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

Abstract In June 2023, Pyrophacus horologium, known to be an autotrophic thecate dinoflagellate, was isolated from the Masan Bay, Korea. In this study, we investigated the morphology, novel molecular characteristics, and feeding characteristics of P. horologium in a successfully established clonal culture strain. Morphological analysis was conducted by capturing detailed images of living and fixed cells using 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, X, 5′, 0a, 9′′, 9c, Pc, 8s, 9′′′, 1p, and 3′′′′. We investigated the previously unreported molecular characteristics of P. horologium by examining interspecies differences using the small subunit, internal transcribed spacer, and large subunit ribosomal rDNA regions. A significant genetic difference was observed between P. horologium and Pyrophacus steinii, demonstrating that P. horologium is distinct from P. steinii. The feeding characteristics showed that P. horologium primarily preys on thecate dinoflagellates with an equivalent spherical diameter of 10.1–31.5 µm, including toxic species, i.e., Alexandrium tamarense, Alexandrium catenella, and Gonyaulax spinifera. Additionally, P. horologium strongly preferred G. spinifera and Heterocapsa niei as its prey. It brought the prey directly to its sulcal plates for ingestion or captured it using a tow filament before feeding and ingested its prey through direct engulfment. Thus, based on the results obtained in this study, we propose that P. horologium, earlier classified as an autotrophic dinoflagellate, is a mixotrophic dinoflagellate.