Decreased phosphorus incorporation explains the negative effect of high iron concentrations in the green microalga Chlamydomonas acidophila

Highlights: • Iron accumulated to up to 38 pg Fe cell⁻¹ in Chlamydomonas acidophila. • Accumulated iron did not result in oxidative stress response on photosynthesis. • Iron-phosphate complexes decrease bio-availability of phosphorus for uptake. • 260 mg iron per litre resulted in 50% inhibition of phosphate incorporation at pH 2.4. • Unfavourable chemical phosphorus speciation reduced algal growth. The green microalga Chlamydomonas acidophila is an important primary producer in very acidic lakes (pH 2.0–3.5), characterized by high concentrations of ferric iron (up to 1 g total Fe L⁻¹) and low rates of primary production. It was previously suggested that these high iron concentrations result in high iron accumulation and inhibit photosynthesis in C. acidophila. To test this, the alga was grown in sterilized lake water and in medium with varying total iron concentrations under limiting and sufficient inorganic phosphorus (Pi) supply, because Pi is an important growth limiting nutrient in acidic waters. Photosynthesis and growth of C. acidophila as measured over 5 days were largely unaffected by high total iron concentrations and only decreased if free ionic Fe³⁺ concentrations exceeded 100 mg Fe³⁺ L⁻¹. Although C. acidophila was relatively rich in iron (up to 5 mmol Fe: mol C), we found no evidence of iron toxicity. In contrast, a concentration of 260 mg total Fe L⁻¹ (i.e. 15 mg free ionic Fe³⁺ L⁻¹), which is common in many acidic lakes, reduced Pi-incorporation by 50% and will result in Pi-limited photosynthesis. The resulting Pi-limitation present at high iron and Pi concentrations was illustrated by elevated maximum Pi-uptake rates. No direct toxic effects of high iron were found, but unfavourable chemical Pi-speciation reduced growth of the acidophile alga.