Ewa Lukomska

Enhancement of neutral lipid productivity in the microalga Isochrysis affinis Galbana (T‐Iso) by a mutation‐selection procedure

Microalgae offer a high potential for energetic lipid storage as well as high growth rates. They are therefore considered promising candidates for biofuel production, with the selection of high lipid‐producing strains a major objective in projects on the development of this technology. We developed a mutation‐selection method aimed at increasing microalgae neutral lipid productivity. A two step method, based on UVc irradiation followed by flow cytometry selection, was applied to a set of strains that had an initial high lipid content and improvement was assessed by means of Nile‐red fluorescence measurements. The method was first tested on Isochrysis affinis galbana (T‐Iso). Following a first round of mutation‐selection, the total fatty acid content had not increased significantly, being

Transient initial phase in continuous culture of Isochrysis galbana affinis Tahiti

262\pm 21\,{\rm mgTFA}\,{\rm (gC)}^{- {\rm 1}}

Effects of Nitrogen Limitation on Dunaliella sp.–Alteromonas sp. Interactions: From Mutualistic to Competitive Relationships

for the wild type (WT) and

Response of CO 2 -starved diatom Phaeodactylum tricornutum to light intensity transition

269\pm 49\,{\rm mgTFA}\,{\rm (gC)}^{- {\rm 1}}

High‐affinity nitrate/nitrite transporter genes (Nrt2) in Tisochrysis lutea: identification and expression analyses reveal some interesting specificities of Haptophyta microalgae

for the selected population (S1M1). Conversely, fatty acid distribution among the lipid classes was affected by the process, resulting in a 20% increase for the fatty acids in the neutral lipids and a 40% decrease in the phospholipids. After a second mutation‐selection step (S2M2), the total fatty acid content reached

Optimizing conditions for the continuous culture of Isochrysis affinis galbana relevant to commercial hatcheries

409\pm 64\,{\rm mgTFA}\,{\rm (gC)}^{- {\rm 1}}

Screening and selection of growth-promoting bacteria for Dunaliella cultures

with a fatty acid distribution similar to the S1M1 population. Growth rate remained unaffected by the process, resulting in a 80% increase for neutral lipid productivity. Biotechnol. Bioeng. 2012; 109: 2737–2745.

Effects of blue light on the biochemical composition and photosynthetic activity of Isochrysis sp. (T-iso)

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 o f5◊1 0 5 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 ±1 0 ‐5 µmol O2 ml ‐1 min ‐1 , respectively. At steady-state, nutrient cell uptakes were qN = 83.3 ± 2.0 fmol N cell ‐1 d ‐1 for NO3 ‐ and qP = 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.

Use of a lipid rich strain reveals mechanisms of nitrogen limitation and carbon partitioning in the haptophyte Tisochrysis lutea

Interactions between photosynthetic and non-photosynthetic microorganisms play an essential role in natural aquatic environments and the contribution of bacteria and microalgae to the nitrogen cycle can lead to both competitive and mutualistic relationships. Nitrogen is considered to be, with phosphorus and iron, one of the main limiting nutrients for primary production in the oceans and its availability experiences large temporal and geographical variations. For these reasons, it is important to understand how competitive and mutualistic interactions between photosynthetic and heterotrophic microorganisms are impacted by nitrogen limitation. In a previous study performed in batch cultures, the addition of a selected bacterial strain of Alteromonas sp. resulted in a final biomass increase in the green alga Dunaliella sp. as a result of higher nitrogen incorporation into the algal cells. The present work focuses on testing the potential of the same microalgae–bacteria association and nitrogen interactions in chemostats limited by nitrogen. Axenic and mixed cultures were compared at two dilution rates to evaluate the impact of nitrogen limitation on interactions. The addition of bacteria resulted in increased cell size in the microalgae, as well as decreased carbon incorporation, which was exacerbated by high nitrogen limitation. Biochemical analyses for the different components including microalgae, bacteria, non-living particulate matter, and dissolved organic matter, suggested that bacteria uptake carbon from carbon-rich particulate matter released by microalgae. Dissolved organic nitrogen released by microalgae was apparently not taken up by bacteria, which casts doubt on the remineralization of dissolved organic nitrogen by Alteromonas sp. in chemostats. Dunaliella sp. obtained ammonium-nitrogen more efficiently under lower nitrogen limitation. Overall, we revealed competition between microalgae and bacteria for ammonium when this was in continuous but limited supply. Competition for mineral nitrogen increased with nitrogen limitation. From our study we suggest that competitive or mutualistic relationships between microalgae and bacteria largely depend on the ecophysiological status of the two microorganisms. The outcome of microalgae–bacteria interactions in natural and artificial ecosystems largely depends on environmental factors. Our results indicate the need to improve understanding of the interaction/s between these microbial players.

Microalgae and Biotechnology

In this study, we investigated the responses of Phaeodactylum tricornutum cells acclimated to 300 µmol m−2 s−1 photon flux density to an increase (1000 µmol m−2 s−1) or decrease (30 µmol m−2 s−1) in photon flux densities. The light shift occurred abruptly after 5 days of growth and the acclimation to new conditions was followed during the next 6 days at the physiological and molecular levels. The molecular data reflect a rearrangement of carbon metabolism towards the production of phosphoenolpyruvic acid (PEP) and/or pyruvate. These intermediates were used differently by the cell as a function of the photon flux density: under low light, photosynthesis was depressed while respiration was increased. Under high light, lipids and proteins accumulated. Of great interest, under high light, the genes coding for the synthesis of aromatic amino acids and phenolic compounds were upregulated suggesting that the shikimate pathway was activated. This article is part of the themed issue ‘The peculiar carbon metabolism in diatoms’.