Photoacclimatation chez une diatomée arctique (Thalassiosira gravida) dans un contexte de fonte précoce des glaces en arctique

Les floraisons printanières phytoplanctoniques sont plus précoces dans environ 11% de l’océan Arctique dû à un retrait plus hâtif des glaces au printemps. Quelles conditions d’éclairement et de photopériode sont nécessaires à l’initiation de cette floraison ? L’étude présentée vise à quantifier l’effet de l’éclairement et de la durée du jour sur la croissance de la Thalassiosira gravida. La T.gravida a été cultivée en laboratoire sous quatre régimes lumineux représentant une fonte précoce de la banquise aux mois de mars, avril, mai et juin dans la baie de Baffin. La T.gravida a montré une capacité de photoacclimatation lui permettant de maintenir un taux de croissance plus élevé et constant d’avril à juin. Les propriétés photosynthétiques de cette diatomée permettent sa croissance si la fonte printanière de la banquise survient au mois de mars. Toutefois, les conditions environnementales favorables à une floraison plus importante seraient celles des mois d’avril et mai

The Role of Sustained Photoprotective Non-photochemical Quenching in Low Temperature and High Light Acclimation in the Bloom-Forming Arctic Diatom Thalassiosira gravida

Thalassiosira gravida is a major Arctic diatom responsible for the under-ice spring bloom. We investigated T. gravida physiological plasticity growing it at two temperatures (0 and 5°C) and under different light intensities typically found in its natural environment. T. gravida showed remarkable thermal- and photo-acclimatory plasticity including: low light saturation parameter for growth (KE) and photosynthesis (EK), low μmax but relatively high Chl a/C, low C/N, and decreasing light-saturated carbon fixation rate (PmC) with increasing growth irradiance. T. gravida also showed remarkable photoprotective features, namely a strong sustained non-photochemical quenching (NPQs, hour kinetics relaxation) supported by a high amount of xanthophyll cycle pigments. T. gravida growth remained possible under a wide range of irradiances but photosynthetic plasticity was higher at moderately low light (up to ~50 μmol photons m−2 s−1), nevertheless corresponding to the mean in situ conditions under which it predominates, i.e., underneath the spring thin-ice punctuated with melting ponds. The potential role of NPQs in the photophysiological plasticity of T. gravida is discussed

Green Edge ice camp campaigns : understanding the processes controlling the under-ice Arctic phytoplankton spring bloom

The Green Edge initiative was developed to investigate the processes controlling the primary productivity and fate of organic matter produced during the Arctic phytoplankton spring bloom (PSB) and to determine its role in the ecosystem. Two field campaigns were conducted in 2015 and 2016 at an ice camp located on landfast sea ice southeast of Qikiqtarjuaq Island in Baffin Bay (67.4797∘ N, 63.7895∘ W). During both expeditions, a large suite of physical, chemical and biological variables was measured beneath a consolidated sea-ice cover from the surface to the bottom (at 360 m depth) to better understand the factors driving the PSB. Key variables, such as conservative temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured at the ice camp. Meteorological and snow-relevant variables were also monitored. Here, we present the results of a joint effort to tidy and standardize the collected datasets, which will facilitate their reuse in other Arctic studies

The effect of Open Access mandate strength on deposit rate and latency

Access-denial because of the high cost of journal subscriptions is a major obstacle to the progress of research, universities and research funders need to adopt mandates that require their researchers to make their published papers Open Access (OA) by depositing them in their Institutional Repositories. To measure the effectiveness of these mandates MELIBEA has ranked and weighted OA mandates according to their specific requirements, and assigned them an overall score for strength. There is a weak but significant positive correlation between the MELIBEA overall weighted score for mandate strength and the Public Access (PA) deposit rate. If the policy stipulates that deposit is mandatory “For internal use” (e.g., research performance evaluation) deposit rate is significantly higher for PA and RA (Restricted Access) combined; deposit latency for PA alone is also significantly shorter. Finally, if the policy requires that the deposit must be done “At time of acceptance,” deposit rate is significantly higher for combined PA and RA deposits, compared to requiring deposit “At time of publication” or “Unspecified.” This effect is significant only for 2011 and almost significant for all years combined

Photoacclimation of the polar diatom Chaetoceros neogracilis at low temperature

Polar microalgae face two major challenges: 1- growing at temperatures (-1.7 to 5°C) that limit enzyme kinetics; and 2- surviving and exploiting a wide range of irradiance. The objective of this study is to understand the adaptation of an Arctic diatom to its environment by studying its ability to acclimate to changes in light and temperature. We acclimated the polar diatom Chaetoceros neogracilis to various light levels at two different temperatures and studied its growth and photosynthetic properties using semi-continuous cultures. Rubisco content was high, to compensate for low catalytic rates, but did not change detectably with growth temperature. Contrary to what is observed in temperate species, in C. neogracilis, carbon fixation rate (20 min 14C incorporation) equaled net growth rate (μ) suggesting very low or very rapid (<20 min) re-oxidation of the newly fixed carbon. The comparison of saturation irradiances for electron transport, oxygen net production and carbon fixation revealed alternative electron pathways that could provide energy and reducing power to the cell without consuming organic carbon which is a very limiting product at low temperatures. High protein contents, low re-oxidation of newly fixed carbon and the use of electron pathways alternative to carbon fixation may be important characteristics allowing efficient growth under those extreme environmental conditions

S8 Fig -

EKNPQ (A) and NPQmax (B) versus (Dt +Dd)/Chl a at 0°C (circles) and 5°C (circles). In A and B a regression line was fitted on the whole dataset (both 0 and 5°C) with the exception of the datapoints corresponding to 150 and 400 μmol photon m-2 s-1 for which NPQmax and EKNPQ are underestimated (see text). (DOCX)</p

Acclimation of <i>C</i>. <i>neogracilis</i> NPQ.

NPQ versus incubation irradiance at various growth irradiances at 0°C (A) and 5°C (B). Each data point is the mean of 3 cultures measured each day during 3 consecutive days (23, 50, 80, 150, 400 μmol quanta m-2 s-1) or 2 days (10 μmol quanta m-2 s-1). Error bars represent standard deviations. (DOCX)</p

Fig 1 -

(A) Growth rate of C. neogracilis cultures versus growth irradiance at 0°C (circles) and 5°C (triangles). A Poisson function was fitted to the data and gives an estimate of μmax of 0.63 d-1 and a KE of 19 μmol photon m-2 s-1 at 0°C and a μmax of 1.1 d-1 and a KE of 35 μmol photon m-2 s-1 at 5°C. (B) Chl a: C ratio of C. neogracilis cells versus growth irradiance at 0°C (circles) and 5°C (triangles). In (A) each data point is the mean growth rate of 3 cultures measured each day over 10 consecutive days. In (B), each data point is the mean of 3 cultures measured each day over 3 consecutive days (23, 50, 80, 150, 400 μmol photon m-2 s-1) or 2 days (10 μmol photon m-2 s-1). Error bars represent standard deviations.</p

Fig 5 -

EKC versus EKETR (A), EKO2 versus EKETR (B) and EKC versus EKO2 (C) at 0°C (circles) and 5°C (triangles). In A, B, C, the dotted lines represent the 1:1 line. The growth irradiance at which growth rate (μ), carbon fixation (14C), O2 net production, ETR and NPQ saturate (E = EKi) is presented in D at 0°C (white bars) and 5°C (grey bars). In D, horizontal lines indicate growth irradiances (10, 23, 50, 80, 150, 400 μmol photon m-2 s-1).</p

Photoacclimation of carbon fixation.

EKC (A), and α* (B) versus growth irradiance at 0°C (circles) and 5°C (triangles). PCm (squares), PCe (diamonds) and μ (hexagons) versus growth irradiance at 0°C (C) and 5°C (D). PCe versus μ (E) and PCm versus μ at 0°C (circles) and 5°C (triangle). Each data point is the mean of 3 cultures measured each day during 3 consecutive days (50, 80, 400 μmol photon m-2 s-1) or 2 days (10 μmol photon m-2 s-1). In A, E, F, the dotted lines represent the 1:1 line Error bars represent standard deviations.</p