Photosynthetic and ultrastructural responses of the chlorophyte Lobosphaera to the stress caused by a high exogenic phosphate concentration

Abstract

Biotechnology of microalgae holds promise for sustainable using of phosphorus, a finite non-renewable resource. Responses of the green microalga Lobosphaera sp. IPPAS C-2047 to elevated inorganic phosphate (P_i) concentrations were studied. Polyphosphate (PolyP) accumulation and ultrastructural rearrangements were followed in Lobosphaera using light and electron microscopy and linked to the responses of the photosynthetic apparatus probed with chlorophyll fluorescence. High tolerance of Lobosphaera to ≤ 50 g L^-1 P_i was accompanied by a retention of photosynthetic activity and specific induction of non-photochemical quenching (NPQ up to 4; F_v/F_m around 0.7). Acclimation of the Lobosphaera to the high P_i was accompanied by expansion of the thylakoid lumen and accumulation of the carbon-rich compounds. The toxic effect of the extremely high (100 g L^-1) P_i inhibited the growth by ca. 60%, induced a decline in photosynthetic activity and NPQ along with contraction of the lumen, destruction of the thylakoids, and depletion of starch reserves. The Lobosphaera retained viability at the P_i in the range of 25-100 g L^-1 showing moderate an increase of intracellular P content (to 4.6% cell dry weight). During the initial high P_i exposure, the vacuolar PolyP biosynthesis in Lobosphaera was impaired but recovered upon acclimation. Synthesis of abundant non-vacuolar PolyP inclusions was likely a manifestation of the emergency acclimation of the cells converting the P_i excess to less metabolically active PolyP. We conclude that the remarkable P_i tolerance of Lobosphaera IPPAS C-2047 is determined by several mechanisms including rapid conversion of the exogenic P_i into metabolically safe PolyP, the acclamatory changes in the cell population structure. Possible involvement of NPQ in the high Pi resilience of the Lobosphaera is discussed.

Keywords: High exogenic phosphate; Microalgae; Non-photochemical quenching; Stress tolerance.