Characterization of the proteomic profiles of the brown tide alga Aureoumbra lagunensis under phosphate- and nitrogen-limiting conditions and of its phosphate limitation-specific protein with alkaline phosphatase activity

The persistent bloomof the brown tide alga Aureoumbra lagunensis has been reported in coastal embayments along southern Texas, but themolecularmechanisms that sustain such algal bloomare unknown.We compared the proteome and physiological parameters of A. lagunensis grown in phosphate (P)-depleted, P- and nitrogen (N)-depleted, and nutrient-replete cultures. For the proteomic analysis, samples fromthree conditions were subjected to two-dimensional electrophoresis and tandemmass spectrometry analysis. Because of the paucity of genomic resources in this species, a de novo cross-species protein search was used to identify the differentially expressed proteins, which revealed their involvement in several key biological processes, such as chlorophyll synthesis, antioxidative protection, and protein degradation, suggesting that A. lagunensis may adopt intracellu-lar nutrient compensation, extracellular organic nutrient regeneration, and damage protection to thrive in P-depleted environments. A highly abundant P limitation-specific protein, tentatively identified as a putative alkaline phosphatase, was further characterized by enzyme activity assay on nondenaturing gel and confocalmicroscopy, which confirmed that this protein has alkaline phosphatase activity, is a cytoplasmic protein, and is closely associated with the cellmembrane. The abundance, location, and functional expression of this alkaline phosphatase all indicate the importance of organic P utilization for A. lagunensis under P limitation and the possible role of this alkaline phosphatase in regenerating phosphate fromextra- or intracellular organic phosphorus.