Christophe Brunet

GROWTH INHIBITION AND TOXICITY OF THE DIATOM ALDEHYDE 2‐TRANS, 4‐TRANS‐DECADIENAL ON THALASSIOSIRA WEISSFLOGII (BACILLARIOPHYCEAE)

A common aldehyde present in marine and freshwater diatoms, 2‐trans, 4‐trans‐decadienal (A3), is involved in the wound‐activated response of diatoms to copepod grazing. Upon breakage of the diatom cell membrane, aldehydes are enzymatically produced by the rapid conversion of precursors and strongly impact copepod reproduction by impairing egg production and hatching success, inducing teratogenic embryos modifications. In this study, A3 was assayed with the marine diatom Thalassiosira weissflogii (Grunow) Fryxell et Hasle. The aldehyde concentration necessary to reduce 50% growth rate (EC50) was 0.29 mg·L−1. Decadienal was found to inhibit T. weissflogii cell growth in a dose‐ and time‐dependent manner, with irreversible effects after 24 h of exposure. Decadienal induced a degenerative process, through modifications of cell membrane characteristics, interference with cell cycle progression, and with cell metabolic activity, leading to cell death. A preferential action of A3 on dividing cells was observed. Photosynthetic efficiency significantly decreased upon exposure to the aldehyde, paralleled by an increase in diatoxanthin, suggesting a protective role of this xanthophyll, usually involved in photoprotection. Dying cells exhibited the morphological and biochemical features that bear close resemblance to apoptosis of mammalian cells, including cell shrinkage, chromatin condensation, and degradation of nuclear DNA to nucleosomal size fragments. These data are the first direct evidence to show aldehydes are toxic to diatoms. We suggest a possible nontoxic role of such compounds as chemical signals of unfavorable conditions within the phytoplankton communities, which may be relevant for the population dynamics of diatoms during blooms.

Photoprotection and xanthophyll‐cycle activity in three marine diatoms1

Light is one of the most important factors affecting marine phytoplankton growth, and its variability in time and space strongly influences algal performance and success. The hypothesis tested in this work is that the activity of the xanthophyll cycle and the development of nonphotochemical quenching could be considered a functional trait of algal diversity. If this hypothesis is true, a relationship must exist between fast‐activated pigment variations linked to photoprotective behavior and the ecology of the species. This assumption was tested on three diatoms: Skeletonema marinoi Sarno et Zingone, Thalassiosira rotula Meunier, and Chaetoceros socialis Lauder. These three diatoms occupy different ecological niches. Strains of these diatoms were subjected to five changes in irradiance. Xanthophyll‐cycle activity, quantum yield of fluorescence, and electron transport rate were the main parameters determined. There were marked interspecific differences in xanthophyll‐cycle activity, and these differences were dependent on the light history of the cells. Chaetoceros socialis responded efficiently to changing irradiance, which might relate to its dominance during the spring bloom in some coastal areas. In contrast, T. rotula responded with a slower photoprotection activation, which seems to reflect its more offshore ecological distribution. The photoresponse of S. marinoi (a late‐winter coastal species blooming in the Adriatic Sea) was light‐history dependent, becoming photoinhibited under high light when acclimated to low light, but capable of reaching a high photoprotection level when acclimated to moderate light. Our hypothesis on the photoprotection capacity as a functional trait in microalgae seems to be validated given the results of this study.

Comparative Ecophysiology of the Xanthophyll Cycle in Six Marine Phytoplanktonic Species

The ecophysiology of the photoprotective xanthophyll cycle (XC) was compared in six chlorophyll c-containing pico- and nano-phytoplankton species. Different accessory pigment combinations and ecological properties characterize the six studied species, Bolidomonas mediterranea, Pelagomonas calceolata, Phaeocystis cordata, Phaeocystis sp. (strain RCC186), Mesopedinella arctica and Ochromonas sp. The first experimental set consisted in the study of the activity of the xanthophyll cycle and non-photochemical quenching (NPQ) during two gradual light increases, with photon flux density (PFD) ranging from 40 to 400 micromol photons m(-2)s(-1). Pigments, absorption spectra, flow cytometry measurements for cell counts and chlorophyll a autofluorescence and Electron Transport Rate (ETR) vs. light curves were determined at different times during the experiment. The second set of experiments consisted in using two inhibitors: carotenogenesis inhibitor (norflurazon) and de-epoxidation step (occurring in the xanthophyll cycle) inhibitor (dithiotreitol) during high-light shift, to compare the functioning of the xanthophyll cycle among the different species. Results highlighted a large diversity in the xanthophyll cycle functioning, possibly related to the ecological traits of the species. In view of the results, three groups of species have been identified as (i) high light-adapted, (ii) low light-adapted and (iii) variable light-adapted species.

Environmental characteristics of Agulhas rings affect interocean plankton transport

Agulhas rings provide the principal route for ocean waters to circulate from the Indo-Pacific to the Atlantic basin. Their influence on global ocean circulation is well known, but their role in plankton transport is largely unexplored. We show that, although the coarse taxonomic structure of plankton communities is continuous across the Agulhas choke point, South Atlantic plankton diversity is altered compared with Indian Ocean source populations. Modeling and in situ sampling of a young Agulhas ring indicate that strong vertical mixing drives complex nitrogen cycling, shaping community metabolism and biogeochemical signatures as the ring and associated plankton transit westward. The peculiar local environment inside Agulhas rings may provide a selective mechanism contributing to the limited dispersal of Indian Ocean plankton populations into the Atlantic.

Phytoplankton Pigments: Pigments and photoacclimation processes

This chapter reviews the nature of pigment variations in phytoplankton in response to changes in light regime (irradiance, spectral composition and daylength). These changes belonging to processes called acclimation and/or adaptation maximize the evolutionary fitness of a species, within the constraints set by the environmental conditions (Raven and Geider, 2003). In general, adaptation indicates long-term evolutionary outcome based on the genes a given species obtains (genetic adaptation) while acclimation denotes adjustments in response to variation in key-environmental variables (physiological acclimation). Photo-acclimation corresponds to a mosaic of processes involving many cellular components and occurring over a broad range of time scales, from seconds to days. These processes, covering many physiological, biochemical, biophysical and biological changes, allow the optimization of cell activities, such as photosynthesis, respiration, growth and division when faced with changing irradiance (e.g., Herzig and Dubinsky, 1993; Anning et al., 2000; Raven and Geider, 2003). This is an important issue in phytoplankton ecology because of the fluctuating light environment experienced by pelagic algae, related to daylight variations together with the exponential decrease of light and the vertical – active or passive – movements of algae along the water column. In order to cope with these never-ending fluctuations in light-regime, marine phytoplankton can adjust their pigment pool, which is mainly constituted by two functional categories, namely pigments used for light harvesting and for photoprotection. Many accessory pigments constituting the light-harvesting complexes are photosynthetically active i.e. they are able to transfer the energy absorbed from sunlight to the photosynthetic reaction centers (RC) of photosystems (PS) II and I. They are called light harvesting pigments and include the photosynthetic carotenoids. However, some carotenoids are not involved in photosynthesis and do not transfer the absorbed energy to the RC. These non-photosynthetically active carotenoids are also called photoprotective carotenoids (PPC). The function and dynamics of long-term (hours-days) and short-term (minutes-hours) photo-acclimation are described in the following two sections (4.4.1 and 4.4.2, respectively). The long-term photo-acclimation response mainly consists in changes of structure and composition of the photosystems while the short-term photo-acclimation process mainly concerns the xanthophyll cycle (XC) activation and the associated non-photochemical fluorescence quenching (NPQ). In the third section (4.4.3), the ecophysiological variability of XC and its use as a biological tracer in oceanographic studies is reported.

Distribution and Growth of Aerobic Anoxygenic Phototrophs in the Mediterranean Sea

The distribution of aerobic anoxygenic phototrophs (AAPs) was surveyed in various regions of the Mediterranean Sea in spring and summer. These phototrophic bacteria were present within the euphotic layer at all sampled stations. The AAP abundances increased with increasing trophic status ranging from 2.5 × 10(3) cells per ml in oligotrophic Eastern Mediterranean up to 90 × 10(3) cells per ml in the Bay of Villefranche. Aerobic anoxygenic phototrophs made up on average 1-4% of total prokaryotes in low nutrient areas, whereas in coastal and more productive stations these organisms represented 3-11% of total prokaryotes. Diel bacteriochlorophyll a decay measurements showed that AAP community in the Western Mediterranean grew rapidly, at rates from 1.13 to 1.42 day(-1). The lower AAP abundances registered in the most oligotrophic waters suggest that they are relatively poor competitors under nutrient limiting conditions. Instead, AAPs appear to be metabolically active organisms, which thrive better in more eutrophic environments providing the necessary substrates to maintain high growth rates.

PHYLOGENETIC POSITION OF CRUSTOMASTIX STIGMATICA SP. NOV. AND DOLICHOMASTIX TENUILEPIS IN RELATION TO THE MAMIELLALES (PRASINOPHYCEAE, CHLOROPHYTA)1

A new marine microalga from the Mediterranean Sea, Crustomastix stigmatica Zingone, is investigated by means of LM, SEM, TEM, and pigment and molecular analyses (nuclear‐encoded small subunit [SSU] rDNA and plastid‐encoded rbcL). Pigment and molecular information is also provided for the related species Dolichomastix tenuilepis Throndsen et Zingone. Crustomastix stigmatica has a bean‐shaped cell body 3–5 μm long and 1.5–2.8 μm wide, with two flagella four to five times the body length. The single chloroplast is pale yellow‐green, cup‐shaped, and lacks a pyrenoid. A small bright yellow stigma is located in the mid‐dorsal part of the cell under the chloroplast membrane. An additional accumulation of osmiophilic globules is at times seen in a chloroplast lobe. Cells lack flat scales, whereas three different types of hair‐like scales are present on the flagella. The main pigments of C. stigmatica are those typical of Mamiellales, though siphonein/siphonaxanthin replaces prasinoxanthin and uriolide is absent. The pigment pool of D. tenuilepis is more similar to that of Micromonas pusilla (Butcher) Manton et Parke and of other Mamiellales. The nuclear SSU rDNA phylogeny shows that the inclusion of C. stigmatica and D. tenuilepis in the Mamiellales retains monophyly for the order. The two species form a distinct clade, which is sister to a clade including all the other Mamiellales. Results of rbcL analyses failed to provide phylogenetic information at both the order and species level. No unique morphological or pigment characteristics circumscribe the mamiellalean clade as a whole nor its two daughter clades.

Spectral radiation dependent photoprotective mechanism in the diatom Pseudo-nitzschia multistriata.

Phytoplankton, such as diatoms, experience great variations of photon flux density (PFD) and light spectrum along the marine water column. Diatoms have developed some rapidly-regulated photoprotective mechanisms, such as the xanthophyll cycle activation (XC) and the non-photochemical chlorophyll fluorescence quenching (NPQ), to protect themselves from photooxidative damages caused by excess PFD. In this study, we investigate the role of blue fluence rate in combination with red radiation in shaping photoacclimative and protective responses in the coastal diatom Pseudo-nitzschia multistriata. This diatom was acclimated to four spectral light conditions (blue, red, blue-red, blue-red-green), each of them provided with low and high PFD. Our results reveal that the increase in the XC pool size and the amplitude of NPQ is determined by the blue fluence rate experienced by cells, while cells require sensing red radiation to allow the development of these processes. Variations in the light spectrum and in the blue versus red radiation modulate either the photoprotective capacity, such as the activation of the diadinoxanthin-diatoxanthin xanthophyll cycle, the diadinoxanthin de-epoxidation rate and the capacity of non-photochemical quenching, or the pigment composition of this diatom. We propose that spectral composition of light has a key role on the ability of diatoms to finely balance light harvesting and photoprotective capacity.

The Challenge of Ecophysiological Biodiversity for Biotechnological Applications of Marine Microalgae

In this review, we aim to explore the potential of microalgal biodiversity and ecology for biotechnological use. A deeper exploration of the biodiversity richness and ecophysiological properties of microalgae is crucial for enhancing their use for applicative purposes. After describing the actual biotechnological use of microalgae, we consider the multiple faces of taxonomical, morphological, functional and ecophysiological biodiversity of these organisms, and investigate how these properties could better serve the biotechnological field. Lastly, we propose new approaches to enhancing microalgal growth, photosynthesis, and synthesis of valuable products used in biotechnological fields, mainly focusing on culture conditions, especially light manipulations and genetic modifications.

PHOTOPHYSIOLOGICAL PROPERTIES OF THE MARINE PICOEUKARYOTE PICOCHLORUM RCC 237 (TREBOUXIOPHYCEAE, CHLOROPHYTA)1

The photophysiological properties of strain RCC 237 belonging to the marine picoplanktonic genus Picochlorum, first described by Henley et al., were investigated under different photon flux densities (PFD), ranging from 40 to 400 μmol photons· m−2·s−1, mainly focusing on the development of the xanthophyll cycle and its relationship with the nonphotochemical quenching of fluorescence (NPQ). The functioning of the xanthophyll cycle and its photoprotective role was investigated by applying a progressive increase of PFD and using dithiotreitol and norflurazon to block specific enzymatic reactions in order to study in depth the relationship between xanthophyll cycle and NPQ. These two processes were significantly related only during the gradually increasing light periods and not during stable light periods, where NPQ and zeaxanthin were decoupled. This result reveals that NPQ is a photoprotective process developed by algae only when cells are experiencing increasing PFD or in response to stressful light variations, for instance after a sudden light shift. Results showed that the photobiological properties of Picochlorum strain RCC 237 seem to be well related to the surface water characteristics, as it is able to maintain its photosynthetic characteristics under different PFDs and to quickly activate the xanthophyll cycle under high light.