Chlorella biomaterials: phosphate, starch, oil and possible involvement of autophagy

Abstract

Phosphorus is an essential element for life on the earth and is also crucial for modern agriculture, which is currently dependent on inorganic phosphate fertilizers produced from phosphate rock. Polyphosphate-accumulating organisms (PAOs) have been used for an application of biological phosphorus removal from wastewater. Chlorella is well-known trebouxiophycean green microalgae. When we observed Chlorella cells cultured under the sulfur-depleted condition using transmission electron microscopy, electron dense bodies (DBs) are found in the subcellular region. The commonly-used fluorescent dye, 4', 6-diamidino-2-phenylindole (DAPI), can also be used for polyphosphate detection. Fluorescent microscopic observation shows that structure and subcellular distribution of the DBs are resemble with those of polyphosphate granules detected by DAPI. However, the relationship between polyphosphate, DBs and P-accumulation dynamics has been still unclear. Based on the energy dispersive X-ray analysis, the P signal is detected only in DBs. Molybdenum blue assay and 3D-TEM analysis shows that the DB is the site of polyphosphate accumulation during early and middle age of culture, indicating that Chlorella has a potential as a phosphate-accumulating organism.