered faster than when the duration was several cell generations. Similarly, the recovery time was shorter when the light intensity was lower. Both speciesrecovered nearly completely after 1.5 h exposures to ML, and T. pseudonanno to HL treatment following several cell generations. There was a difference in the recovery response between the two species examined. T. pseudonana (Demers et al. 1991), as well as other diatoms (see Lavaud et al. 2002), are able to synthesize large quantities of de-epoxidizable xanthophylls. This ability allows them to adjust their photosynthetic mechanism to fluctuating light regime associated with turbulent environments. The high recovery during the first minutes in the dark observed is the consequence of this capacity. The recovery of I. Galbana pointed out that this species has a much slower response to changing light regime than the diatom. Also, we observed chronic photoinhibition (no recovery after 24 h; data not shown) only in the case of I. galbana kept under high light for several generations, but not for the shorter exposure at that light. T. pseudonana, on the other hand,did not suffer chronic photoinhibition in any treatment. Laboratory experiments: Semi-continuous bath cultures of the diatom Thalassiosira pseudonana (clone 3H) and the prymnesiophyte Isochrisys galbana were grown at 20 ± 0.5° C and 40-50 µmol quanta m-2s-1 light. The cultures were exposed to different light doses, varying duration and intensity. “High-light” (HL) and “Medium-light” (ML) intensities were 350 µmol quanta m-2s-1 and 110 µmol quanta m-2s-1, respectively. The cultures were exposed to “long” (several generations) and “short” (30 min) durations at each of the light intensities. During long exposures, new medium was replaced each day to ensure adequate nutrient supply. Conclusion Covariations between the initial slope of the photosynthesis-irradiance curve and fluorescence and photoadaptation absorption parameters of phytoplankton suggest a new pathway to estimate primary production in the marine environment, including the effect of the recent light history. References Demers, S., S. Roy, R. Gagnon, and C. Vignault. 1991. Rapid light-induced changes in cell fluorescence and in xanthophyll-cycle pigments of Alexandrium excavatum (Dinophyceae) and Thalassiosira pseudonana (Baciollariophyceae): a photo-protection mechanism . Mar. Eco. Prog. Ser. 76: 185-193. Eisner, L. B., M. B. Twardowski, and T. J. Cowles, 2003. Resolving phytoplankton photoprotective: photosynthetic carotenoid ratios on fine scales using in situ spectral absorption measurements. Limnol. Oceanogr. 48: 632-646. Fuchs, E., R. C. Zimmerman, and J. S. Jaffe, 2002. The effect of elevated levels of phaeophytin in natural water on variable fluorescence measured from phytoplankton. J. Plankton Res. 24: 1221-1229. Lavaud, J., B. Rousseau, H. J. van Gorkom, and A.-L. Etienne. 2002. Influence of the diadinoxanthin pool size on protoprotection in the marine planktonic diatom Phaeodactylum tricornutum. Plant Physio. 129: 1398-1406.