2). This cannot be attributed to a difference in iron bioavailability, since acetate does not impact Fe speciation significantly, nor can it be attributed to a larger cell size, since phototrophically grown cells were actually 10–20% smaller in diameter than photoheterotrophically grown cells

(data not shown). Fig. 2 Iron content of photoheterotrophic versus phototrophic cells in various iron concentrations. Cells were grown in the presence (A) and absence (B) of acetate in various concentrations of iron, and iron content was determined by ICP-MS. Error based on three independent experiments. Asterisk (*) denotes statistically significant differences between acetate BB-94 molecular weight and CO2 (one-way ANOVA, P < 0.05) Photosynthetic and respiratory capacity of photoheterotrophic versus phototrophic cells Because photosynthesis and respiration are the two most iron-rich processes in the cell, photosynthetic and respiratory rates were measured to assess the impact of Fe nutrition on these bioenergetic pathways. Our estimates of in situ photosynthetic rates showed that the oxygen evolution rates this website of photoheterotrophically grown cells (+acetate)

decreased as a function of iron nutrition (Table 2). In phototrophic conditions (−acetate), oxygen evolution rates remained comparable to those in iron-replete acetate-grown cells (approximately 6 nmol ml−1 min−1 per million cells), even under severe iron VX-680 limitation. Similarly, chlorophyll a levels remained steady over a range of iron concentrations in phototrophically grown cells (approximately 5 fmol chl a/cell), whereas in the presence of acetate, chlorophyll a levels correlated with the amount of iron provided in the medium (Fig. 3). The amount of chlorophyll a accumulated in phototrophically grown cells was equivalent to the chlorophyll a level of iron-deficient acetate-grown cells (1-μM Fe). Respiration rates were unaffected by iron nutrition, but were affected instead by carbon source. Acetate-grown cells

had the ability to respire at a rate approximately two times greater than CO2-grown cells (2 nmol ml−1 min−1 per million cells vs. 0.7 nmol ml−1 min−1 per million Florfenicol cells). This is consistent with the increased abundance of respiratory chain components in acetate-grown cells (Naumann et al. 2007). The mechanism contributing to increased abundance of respiratory components in acetate-grown cells is not known. Whole transcriptome analyses (M. Castruita, unpublished) do not give an indication of a specific increase in the expression of genes encoding respiratory components. Table 2 Photosynthetic and respiratory rates of acetate versus CO2-grown cells in various iron concentrations Fe (μM) Acetate CO2 Photosynthetic ratea Respiration ratea Photosynthetic ratea Respiration ratea 0.1 3.1 ± 0.8 −2.1 ± 0.4 5.2 ± 1.4 −0.8 ± 0.1 0.2 3.4 ± 0.7 −1.9 ± 0.2 5.9 ± 0.8 −0.8 ± 0.2 1 4.9 ± 1.2 −1.9 ± 0.6 6.0 ± 0.6 −0.6 ± 0.0 20 6.7 ± 0.8 −2.