Shoshana Didi-Cohen

Effects of Light and Nitrogen Starvation on the Content and Composition of Carotenoids of the Green Microalga Parietochloris incisa

The changes in pigment content and composition of the unicellular alga Parietochloris incisa comb. nov (Trebouxiophyceae, Chlorophyta) were studied. This alga is unique in its ability to accumulate high amounts of arachidonic acid in the cell during cultivation under different irradiances and nitrogen availability in the medium. Under low irradiance of 35 μE/(m2 s) photosynthetically active radiation the P. incisa cultures possessed slow growth and a relatively low carotenoid-to-chlorophyll ratio. At higher irradiances (200 and 400 μE/(m2 s)) on complete medium, the alga displayed higher growth rate and an increase in the carotenoid content, especially that of β-carotene and lutein. Both on nitrogen-free (regardless of illumination intensity) and nitrogen-replete medium (under high light), a considerable increase in the ratio of carotenoid and chlorophyll contents was recorded. Predominant accumulation of xanthophylls took place in thylakoid membranes, whereas β-carotene deposition occurred mainly in the cytoplasmic lipid globules (oil bodies); lower amounts of carotenoids were accumulated in the absence of nitrogen. Under high light and nitrogen-deficiency conditions, an increase in violaxanthin de-epoxidation and nonphotochemical quenching was recorded together with a decline in variable chlorophyll fluorescence (Fv/Fm) level. A possible photoprotective role of carotenoids in adaptation of P. incisa to high light under nitrogen starvation conditions is discussed.

BIOSYNTHESIS OF EICOSAPENTAENOIC ACID (EPA) IN THE FRESHWATER EUSTIGMATOPHYTE MONODUS SUBTERRANEUS (EUSTIGMATOPHYCEAE)1

In an attempt to elucidate the biosynthesis of the polyunsaturated fatty acid eicosapentaenoic acid (20:5ω3, EPA), we treated cultures of the eustigmatophyte Monodus subterraneus Peterson with either salicylhydroxamic acid or the herbicide SAN 9785. Labeled linoleic acid was incorporated into the cultures in the presence and absence of the latter inhibitor, and the redistribution of label was followed. Our results suggest that the major biosynthetic pathway leading to EPA involves fatty acids of the ω6 family. In the early stages of the biosynthesis, 18:1 is predominantly incorporated to the sn‐2 position of phosphatidylcholine, where it is stepwise desaturated by the Δ12 and Δ6 desaturases to 18:3ω6. The latter is released from the lipid, elongated to 20:3ω6 and reincorporated to both positions of phosphatidylethanolamine (PE) where it is further desaturated by the Δ5 and ω3 desaturases to EPA. We suggest that PE is the donor of the 20:5/20:5 diacylglycerol that is imported to the chloroplast to form the eukaryotic‐like molecular species of monogalactosyldiacylglycerol. Likewise, 20:3ω6 can be also incorporated into diacylglyceryltrimethylhomoserine, mostly to the sn‐2 position and similarly desaturated to 20:4ω6 and 20:5ω3. These fatty acids can be exported and incorporated into the sn‐1 position of the prokaryotic‐like molecular species of the chloroplastic lipids. We thus suggest that both the eukaryotic‐like and the prokaryotic‐like molecular species are biosynthesized by different extraplastidial lipids.

Triacylglycerols of the red microalga Porphyridium cruentum can contribute to the biosynthesis of eukaryotic galactolipids.

A mutant of the red microalga Porphyridium cruentum was selected on the basis of impaired growth at suboptimal temperatures (15 vs. 25°C). Fatty acid and lipid analyses revealed diminished proportions of eicosapentaenoic acid (from 41 to 30%) and of the eukaryotic molecular species (from 38 to 28% of monogalactosyldiacylglycerol (MGDG) and elevated proportion (10 vs. 2%) of triacylglycerols (TAG) in the mutant, as compared with the wild type. Pulse labeling of the wild type cells with radioactive fatty acid precursors indicated an initial incorporation of the fatty acids into phosphatidylcholine (PC) and TAG. Following the pulse, the label of PC and TAG decreased with time (from 25 to 5% of the total dpm in TAG) while that of chloroplastic polar lipids, mainly MGDG, continued to increase. In the mutant, however, the labeling of TAG after the pulse was higher (30% of the total dpm) than that of the wild type and decreased only slightly to 20%. This may indicate that in P. cruentum, TAG can contribute to the biosynthesis of eukaryotic species of MGDG.

Downregulation of a putative plastid PDC E1α subunit impairs photosynthetic activity and triacylglycerol accumulation in nitrogen-starved photoautotrophic Chlamydomonas reinhardtii

Summary Impaired carbon precursor supply through cpPDC is detrimental for TAG biosynthesis in Chlamydomonas reinhardtii under conditions of photoautotrophy and nitrogen starvation.

Effects of illumination and nitrogen starvation on accumulation of arachidonic acid by the microalga Parietochloris incisa

Parietochloris incisa is a unicellular freshwater green alga capable of accumulating high amounts of the valuable long-chain polyunsaturated arachidonic acid (AA) in triacylglycerols (TAG) of cytoplasmic oil bodies. To find the cultivation conditions providing maximum AA yield, the effects of illumination and N-availability on the dry weight (DW), chlorophyll, carotenoid, and AA content were studied. Under nitrogen starvation, TAG accounted for over 30% of dry weight (DW) and the AA content became as high as about 55% of total fatty acids. For biomass accumulation, light intensity of ca 400 μE m−2 s−1 was found to be optimal for growing P. incisa on a complete medium. Lower light intensities (or a higher cell density of inoculum) resulted in a higher AA yield when the alga was cultivated on nitrogen-free media. In the absence of nitrogen, algal cells were unable to cope with high illumination and suffered from photooxidative damage, whereas the nutrientsufficient culture survived under such illumination conditions, probably due to accumulation of carotenoids. Nitrogen-deprived P. incisa cells displayed elevated sensitivity to light.