Federico Corato

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.

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.

SYMPLEX experiment: first results on oceanic mesoscale dynamics and related primary production from AVHRR and SeaWIFS satellite data and field experiments

Upper ocean dynamics is characterized by a strong variability, at different scales, both in direction and structure of the flow. Mesoscale variability, which is ubiquitous in the world ocean, is often the dominant component in the variance spectrum of velocity with relevant implications on water mass mixing and transformation and on the carbon transfer in the marine food web. Mesoscale activity is manifested through the formation of instabilities, meanders and eddies. Eddies generate either a doming of isopycnals (cyclones) or a central depression (anticyclones). This in turn modifies, among the others, nutrient and organism distributions in the photic zone eventually enhancing or depressing photosynthetic activity and other connected biological responses. The mechanism is similar to what has been thoroughly studied for the warm and cold core rings but at different spatial and temporal scales. The enhancement of phytoplankton growth and the modification of photosynthetic parameters has been shown to occur in situ by means of a modulated fluorescence probe. More recently, an attempt to estimate the magnitude of this specific forcing on nutrient fluxes and primary production has also been conducted at different scales by modeling exercises, though with contrasting estimates the relative importance concerns. Because phytoplankton growth takes place when light, nutrients and cells are found at the same place, the increase in primary production favored by mesoscale eddies cannot be easily predicted. The incident light, the seasonality, the life-time of the structure, its intensity etc. can all influence the final yield. In addition, it has still to be determined which component of the community reacts faster and takes advantage of the new nutrients and how efficiently the new carbon is channeled in the food web. For what remote sensing is concerned, the detectability form the space of such structures is certainly dependent on the depth at which the upward distortion of isopycnals takes places. It can be supposed that a change in bio-optical signature of the whole structure could occur because of the 3-D dynamics of the eddy. If this holds true, then color remote sensing coupled with sea level topography and sea surface temperature should be a powerful tool to track such transient structures. The ALT-SYMPLEX program has been designed to better understand the relationship between short living eddies and carbon transfer in the food web. This is based on several experiments aimed to integrate remote sensing data (ocean color and surface topography) and in situ data in order to evaluate the relationship between surface and sub-surface physical dynamics and its relations on chemical and biological aspects in presence of mesoscale features.