Hints for understanding microalgal phosphate-resilience from Micractinium simplicissimum IPPAS C-2056 (Trebouxiophyceae) isolated from a phosphorus-polluted site

A novel chlorophyte algae strain with outstanding resilience to high inorganic phosphate (P i ) concentrations in the medium was isolated from a phosphorus-polluted site near a rock phosphate mine. According to the morphological, ultrastructural and genetic criteria the strain was assigned to the species Micractinium simplicissimum H.Chae, H.-G. Choi & J.H.Kim. This strain retained cell viability and growth capacity in the presence of P i concentrations up to 14 g L –1 . The uptake of P i by the cells was moderate (equal to ca. 0.7% increase in cell dry weight P percentage) regardless of the amount of the exogenic P i added to the culture. At the same time, approximately a half of the P i removed by the M. simplicissimum from the culture was reversely adsorbed by the cell surface and/or the intercellular matrix and cell debris. The ultrastructural studies indicated the metabolically active status of the cells together with the presence of phosphorus-rich (likely, polyphosphate) inclusions outside and inside the cells (mainly in vacuoles). We hypothesized that the P i resilience of the studied strain stems from its high P i adsorption capacity together with its ability to throttle the P i influx into the cell preventing the rapid buildup of intracellular P i and potentially toxic short-chain polyphosphate.