Cnidarians containing symbiotic microalgae often inhabit highly variable light environments where successful growth requires that, during transient (potentially stressful) periods of high light (HL), the microalgal cells invest energy in photoprotection to minimise photodamage, or allow for photodamage to occur and invest in photorepair; however, the relative contribution of photoprotection and photorepair remains uncharacterised. Here we determined the light dependence of Photosystem II (PSII) photoinhibition and photorepair in 2 phylotypes of Symbiodinium displaying different susceptibilities to thermal stress. Upon exposure to photon flux densities (PFDs) >500 mu mol photons m(-2)s(-1) the thermally 'sensitive' Strain A1.1 displayed higher net photoinhibition, measured as a decrease in maximum PSII efficiency (F-v/F-m), than the thermally 'tolerant' Strain A1. In contrast, gross photoinhibition, assessed as the decline of F-v/F-m in the presence of an inhibitor of D1 protein synthesis, was similar in the 2 strains. Therefore, photorepair was considered to be the key mechanism minimising net photoinhibition in Strain A1. Consistent with this conclusion, the 2 strains displayed similar capacities for other mechanisms of avoiding photodamage, specifically, photochemical (q(p)) and non-photochemical (NPQ) excitation energy quenching. Measurements on Strain A1 grown under 2 PFDs (100 and 650 mu mol photons m(-2) s(-1)) revealed that photoacclimation to HL involved the upregulation of qp, which minimised gross photoinhibition by maintaining PSII in a more oxidised state. We conclude that both interspecific (e.g. phylotype diversity) and intraspecific (e.g. photoacclimation state) factors affect the susceptibility of Symbiodinium to light stress.

The response in intracellular dimethyl sulfoniopropionate (DMSP) and dissolved DMSP and dimethyl sulfide (DMS) concentrations when Emiliania huxleyi was exposed to acute (1‐h) increases in photon flux densities of photosynthetically active radiation (PAR) and ultraviolet (UV) radiation was examined in cells acclimated to low light (LL, 30 µmol photons m−2 s−1) and high light (HL, 300 vmol photons m−2 s−1). LL‐acclimated cells displayed greater photoinhibition, assessed as a decrease in maximum photochemical efficiency (Fv : Fm). Photoinhibition was increased by exposure to UV wavelengths. LL‐acclimated cells also exhibited more light dissipation through the xanthophyll cycle, evident as changes in de‐epoxidation state. Greater photoinhibition in LL‐acclimated cells corresponded with increased accumulation of DMSP of 21% ± 4% relative to initial concentrations, contrasting with a slight decrease of 5% ± 6% in HL‐acclimated cells. Exposure to UV appeared to decrease the rates of intracellular accumulation of DMSP. Conversely, PAR + UV exposure stimulated the net production of dissolved DMSP and DMS in both HL‐acclimated and LL‐acclimated cultures, compared with high PAR alone. The results indicate a direct link between acute photo‐oxidative stress and DMSP synthesis by E. huxleyi. The physiological basis for increased release of DMSP and DMS from cells due to high PAR + UV exposure is unclear. However, the timescales of changes in intracellular DMSP, dissolved DMSP, and DMS are consistent with variations in light intensity experienced by phytoplankton in a turbulent mixed layer and are similar to rates of change in photosynthetic parameters associated with photoacclimation.

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low‐light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in Fv/Fm, than high‐light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de‐epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross‐section (σPSII) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σPSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a “lake” organization of the light‐harvesting complex.

Circadian variations of pigment content in the diatom Phaeodactylum tricornutum were analyzed in different light regimes. The study was aimed at discerning the role of putative endogenous controls from the constraint imposed by the alternation of light (L) and dark (D) periods. Our experiments showed that in a typical LD cycle of illumination, pigment synthesis follows the somatic growth of the cell, both arresting during D periods. In particular, the diurnal increase of chlorophyll a content was proportional to the increase in cell size and preceding cell division, occurring at night. By contrast, diadinoxanthin and beta-carotene displayed different phases, which is likely to be related to their involvement in photoprotection mechanisms. The experiments also showed that the synthesis of both photosynthetic and photoprotective pigments was dependent not only on light availability and the phasing of somatic growth, but also responded to other internal regulation. Over the time scale of the experiments (hours to days), the removal of LD-DL triggers impaired cell physiology, whereas the circadian patterns in pigment synthesis persisted. Our results support the hypothesis that an internal regulation of cell biosynthetic machinery can improve phytoplankton fitness, even in high variable environments such as the oceanic mixed layers. Therefore, we suggest that phytoplankton growth depends not only on the availability of external resources, but also on internal regulatory mechanisms whose unveiling would further our understanding of phytoplankton diversity and dynamics.

Satellite ocean color measurements can contribute, better than any other source of data, to quantify the spatial and time variability of ocean productivity and, tanks to the success of several satellite missions starting with CZCS up to SeaWiFS, MODIS and MERIS, it is now possible to start doing the investigation of interannual variations and compare level of production during different decades ([1],[2]). The interannual variability of the ocean productivity at global and regional scale can be correctly measured providing that chlorophyll estimate are based on well calibrated algorithms in order to avoid regional biases and instrumental time shifts. The calibration and validation of Ocean Color data is then one of the most important tasks of several research projects worldwide ([3], [4]). Algorithms developed to retrieve chlorophyll concentration need a specific effort to define the error ranges associated to the estimates. In particular, the empirical algorithms, calculated on regression with in situ data, require independent records to verify the degree of uncertainties associated. In addition several evidences demonstrated that regional algorithms can improve the accuracy of the satellite chlorophyll estimates [5]. In 2002, Santoleri et al. (SIMBIOS) first showed a significant overestimation of the SeaWiFS derived chlorophyll concentration in Mediterranean Sea when the standard global NASA algorithms (OC4v2 and OC4v4) are used. The same authors [6] proposed two preliminary new algorithms for the Mediterranean Sea (L-DORMA and NL-DORMA) on a basis of a bio-optical data set collected in the basin from 1998 to 2000. In 2002 Bricaud et al., [7] analyzing other bio-optical data collected in the Mediterranean, confirmed the overestimation of the chlorophyll concentration in oligotrophic conditions and proposed a new regional algorithm to be used in case of low concentrations. Recently, the number of in situ observations in the basin was increased, permitting a first evaluation of the DORMA algorithms with data not used for the algorithms empirical coefficient retrieval and an extensive validation of the new MERIS algorithm. Up to now two possible explanations of the fact that the oligotrophic waters of the Mediterranean Sea are greener than would result from their phytoplankton content alone have been proposed. Claustre et al. [8] suggested that the observed distortion effect on the blue to green ratio can be due to the presence of Saharan dust in the upper layers that enhance absorption in the blue and backscattering in the green. DOR2002 also proposed a tentative explanation of the observed distortion effect based on the hypothesis that enhanced CaCO3 concentration, due to relative abundance of coccolithophores in the oligotrophic waters of the Mediterranean Sea, can cause similar results. In this note, we show a first analysis of the performances of two regional Mediterranean OC algorithms based on SeaWiFS bands (the NL-DORMA and Bricaud et al. 2002 algorithms), the standard OC4v4 NASA algorithm and the MERIS algorithm currently used to produce chlorophyll maps for case I waters. Finally we revisit the Claustre et al. (2002) and DOR2002 hypothesis in the light of new measurements recently published by Malinverno et al. [9] and the results of the ADIOS project of EC.

Primary Production (PP) has been estimated in the Mediterranean Sea from SeaWiFS data for the years 1998-2001 adapting a model developed by Antoine et al. (1996)(1) to Mediterranean conditions. The tuning is based on the use of a new large data set of chlorophyll profiles acquired during the last 10 years in the Mediterranean Sea. Moreover a new data set of cloud cover and temperature has been used and SeaWiFS derived chlorophyll has been estimated using a Mediterranean algorithm recently proposed by D'Ortenzio et al. (2002)(2). Comparison between present model estimates, previous models estimates and in situ data gives satisfactory results and show that the adapted model better reproduces the PP seasonal trend of Mediterranean Sea. The application of the procedure to each pixel of single SeaWiFS images allows the creation of daily, weekly and monthly averaged estimates of depth-integrated PP.

Light is a source of both energy and information for the biota. The spatial, temporal and spectral variability of light experienced by marine phytoplankton differs significantly from that experienced by terrestrial plants, due to the selective attenuation of solar irradiance in the aquatic medium. In the present study we analysed such variability and focused, in particular, on those bands within the spectrum that may act as potential signals for physiological responses. Our results demonstrate that the spectral variation of the light field carries information on the time of day, the vertical position and the presence of very close neighbours, also underwater. This is consistent with the recent findings of a widespread occurrence of photoreceptors in marine algae. We show also that red photoreceptors, whose presence in marine algae was difficult to reconcile with the strong attenuation of long wavelengths by water, may be triggered at depth by the red light generated by transpectral processes.

The possible impact of the Eastern Mediterranean Transient (EMT) on the autotrophic biomass distribution has been investigated through a detailed analysis of remote sensing observations on a basin scale. Since the EMT effect was circumscribed in time, satellite data from Coastal Zone Color Scanner (CZCS) and Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS), relative to the pre‐ and post‐EMT period, respectively, were utilized for the study. The results of the analysis demonstrate that the changes in the circulation of the eastern Mediterranean Sea did not affect the general patterns of biomass distribution in the basin. The chlorophyll a fields at the surface were substantially similar in the CZCS and SeaWiFS periods, showing similar spatial patterns and only a slight difference in the timing of the main events. On the other hand, in the SeaWiFS period a recurrent and large patch of chlorophyll a was detectable in the northwestern Ionian Sea. The analysis of existing data supports the possibility that this new structure is the result of changes related to the EMT, though the observed subregional enhancement of biomass occurred only in an area where concurrent factors such as doming and convection played a synergistic role with the EMT‐induced changes.

The major aim of this paper is the validation of SeaWiFS-derived chlorophyll-a concentration in the Mediterranean Sea. A data set containing in situ chlorophyll-a profiles and optical measurements of in-water and above-water radiances was used to evaluate the performances of several ocean color algorithms in the Mediterranean Sea. The analysis revealed a systematic overestimation of chlorophyll-a concentration by National Aeronautics and Space Administration (NASA) global algorithms (OC2v4 and OC4v4). The error appears to be correlated with chlorophyll-a concentration, by exhibiting marked differences at low values (C<0.15 mg/m3). In particular at low concentration, the bias observed for OC2v4 is about twice that observed for OC4v4. The same analysis made using the Gitelson et al. [J. Mar. Syst. 9 (1996) 283.] Coastal Zone Color Scanner (CZCS) regional algorithm (GIT) revealed that this model underestimates the pigments concentration but it does not exhibit a correlation between the error and the measures. On the other hand, when the NASA standard algorithms are applied to remotely sensed data, the behavior appears reversed: the OC2v4 algorithm exhibits better estimates than OC4v4, which is probably more affected by atmospheric correction problems. When applied to satellite data, the GIT algorithm performs better than the NASA global algorithms, although the estimates are very poor in the high chlorophyll-a range. Two new Mediterranean algorithms are then proposed by fitting our Mediterranean bio-optical data set with linear and OC2-like functional forms. The new algorithms perform well when applied either to the bio-optical measurements or to satellite data. The different behavior of the same algorithm when applied to bio-optical measurements or to remotely sensed data demonstrates that the atmospheric correction is still the main source of error in ocean color data. Due to the relatively small number of available in situ data, the algorithms that we generated have to be considered very preliminary. Discussion was carried out on the reasons of the global algorithm misfit, providing possible explanations and some preliminary result. The influence of coccolithophores and of the yellow substance on the optical response of the Mediterranean waters is investigated, showing that they can at least partially explain the systematic misfit. All the above shows that a region like the Mediterranean Sea requires an independent treatment of the atmospheric and of the in-water bio-optical term to obtain reliable estimates of phytoplankton activity.

A Mediterranean data set containing coincident in situ chlorophyll and Sea WiFS remote sensing reflectance was used to evaluate the performances of several ocean color empirical algorithms. The validation of the algorithms is based on a match up file of 63 coincident in situ and Sea WiFS measurements selected on the basis of a sequence of mask criteria that prevents the use of contaminated pixels. All the algorithms calibrated to global or non-Mediterranean data sets overestimate the chlorophyll concentration in the Mediterranean Sea for low values of chlorophyll-a concentration. A total pigment concentration algorithm based on a regression with Mediterranean data, proposed by Gitelson et al. For CZCS, gives good results for low chlorophyll concentrations but exhibits a scatter larger than OC2 for concentrations higher than 0.2 mg/m3. In this paper we propose a preliminary version of a new Mediterranean ocean color algorithm based on the OC2 functional form with coefficients estimated from a regression with our Mediterranean in situ data.