Effect of iron on growth and ultrastructure of Acaryochloris marina

Abstract

The cyanobacterial genus Acaryochloris is the only known group of oxygenic phototrophs that contain chlorophyll d rather than chlorophyll a as the major photosynthetic pigment. Studies on this organism are still in their earliest stages, and biochemical analysis has rapidly outpaced growth optimization. We have investigated culture growth of the major strains of Acaryochloris marina (MBIC11017 and MBIC10697) by using several published and some newly developed growth media. It was determined that heavy addition of iron significantly enhanced culture longevity. These high-iron cultures showed an ultrastructure with thylakoid stacks that resemble traditional cyanobacteria (unlike previous studies). These cultures also show a novel reversal in the pigment ratios of the photosystem II signature components chlorophyll a and pheophytin a, as opposed to those in previous studies.

FIG. 1.

A. marina MBIC11017 growth in the 48 composite media (MBG-11 supplemented with 0.5×, 1×, or 2× concentrations of each K+ESM component). The positive control (CTRL+) was inoculated with contaminated culture stocks and the negative control (CTRL−) with mechanically cleaned cells (as described in Materials and Methods). Both control cultures were grown on standard MBG-11. All 48 test cultures were inoculated with mechanically cleaned cells. The Fe-EDTA additions are labeled relative to K+ESM concentration (0.5× and 2×). The “Average” line represents all growth curves that show no significant features (n = 46). The y-axis absorbance (a.u.) was taken at 730 nm.

FIG. 2.

Growth rate comparison of the two major A. marina strains (n = 4 for each line), both axenic, grown on FeMBG-11. Doubling times as shown are ∼55 h for MBIC11017 and ∼70 h for MBIC10697. Error bars indicate standard deviations.

FIG. 3.

Growth comparison of axenic A. marina MBIC11017 in different standard media (n = 5 for each line). All media are based on published recipes, with the exception of FeMBG-11, which is described herein. The y-axis absorbance (a.u.) was taken at 730 nm. Error bars indicate standard deviations.

FIG. 4.

Full cell electron micrographs of A. marina MBIC11017 cells fixed with osmium tetroxide. Cells grown with their contaminating bacteria on MBG-11 (A and B) have thylakoids which are ubiquitously evenly spaced by an electron-dense material (arrowheads). When seen in cross-section, cells often have thylakoid membranes that are pinched into “corners” (asterisks). Cells grown axenically on MBG-11 with no iron (C) have only sparse areas of electron-dense material spacing thylakoids which are generally appressed (brackets).

FIG. 5.

Electron micrographs of axenic A. marina MBIC11017 cells grown in different published media fixed with potassium permanganate. Cells grown on K+ESM (A), modified K (B), and MBG-11 (C) all have erratically spaced thylakoid membranes. Most thylakoids seen in these cells were appressed peripherally, with interspersed areas of electron-dense, interthylakoid space. Cells grown on FeMBG-11 (D) have thylakoids which are evenly spaced, with a ∼20-nm gap of electron-dense material separating them.