4 6.1 ± 0.6 −0.7 ± 0.1 Photosynthesis measured at 217 μmol

photons m−2 s−1. Standard deviation based on biological triplicates anmol ml−1 min−1 106 cells−1 Fig. 3 C59 wnt Chlorophyll a content of photoheterotrophic versus phototrophic cells. Cells were grown in the presence (A) and absence (B) of acetate in various concentrations of iron and chlorophyll a abundance was determined by HPLC. Standard deviation based on biological triplicates Photosynthetic efficiency of photoheterotrophic versus phototrophic cells Photosynthesis was further assessed by determination of photosynthesis–irradiance curves. In the presence of acetate, the maximum photosynthetic rate (P max) was decreased with respect to decreased iron nutrition (Table 3). Conversely, P max was increased in phototrophically grown severely iron-limited cells (0.1-μM Fe). On the other hand, the relative quantum efficiency of oxygen evolution (α) was decreased in response to decreased iron concentration in both photoheterotrophically and phototrophically grown cells; although, in phototrophic cells the decrease in α is not seen until severe iron limitation (0.1-μM Fe), concomitant with the increase in P max. The increase in P max of phototrophic cells at 0.1-μM Fe results in an increase in the light saturation index

(E k , defined as P max/α). Table 3 Photosynthetic parameters Fe (μM) Acetate CO2 P max a αb E k (P max/α) c P max a αb E k (P max/α) c 0.1 1.7 ± 0.1 0.014 ± 0.001 130 ± 6 3.6 ± 0.2 0.013 ± 0.004 300 ± 140 Akt inhibitor 0.2 1.9 ± 0.0 0.014 ± 0.002 140 ± 21 2.8 ± 0.2 0.021 ± 0.001 140 ± 16 1 2.3 ± 0.2 0.024 ± 0.002 95 ± 1 2.5 ± 0.2 0.022 ± 0.003 120 ± 11 20 2.7 ± 0.4 0.023 ± 0.002 120 ± 25 2.7 ± 0.2 0.022 ± 0.002 120 ± 7 Standard deviation based on biological triplicates anmol O2 (nmol Chl a)−1 min−1 b(nmol O2 [nmol Chl a]−1 min−1)/(μmol photons m−2 s−1) cμmol photons m−2 s−1 Maximum quantum efficiency of PSII in photoheterotrophic versus phototrophic cells F v /F m

is relatively constant, but decreased in stressed cells (Björkman and Demmig ASK1 1987), such as those under iron limitation (Morales et al. 1990, 2000). This parameter, which assesses the maximum quantum efficiency of PSII photochemistry, was lower in iron-limited relative to iron-replete Chlamydomonas cells in the presence of acetate, but remained high in iron-limited cells growing phototrophically, indicating maintenance of light reactions and photochemistry (Table 4). Non-photochemical quenching (NPQ) was likewise decreased in iron-limited acetate-grown cells, but remained high (and perhaps slightly increased) in iron-limited cells without acetate (Fig. 4). This suggests that increased NPQ contributes to the ability of phototrophic iron-limited cells to maintain photosynthesis. At a biochemical level, the abundance of photoprotective xanthophyll cycle pigments was increased in photoheterotrophic iron-limited cells when compared to phototrophic iron-limited cells (Fig. 5).