Chemical composition and quantitative cytological measurements were determined for the eustigmatophyte Nannochloropsis sp. Cultures were grown in turbidostats at three irradiance levels: growth‐limiting light, growth‐saturating light and photoinhibiting light. Cellular chlorophyll a content decreased as irradiance level increased, concomitant with a disproportionate reduction in carotenoid content. Nannochloropsis sp. grown in saturating light was characterized by a high content of lipid, fatty acids and carbohydrate compared with cells grown in light‐limiting conditions. The increase in cellular lipid content coincided with a reduction in the percentage of eicosapentaenoic acid (C20:5) and arachidonic acid (C20:4), fatty acids that are mainly associated with galactolipids, and with an increase in the relative abundance of palmitic acid (C16:0) and palmitoleic acid (C16:1). At growth limiting light conditions, Nannochloropsis sp. preferentially synthesized galactolipids; however, as growth became light saturated, relatively more neutral lipids, mainly triacylglycerols, were synthesized. Changes in lipid content and composition were qualitatively related to changes in cell morphology. Cells grown under low light conditions were characterized by a large relative volume of chloroplast, high surface density of thylakoid membranes and low relative volume of lipid storage bodies. The physiological implications of the changes in cellular lipid composition and ultrastructure are discussed in relation to light/shade adaptation.

In the marine unicellular chlorophyte, Dunaliella tertiolecta Butcher, the spectrally averaged m vivo absorption cross section, normalized to chlorophyll a (so‐called a* values), vary two‐fold in response to changes in growth irradiance. We used a kinetic approach to examine the specific factors which account for these changes in optical properties as cells photoadapt. Using Triton X‐100 to solubilize membranes, we were able to differentiate between “package” effects and pigmentation effects. Our analyses suggest that 43–49% of the variability in a* is due to changes in pigmentation, whereas 51–57% is due to the “package” effect. Further analyses revealed that changes in cell sue did not significantly affect packaging, while thylakoid stacking and the transparency of thylakoid membranes were important factors. Our results suggest that thylakoid membrane protein/lipid ratios change during photoadaptation, and these changes influence the effective rate of light harvesting per unit chlorophyll a.

Hypolithic algae were found under flint pebbles in the northern part of the Negev desert in Israel. The algae appeared in the contact area between the stone and the loess soil in which it is partially buried. Two types of flint were found in the research area: dark transparent ones and light opaque ones. A significant difference was found in the distribution of algae in these two types: 46.8% of the dark flints bear algae up to a thickness of 40 mm in the main range of 5 to 15 mm, whereas the respective figure for the light flints was 20.9% up to 30 mm only and in the thickness range of mainly 5 to 10 mm. No significant difference could be shown concerning the temperature underneath the two types of flint stones. The dark flint had a higher extinction coefficient than the light flint and this means that a greater amount of light penetrates to a deeper thickness in the dark flint than in the light. It may be assumed that the differences in light penetration explain the wider distribution under the dark flint than the light flint.

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In this study we report the kinetics of photoacclimation of the unicellular alga Nannochloropsis sp. grown under high light (HL), and subsequently transferred to low light (LL). We examined the changes in ultrastructural features, pigmentation, and photosynthetic parameters over short intervals until the LL steady state was reached. The ultrastructural changes were followed by quantitative morphometric measurements of transmission electron micrographs. We found that the increase in the relative volume of the chloroplast during acclimation to LL (twofold) was accompanied by an increase in number of stacks (twofold) and in the surface area of thylakoids per cell (2.5‐fold). The increase in photosynthetic unit (PSU) density was about 2.15‐fold. Maximal density was about 84 PSU·μm−2 in LL cells, and minimal density was 39 PSU·μm−2 in HL cells. The HL/LL ratio of the in vivo optical absorption cross‐section of PSU (σPSU) was 2.8, whereas in the in vivo optical absorption cross‐section of the cell (σcell), the trend of change was in the opposite direction: 1.7‐fold higher in LL‐acclimated cells than in HL‐acclimated cells. We propose a partial sequence of the photoacclimation processes based on our data and the derived rate constants.