Alkaline phosphatase gene sequence characteristics and transcriptional regulation by phosphate limitation in Karenia brevis (Dinophyceae)

Abstract

The toxic dinoflagellate Karenia brevis Davis forms harmful algal blooms and has caused devastating consequences in the Gulf of Mexico in the past decades. While phosphorus nutrient is believed to play an important role in the bloom formation, the molecular mechanism is still poorly understood. In this study, we explored alkaline phosphatase (AP) in K. brevis, an enzyme known to facilitate the utilization of dissolved organic phosphorus (DOP) in other phytoplankton when the dissolved inorganic phosphorus (DIP) is limited in the environment. We isolated the full-length cDNAs of AP gene (kbrap) from this species and characterized its expression pattern under contrasting DIP conditions. Analysis of the deduced amino acid sequence revealed that K. brevis AP is a membrane-associated protein, consistent with the predominant cell surface localization of the AP activity detected by enzyme labeled fluorescence labeling. Phylogenetic analyses indicate that kbrap is closest to the homolog from another toxic dinoflagellate, Amphidinium carterae, at both the nucleotide and amino acid levels. The two dinoflagellate APs share similarity in the key domains with diatom APs, and are phylogenetically more closely related to counterparts from diatoms than those from green algae and bacteria. When K. brevis cells were cultured in a DIP-limited medium, the AP activity increased markedly (5–6-fold higher than the initial values), following a slight and brief increase in kbrap expression level, while the cell concentrations in the cultures decreased significantly. The AP enzyme activity and kbrap transcription level were repressed when DIP was resupplied to the DIP-limited cultures. Our results suggest that AP enables K. brevis to grow in DIP-limited and DOP-rich environments, but its mode of response is different from that in A. carterae.