Transient initial phase in continuous culture of

Starting microalgae continuous culture generally includes a preliminary batch culture to obtain sufficient cell density. It is possible to apply continuous regime from the beginning of the culture (IC mode) rather than to begin by an initial batch (IB mode). It is our purpose to check that both initial modes lead to identical steady-state cell characteristics. The microalga Isochrysis galbana affinis Tahiti was used for this comparative study. With an initial cell density of 5 × 105 cell ml–1 and a dilution rate of about 1.0 d–1, both IB and IC modes led to identical cell density once steady-state is reached between 6 and 8 d after inoculation in the two cases. Cell concentration of chlorophyll a and pheopigment a were found to be similar for IB and IC modes at steady-state. Initial culture conditions did not influence saturation irradiance and oxygen consumption rate, which were found to be 650 ± 143 μmol phot m–2 s–1 and 1.54 × 10–3 ± 10–5 μmol O2 ml–1 min–1, respectively. At steady-state, nutrient cell uptakes were ρN = 83.3 ± 2.0 fmol N cell–1 d–1 for NO3– and ρP = 5.5 ± 0.4 fmol P cell–1 d–1 for H2PO4– and did not exhibit significant differences between IB and IC modes. Under the prevailing experimental conditions, results show that IC mode involved very similar steady-state cell characteristics when compared to IB mode subsequent steady-state. IC mode could be an attractive alternative especially for experimental laboratory studies, as it should lead to higher flexibility in starting continuous cultures

Effect of nickel and Iron stress on photosynthesis and metabolite production of Heterocapsa cf. bohaiensis (dinoflagellate).

International audienceMicroalgae are well known to produce valuable compounds in response to abiotic stresses. Among these stresses, metals can influence the production of primary or specific metabolites related to reactive oxygen species formation (Barra et al. 2014; Chen et al. 2017; Paliwal et al. 2017). Heterocapsa cf. bohaiensis, a newly described dinoflagellate species with haemolytic activity, and associated with shrimp (Penaeus japonicas) and crab (Eriocheir sinensis) mortalities (Xiao et al. 2018; Zhang et al. 2019), has been recently isolated in New Caledonian iron and nickel-rich waters (Merrot et al. 2022; Pelletier 2006). To induce the metallic stress and the production of metabolites, we exposed H. cf. bohaiensis to high concentrations (10-3M) of nickel (Ni2+) and/or iron (Fe2+) and investigate its photosynthetic efficiency in continuous culture using Pulse Amplitude Modulated chlorophyll fluorescence. The results indicates that H. cf. bohaiensis is tolerant to Ni2+ but sensitive to Fe2+ high concentrations, without cell death. JIP-test suggest that the reduction of photosynthesis in response to Fe2+ is due to a disruption in the electron transport chain rather than a defect in the PSII light absorption and trapping which are enhanced by Fe2+(Adamski et al. 2011; Redillas et al. 2011; Strasser et al.). These results highlight the toxic mechanisms of Fe2+ in microalgae and help to understand the production of biomass under metallic stress. Studying this species in this particular environment will also bring knowledge in the field of aquaculture and ecotoxicology, owing to the transfer of metals in the food chain (bioaccumulation, biosorption) and to the possible production of toxins. This study will also provide some insights for potential valorisations in the biotechnology and/or cosmetics fields, through the analysis of primary (pigments, fatty acids / antioxidants) and specific metabolites, with interesting activities, produced under metal stress conditions