Biological efficacy of iron for marine macroalgae

Abstract

Herein, a chemical washing method for iron (Fe) plaques on the macroalgal cell surface and the biological effectiveness of Fe for macroalgal growth is reported. A 3-component complex of Ti, citric acid, and EDTA was used as the chemical washing agent. Iron (<0.03–10 µmol), nutrients, organic ligands, and light intensity (1.0–100 µmol m^–2s^–1) were controlled during macroalgal culture tests. The biological effectiveness of Fe plaques and organic Fe complexes for macroalgal growth was analyzed using photosynthetic activity. The results suggest that the higher the stability constant of organic ligands in the medium (28.5 for Fe-DTPA), the more the Fe uptake by macroalgae is inhibited, leading to lower intracellular Fe (2.20 mmol g^–1 for DTPA at Fe:L = 1:500) and decreased photosynthetic activity (0.44 for DTPA at Fe:L = 1:500); a moderate ability to form complexes with Fe (16.5 for Fe-MGDA) promotes Fe uptake. In addition, trace element contents of the macroalgae were monitored throughout the year, and seasonal fluctuations in the sea area was conducted by supplying Fe from autumn in Mashike Bay, Hokkaido, Japan. In the monthly survey of Grateloupia divaricata, whole-cell Fe content fluctuated (37.9–280.1 mg kg^–1), with a tendency to decrease from May to September and increase again in October. The same tendency was confirmed for other trace elements like aluminium (27.2–470.9 mg kg^–1) and manganese (3.1–12.9 mg kg^–1). Furthermore, the demand for nutrients decreased as growth slowed after the peak season in spring, and Fe content increased in October as most of the plants died and the relative biomass declined. In contrast, arsenic (10.0–18.0 mg kg^–1) and cadmium (0.22–0.50 mg kg^–1), which are highly toxic to living organisms, tended to increase gradually from May to October. This study might be a new insight to formulate appropriate measures for environmental conservation in coastal areas.