Nicolas Hoepffner

Photoacclimation of Prochlorococcus sp. (Prochlorophyta) Strains Isolated from the North Atlantic and the Mediterranean Sea

Two Atlantic (SARG and NATL1) strains and one Mediterranean (MED) strain of Prochlorococcus sp., a recently discovered marine, free-living prochlorophyte, were grown over a range of “white” irradiances (lg) and under low blue light to examine their photoacclimation capacity. All three strains contained divinyl (DV) chlorophylls (Chl) a and b, both distinguishable from “normal” Chls by their red-shifted blue absorption maximum, a Chl c-like pigment at low concentration, zeaxanthin, and [alpha]-carotene. The presence of two phaeophytin b peaks in acidified extracts from both Atlantic strains grown at high lg suggests that these strains also had a normal Chl b-like pigment. In these strains, the total Chl b to DV-Chl a molar ratio decreased from about 1 at 7.5 [mu]mol quanta m-2 s-1 to 0.4 to 0.5 at 133 [mu]mol quanta m-2 s-1. In contrast, the MED strain always had a low DV-Chl b to DV-Chl a molar ratio, ranging between 0.13 at low lg and 0.08 at high lg. The discrepancies between the Atlantic and MED strains could result from differences either in the number of light-harvesting complexes (LHC) II per photosystem II or in the Chl b-binding capacity of the apoproteins constituting LHC II. Photosynthesis was saturated at approximately 5 fg C(fg Chl)-1 h-1 or 6 fg C cell-1 h-1, and growth was saturated at approximately 0.45 d-1 for both MED and SARG strains at 18[deg]C, but saturating irradiances differed between strains. Atlantic strains exhibited increased light-saturated rates and quantum yield for carbon fixation under blue light.

Determination of the major groups of phytoplankton pigments from the absorption spectra of total particulate matter

The contributions of detrital particles and phytoplankton to total light absorption are retrieved by nonlinear regression on the absorption spectra of total particles from various oceanic regions. The model used explains more than 96% of the variance in the observed particle absorption spectra. The resulting absorption spectra of phytoplankton are then decomposed into several Gaussian bands reflecting absorption by phytoplankton pigments. Such a decomposition, combined with high-performance liquid chromatography data on phytoplankton pigment concentrations, allows the computation of specific absorption coefficients for chlorophylls a, b, and c and carotenoids. The spectral values of these in vivo absorption coefficients are then discussed, considering the effects of secondary pigments which were not measured quantitatively. We show that these coefficients can be used to reconstruct the absorption spectra of phytoplankton at various locations and depths. Discrepancies that do occur at some stations are explained in terms of particle size effect. These coefficients can be used to determine the concentrations of phytoplankton pigments in the water, given the absorption spectrum of total particles.

Depth and wavelength dependence of phytoplankton photosynthesis: implications for the remote sensing of marine primary production

A detailed profile of the photosynthetic parameters and the action spectra was obtained in the Sargasso Sea during late summer, when the upper water column was strongly stratified. Significant depth variation in their magnitudes was present in the lower part of the euphotic zone. The consequences of such vertical structure on the estimation of water column primary production by remote sensing on ocean color are evaluated by sensitivity analysis using a spectra irradiance model. The errors in estimated primary production at depth assuming constant values of the photosynthetic parameters can be of several fold. However, for estimates of daily water-column primary production the errors are generally less than 10 percent.

Absorption modeling in case II waters: the need to distinguish colored dissolved organic matter from nonchlorophyllous particulates

The presented study examines, for substantial datasets in different Case II waters, the variability of the spectral absorption of Non-Chlorophyllous Particles (NCP) and Colored Dissolved Organic Matter (CDOM). The investigation has considered the validity of the currently proposed modeling of the absorption of these tow components as a single variable, in applications to Case II water environments. In order to encompass a broad range of environmental situations in the comparison of NCP and CDOM absorption spectra, two very different sites were selected. The locations investigated were the Southern Baltic Proper and a site in the Northern Adriatic close to the Italian coast. These two regions differ both in their basic oceanographic properties and int heir relative proportions of both space and time, combining a series of seasonal oceanographic campaigns in the Baltic with a complete annual time series of monthly measurements at a fixed point in the Adriatic. The analysis has shown that, in the selected European Case II waters, the observed variability of the spectral absorption of NCP and CDOM, both with respect to each other as well as to the total absorption, are independent. It is therefore suggested that, in the frame of Reflectance modeling in Case II waters, attention must be placed on accounting for NCP an CDOM individually when defining the absorption segment.