Morphological and molecular characterization of two potentially toxigenic species of the genus Pseudo-nitzschia (Bacillariophyceae) in Luzon Island, Philippines

Abstract

Harmful algal blooms are a major concern for the environment and industries like fisheries, healthcare, and tourism in coastal countries such as the Philippines. Blooms of diatoms (Bacillariophyceae) seldom cause harm, but half of the species in the cosmopolitan genus Pseudo-nitzschia produce the neurotoxin domoic acid. Only two published Pseudo-nitzschia surveys in the Philippines carried out morphology-based species identification. Molecular taxonomy has revealed a high level of cryptic diversity corresponding to minute morphological differences. This study sought to explore the diversity of and characterize the genus Pseudo-nitzschia in mariculture zones around Luzon Island using morphological and molecular information. Ten culture strains of Pseudo-nitzschia isolated from shellfish farms were used for morphological and molecular species identification. The secondary structure of the internal transcribed spacer 2 region revealed five strains from Pagbilao Bay to be most similar to P. brasiliana, the first record of this species in Luzon. Meanwhile, four Bacoor isolates and one from Pagbilao clustered within a monophyletic clade representing P. pungens, previously found in Manila Bay. Three single nucleotide polymorphisms and one hemi-compensatory base change were found among the P. pungens strains. Morphological characters, specifically cell shape, apex shape, and poroid density confirmed both species identities. Based on band striae, all P. pungens strains were of the nominal variety, P. pungens var. pungens. This study is among the first culture-based taxonomic studies of diatoms in the Philippines, in this case confirming the presence of harmful microalgae in the coastal ecosystems of the country.