Physiological adaptation of the diatom Pseudo-nitzschia delicatissima under copper starvation.

In the open ocean and particularly in iron (Fe)-limited environment, copper (Cu) deficiency might limit the growth of phytoplankton species. Cu is an essential trace metal used in electron-transfer reactions, such as respiration and photosynthesis, when bound to specific enzymes. Some phytoplankton species, such as the diatom Pseudo-nitzschia spp. can cope with Cu starvation through adaptative strategies. In this study, we investigated the physiological strategies of the marine diatom P. delicatissima against Cu starvation. Compared to the control, Cu starvation inhibited growth by 35%, but did not induce any excess mortality. Despite the bottleneck measured in the electron flow of the photosynthetic chain, cells of P. delicatissima conserved their photosynthesis ability. This photosynthesis maintenance was accompanied by structural changes of membranes, where pigments and lipid composition were strongly modified. Diatoms also strongly modified their metabolism, by redirecting their C allocation to energy storage under the form of triglycerides. By maintaining essential metabolic functions and storing energy under the form of lipids, these physiological adaptations might be a strategy enabling this diatom to later bloom under the return of favorable nutritional condition.

A New Protocol Using Acidification for Preserving DMSP in Macroalgae and Comparison with Existing Protocols.

Dimethylsulfoniopropionate (DMSP) plays many important physiological and ecological roles in macroalgae. The most common method to measure DMSP is by gas chromatography analysis of the dimethylsulfide (DMS) produced after NaOH hydrolysis (pH > 12). Storage of DMS, however, is not recommended for more than a week. We investigated if acidification can be a suitable method to preserve DMSP in macroalgal samples over three months of storage, compared to widely used protocols such as drying and freezing at -20°C. The DMSP content of green (Ulva sp. and Ulva compressa), red (Chondrus crispus), and brown (Bifurcaria bifurcata) macroalgae were analyzed: 24 h after NaOH addition (control values); and after acidification (0.2 mol · L HCl-1 ) for 24 h of fresh material, followed by NaOH addition for 24 h. These values were compared to measurements after 3-month storage of samples that had been either dried in a heater (60°C for a night, and storage at room temperature), or frozen at -20°C, or kept in 0.2 mol · L HCl-1 . There was no significant difference between DMSP measurements on freshly collected material and after acidification of the samples, whether 24 h later or after 3 months of storage. This was in contrast to 3-month storage protocols involving overnight drying at 60°C (75-98% DMSP loss), and to a lesser degree freezing at -20°C (37-80% DMSP loss). We thus advise to acidify macroalgal samples for preservation over long periods of time rather than drying or freezing, when assaying DMSP content.

A call for refining the role of humic-like substances in the oceanic iron cycle.

Primary production by phytoplankton represents a major pathway whereby atmospheric CO2 is sequestered in the ocean, but this requires iron, which is in scarce supply. As over 99% of iron is complexed to organic ligands, which increase iron solubility and microbial availability, understanding the processes governing ligand dynamics is of fundamental importance. Ligands within humic-like substances have long been considered important for iron complexation, but their role has never been explained in an oceanographically consistent manner. Here we show iron co-varying with electroactive humic substances at multiple open ocean sites, with the ratio of iron to humics increasing with depth. Our results agree with humic ligands composing a large fraction of the iron-binding ligand pool throughout the water column. We demonstrate how maximum dissolved iron concentrations could be limited by the concentration and binding capacity of humic ligands, and provide a summary of the key processes that could influence these parameters. If this relationship is globally representative, humics could impose a concentration threshold that buffers the deep ocean iron inventory. This study highlights the dearth of humic data, and the immediate need to measure electroactive humics, dissolved iron and iron-binding ligands simultaneously from surface to depth, across different ocean basins.

Iron organic speciation determination in rainwater using cathodic stripping voltammetry.

A sensitive method using Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) has been developed to determine for the first time iron (Fe) organic speciation in rainwater over the typical natural range of pH. We have adapted techniques previously developed in other natural waters to rainwater samples, using the competing ligand 1-nitroso-2-naphthol (NN). The blank was equal to 0.17±0.05 nM (n=14) and the detection limit (DL) for labile Fe was 0.15 nM which is 10-70 times lower than that of previously published methods. The conditional stability constant for NN under rainwater conditions was calibrated over the pH range 5.52-6.20 through competition with ethylenediaminetetraacetic acid (EDTA). The calculated value of the logarithm of ß'(Fe(3+)(NN)(3)) increased linearly with increasing pH according to log ß'(Fe(3+)(NN)(3)) (salinity=2.9, T=20 °C). The validation of the method was carried out using desferrioxamine mesylate B (DFOB) as a natural model ligand for Fe. Adequate detection windows were defined to detect this class of ligands in rainwater with 40 µM of NN from pH 5.52 to 6.20. The concentration of Fe-complexing natural ligands was determined for the first time in three unfiltered and one filtered rainwater samples. Organic Fe-complexing ligand concentrations varied from 104.2±4.1 nM equivalent of Fe(III) to 336.2±19.0 nM equivalent of Fe(III) and the logarithm of the conditional stability constants, with respect to Fe(3+), varied from 21.1±0.2 to 22.8±0.3. This method will provide important data for improving our understanding of the role of wet deposition in the biogeochemical cycling of iron.

Grazing-induced changes in cell wall silicification in a marine diatom.

In aquatic environments, diatoms (Bacillariophyceae) constitute a central group of microalgae which contribute to about 40% of the oceanic primary production. Diatoms have an absolute requirement for silicon to build-up their silicified cell wall in the form of two shells (the frustule). To date, changes in diatom cell wall silicification have been only studied in response to changes in the growth environment, with consistent increase in diatom silica content when specific growth rates decrease under nutrient or light limitations. Here, we report the first evidence for grazing-induced changes in cell wall silicification in a marine diatom. Cells grown in preconditioned media that had contained both diatoms and herbivores are significantly more silicified than diatoms grown in media that have contained diatoms alone or starved herbivores. These observations suggest that grazing-induced increase in cell wall silicification can be viewed as an adaptive reaction in habitats with variable grazing pressure, and demonstrate that silicification in diatoms is not only a constitutive mechanical protection for the cell, but also a phenotypically plastic trait modulated by grazing. In turn, our results corroborate the idea that plant-herbivore interactions, beyond grazing sensu stricto, contribute to drive ecosystem structure and biogeochemical cycles in the ocean.