Hervé Claustre

Variability in the chlorophyll‐specific absorption coefficients of natural phytoplankton: Analysis and parameterization

Variability in the chlorophyll (chl) a-specific absorption coefficients of living phytoplankton aph*(λ) was analyzed using a data set including 815 spectra determined with the wet filter technique in different regions of the world ocean (covering the chlorophyll concentration range 0.02–25 mg m−3). The aph* values were observed to decrease rather regularly from oligotrophic to eutrophic waters, spanning over more than 1 order of magnitude (0.18 to 0.01 m2 mg−1) at the blue absorption maximum. The observed covariation between aph*(λ) and the field chl a concentration (chl) can be explained considering (1) the level or pigment packaging and (2) the contribution of accessory pigments to absorption. Empirical relationships between aph*(λ) and 〈chl〉 were derived by least squares fitting to power functions. These relationships can be used to produce aph* spectra as a function of 〈chl〉. Such a simple parameterization, if confirmed with further data, can be used, e.g., for refining estimates of the carbon fixation rate at global or regional scales, such as those obtained by combining satellite pigment concentration maps with primary production models based on physiological parameters, among which aph* is an important one.

Vertical distribution of phytoplankton communities in open ocean: An assessment based on surface chlorophyll

[1] The present study examines the potential of using the near-surface chlorophyll a concentration ([Chla]surf), as it can be derived from ocean color observation, to infer the column-integrated phytoplankton biomass, its vertical distribution, and ultimately the community composition. Within this context, a large High-Performance Liquid Chromatography (HPLC) pigment database was analyzed. It includes 2419 vertical pigment profiles, sampled in case 1 waters with various trophic states (0.03–6 mg Chla m � 3 ). The relationships between [Chla]surf and the chlorophyll a vertical distribution, as previously derived by Morel and Berthon (1989), are fully confirmed. This agreement makes it possible to go further and to examine if similar relationships between [Chla]surf and the phytoplankton assemblage composition along the vertical can be derived. Thanks to the detailed pigment composition, and use of specific pigment biomarkers, the contribution to the local chlorophyll a concentration of three phytoplankton groups can be assessed. With some cautions, these groups coincide with three size classes, i.e., microplankton, nanoplankton and picoplankton. Corroborating previous regional findings (e.g., large species dominate in eutrophic environments, whereas tiny phytoplankton prevail in oligotrophic zones), the present results lead to an empirical parameterization applicable to most oceanic waters. The predictive skill of this parameterization is satisfactorily tested on a separate data set. With such a tool, the vertical chlorophyll a profiles of each group can be inferred solely from the knowledge of [Chla]surf. By combining this tool with satellite ocean color data, it becomes possible to quantify on a global scale the phytoplankton biomass associated with each of the three algal assemblages.

Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters: Analysis and implications for bio-optical models

Spectral absorption coefficients of total particulate matter ap(λ) were determined using the in vitro filter technique. The present analysis deals with a set of 1166 spectra, determined in various oceanic (case 1) waters, with field chl a concentrations (〈chl〉) spanning 3 orders of magnitude (0.02–25 mg m−3). As previously shown [Bricaud et al., 1995] for the absorption coefficients of living phytoplankton ao(λ), the ap(λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link ap(λ) and ao(λ) to 〈chl〉 show striking similarities. Despite large fluctuations, the relative contribution of nonalgal particles to total absorption oscillates around an average value of 25–30% throughout the 〈chl〉 range. The spectral dependence of absorption by these nonalgal particles follows an exponential increase toward short wavelengths, with a weakly variable slope (0.011±0.0025 nm−1). The empirical relationships linking ap(λ) to (〈chl〉) can be used in bio-optical models. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient based on in situ measurements. This agreement is worth noting as independent methods and data sets are compared. It is stressed that for a given (〈chl〉), the ap(λ) coefficients show large residual variability around the regression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflectance of the ocean, are examined, according to whether or not these variations in ap are associated with concomitant variations in particle scattering. In most situations the deviations in ap actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.

Phytoplankton pigment distribution in relation to upper thermocline circulation in the eastern Mediterranean sea during winter

Using a sampling grid of 67 stations, the influence of basin-wide and subbasin-scale circulation features on phytoplankton community composition and primary and new productions was investigated in the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment-based proxy of the ƒ ratio (new production/total production). For the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m−2, and estimated primary and new production were 0.27 and 0.04 g C m−2 d−1, respectively, when integrated between the surface and the depth of the productive zone (1.5 times the euphotic layer). Nanophytoplankton and picophytoplankton (determined from the pigment-derived criteria) were the dominant size classes and contributed to 60 and 27%, respectively, of Tchl a, while microphytoplankton contributed only to 13%. Subbasin and, to a certain extent, mesoscale structures (cyclonic and anticyclonic gyres) were exceptions to this general trend. Anticyclonic gyres were characterized by low Tchl a concentrations (18.8±4.2 mg m−2, with the lowest value being 12.4 mg m−2) and the highest picophytoplankton contribution (40% of Tchl a). In contrast, cyclonic gyres contained the highest Tchl a concentration (40.3±15.3 mg m−2) with the highest microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyre (cyclonic) region show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplankton. We estimate that the Rhodes gyre is a zone of enhanced new production, which is 9 times higher than that in adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the eastern Mediterranean and emphasize the role of subbasin and mesoscale dynamics in driving phytoplankton biomass and composition and, finally, biogeochemical cycling in this area.

Nitrogen- and irradiance-dependent variations of the maximum quantum yield of carbon fixation in eutrophic, mesotrophic and oligotrophic marine systems

Abstract Natural variability of the maximum quantum yield of carbon fixation ( φ C max ), as determined from the initial slope of the photosynthesis-irradiance curve and from light absorption measurements, was studied at three sites in the northeast tropical Atlantic representing typical eutrophic, mesotrophic and oligotrophic regimes. At the eutrophic and mesotrophic sites, where the mixed layer extended deeper than the euphotic layer, all photosynthetic parameters were nearly constant with depth, and φ C max averaged between 0.05 and 0.03 molC (mol quanta absorbed) −1 , respectively. At the oligotrophic site, a deep chlorophyll maximum (DCM) existed and φ C max varied from ca 0.005 in the upper nutrient-depleted mixed layer to 0.063 below the DCM in stratified waters. firstly, φ C max was found roughly to covary with nitrate concentration between sites and with depth at the oligotrophic site, and secondly, it was found to decrease with increasing relative concentrations of non-photosynthetic pigments. The extent of φ C max variations directly related to nitrate concentration was inferred from variations in the fraction of functional PS2 reaction centers ( f ), measured using fast repetition rate fluorometry. Covariations between f and nitrate concentration indicate that the latter factor may be responsible for a 2-fold variation in φ C max . Moreover, partitioning light absorption between photosynthetic and non-photosynthetic pigments suggests that the variable contribution of the non-photosynthetic absorption may explain a 3-fold variation in φ C max , as indicated by variations in the effective absorption cross-section of photosystem 2 ( σ PS2 ). Results confirm the role of nitrate in φ C max variation, and emphasize those of light and vertical mixing.

Extreme diversity in noncalcifying haptophytes explains a major pigment paradox in open oceans

The current paradigm holds that cyanobacteria, which evolved oxygenic photosynthesis more than 2 billion years ago, are still the major light harvesters driving primary productivity in open oceans. Here we show that tiny unicellular eukaryotes belonging to the photosynthetic lineage of the Haptophyta are dramatically diverse and ecologically dominant in the planktonic photic realm. The use of Haptophyta-specific primers and PCR conditions adapted for GC-rich genomes circumvented biases inherent in classical genetic approaches to exploring environmental eukaryotic biodiversity and led to the discovery of hundreds of unique haptophyte taxa in 5 clone libraries from subpolar and subtropical oceanic waters. Phylogenetic analyses suggest that this diversity emerged in Paleozoic oceans, thrived and diversified in the permanently oxygenated Mesozoic Panthalassa, and currently comprises thousands of ribotypic species, belonging primarily to low-abundance and ancient lineages of the “rare biosphere.” This extreme biodiversity coincides with the pervasive presence in the photic zone of the world ocean of 19′-hexanoyloxyfucoxanthin (19-Hex), an accessory photosynthetic pigment found exclusively in chloroplasts of haptophyte origin. Our new estimates of depth-integrated relative abundance of 19-Hex indicate that haptophytes dominate the chlorophyll a-normalized phytoplankton standing stock in modern oceans. Their ecologic and evolutionary success, arguably based on mixotrophy, may have significantly impacted the oceanic carbon pump. These results add to the growing evidence that the evolution of complex microbial eukaryotic cells is a critical force in the functioning of the biosphere.

BOLIDOMONAS: A NEW GENUS WITH TWO SPECIES BELONGING TO A NEW ALGAL CLASS, THE BOLIDOPHYCEAE (HETEROKONTA)

A new algal class, the Bolidophyceae (Heterokonta), is described from one genus, Bolidomonas, gen. nov., and two species, Bolidomonas pacifica, sp. nov and Bolidomonas mediterranea, sp. nov., isolated from the equatorial Pacific Ocean and the Mediterranean Sea, respectively. Both species are approximately 1.2 μm in diameter and have two unequal flagella; the longer flagellum bears tubular hairs, whereas the shorter is smooth. The flagellar basal apparatus is restricted to two basal bodies, and there is no transitional helix. Cells are naked, devoid of walls or siliceous structures. The internal cellular organization is simple with a single plastid containing a ring genophore and a girdle lamella, one mitochondrion with tubular cristae, and one Golgi apparatus close to the basal bodies. The Mediterranean and the Pacific species differ in the insertion angle between their flagella and their pattern of swimming, these differences possibly being linked to each other. Analyses of the SSU rDNA gene place the two strains as a sister group to the diatoms. Moreover, pigment analyses confirm this position, as fucoxanthin is found as the major carotenoid in both lineages. These data strongly suggest that the ancestral heterokont that gave rise to the diatom lineage was probably a biflagellated unicell.

Is desert dust making oligotrophic waters greener

In situ optical measurements provide evidence that oligotrophic waters of the Mediterranean Sea have a greener color than would result from their phytoplankton content alone. This anomaly, detectable in low chlorophyll waters, remains unnoticed in the chlorophyll-rich waters of the nearby waters of the Moroccan upwelling zone. It is due to enhanced absorption in the blue and enhanced backscattering in the green parts of the visible spectrum likely resulting from the presence of submicron Saharan dust in suspension within the upper layer. This result implies that regional estimations of carbon fixation from ocean color images might be significantly overestimated, not only in the Mediterranean Sea, but also in other oligotrophic areas of the Northern hemisphere, subjected to desert dust deposition.

Specific phytoplankton biomasses and their relation to primary production in the tropical North Atlantic

Abstract Phytoplankton pigment concentrations and primary production rates were measured in the North Tropical Atlantic Ocean (20°N, 31°W) in September–October 1991 and in May–June 1992 to provide new insights into the phytoplankton biomass and dynamics of oligotrophic environments. The overall biomass standing stocks were remarkably constant during both periods (around 23 mg chlorophyll a m −2 ), despite marked differences in the water column stratification. The structure of the autotrophic community was also stable: prochlorophytes, cyanobacteria and flagellates were the dominant autotrophic groups and contributed to 36, 30 and 34% of the chlorophyll a biomass in May–June and 43, 30 and 27% in September–October. The vertical distribution of these taxa was also stable with cyanobacteria dominating at the surface (100−10%) of surface irradiance), prochlorophytes at intermediate depths (10−0.1% of surface irradiance) and flagellates below the euphotic zone (0.1−0.01% of surface irradiance). Despite this qualitative and quantitative stability of the phytoplankton biomass, primary production rates were significantly higher ( p −2 d −1 ) than in September–October (267 ± 53 mg C m −2 d −1 ). The cross-section for photosynthesis per unit chlorophyll a was constant during both periods (0.063 m 2 g Chl a −1 ) suggesting that differences in production rates were mainly governed by variations in irradiance. The photic zone accounted for more than 80% of the integrated production, but less than 50% of the chlorophyll a biomass. Analysis of the photoadaptation characteristics of the dominant populations suggests that cyanobacteria and prochlorophyte distributions are mainly regulated by light, whereas flagellate distribution is mainly linked to nutrient availability. The respective distributions of fucoxanthin, 19′-hexanoyloxyfucoxanthin and 19′-butanoyloxyfucoxanthin suggest that, in such oligotrophic environments, a particular group of 19′-butanoyloxyfucoxanthin-containing flagellates, living close to the nitracline, is responsible for the new production associated with the regular diffusion of nitrate, but that diatoms, generally present at background levels, can be responsible for spatio-temporal events of new production.

Variability in particle attenuation and chlorophyll fluorescence in the tropical Pacific: Scales, patterns, and biogeochemical implications

The variability in particle attenuation (cp) and in chlorophyll in situ fluorescence (Fis) was examined in November 1994 along 150°W in the Pacific Ocean. Two main sources of variation in cp and Fis profiles are identified by analyzing data from a 16°S–1°N transect, and from two 5 day stations (5°S and 16°S). The first source reflects changes in the trophic status resulting from prevailing hydrodynamical regimes at large scales. By using flow cytometric data and some assumptions about the size distribution of the different biological stocks, a decomposition of cp into its vegetal (cveg) and nonvegetal (cnveg) components is attempted. Within the euphotic layer, cveg accounts for 43% of the total cp signal at the equator and for only 20% in the South Pacific gyre. The nonvegetal component is then subdivided into heterotrophic organisms and detritus contributions. The detrital material is an important contributor with 43% of cp at 5°S and 55% at 16°S. A further decomposition of Fis and cveg into the three dominant phytoplanktonic groups (Prochlorococcus, Synechococcus, and picoeucaryotes) confirms that picoeucaryotes are important contributors of the vegetal biomass, especially within and below the deep chlorophyll maximum (DCM) (>50% of the algal stock) at 16°S. The second, and essentially local, source of variation is related to specific rhythms in biological and physiological processes. The prominent signals detected during the time series occur at the daily scale: besides the pronounced fluorescence depression at noon in upper layers, particle attenuation in all the layers examined and fluorescence in the DCM display conspicuous daily oscillations. They result from the balance between daytime accumulation and night removal of particles, of algal cells in particular. Finally, the estimation of cp-based growth rates points out the surprisingly rapid turnover time of the whole particulate matter stock in oligotrophic waters (16°S), not only in the euphotic zone (0.63 d−1) but also within the dimly lit layers of the DCM (0.36 d−1). The corresponding growth rate at 5°S, within a quasi-mesotrophic regime, is 0.47 d−1 within the euphotic zone.