Mixotrophy in Pyrophacus horologium (thecate dinoflagellate): predation on toxic and harmful dinoflagellates and molecular phylogenetics

Abstract

Pyrophacus horologium, known to be an autotrophic thecate dinoflagellate, was isolated from Masan Bay, Korea. In this study, the morphology, novel molecular characteristics, and feeding characteristics of P. horologium were investigated through the use of a successfully established clonal culture strain. Morphological analysis was conducted using detailed images of living and fixed cells obtained via an inverted light microscope and a scanning electron microscope. The morphological analysis confirmed the classification of P. horologium based on the following Kofoidian plate formula: po, pc, 5 ', 0a, 9 '', c9, 8s, 9 ''', 1p, and 3 ''''. The molecular characterization of P. horologium, which had not been previously reported, was revealed by examining genetic differences between species through ribosomal DNA (rDNA) sequences of the small subunit (SSU), internal transcribed spacer (ITS), and large subunit (LSU) regions. A significant genetic difference was observed between P. horologium and P. steinii, demonstrating that P. horologium is distinct from P. steinii. The feeding characteristics exhibited by P. horologium demonstrated a predilection for thecate dinoflagellates with an equivalent spherical diameter (ESD) ranging from 10.1 to 31.5 mu m, including toxic species such as Alexandrium tamarense, A. catenella, and Gonyaulax spinifera. Furthermore, P. horologium exhibited a marked predilection for G. spinifera and Heterocapsa niei as prey. The process of predation was accomplished through two primary mechanisms: direct dragging of prey to sulcal plates or capture with tow filaments, followed by engulfment and consumption. The present study suggests that P. horologium, previously classified as an autotrophic dinoflagellate, is in fact a mixotrophic dinoflagellate.