Concurrent DNA meta‐barcoding and plankton imaging reveal novel parasitic infection and competition in a diatom

Little is known about diatom parasitism in marine systems. Guinardia delicatula, a biomass‐dominant diatom on the Northeast US Shelf (NES), is regularly parasitized by the protistan nanoflagellate, Cryothecomonas aestivalis in this region. While G. delicatula is known to host other protistan parasites, direct observation of these interactions and their dynamics in nature remain elusive. Here, we integrate concurrent DNA meta‐barcoding and automated imaging‐in‐flow cytometry observations to characterize the dynamics of G. delicatula infection by a second parasite, Pirsonia (likely Pirsonia verrucosa). In contrast with C. aestivalis infections, Pirsonia infections are observed sporadically and typically only in a small fraction of the G. delicatula population on the NES. An exception was found in February 2020, when an anomalous co‐infection event was observed in G. delicatula featuring > 20% infection prevalence by Pirsonia and > 10% infection prevalence by C. aestivalis. Investigation of each parasite's infection dynamics' relationship with temperature and salinity suggested that C. aestivalis may consistently dominate G. delicatula infection dynamics due to its wider thermal tolerance range and more cosmopolitan distribution. Pirsonia only appeared capable of dominating G. delicatula infection at temperatures near or below 4°C, a known temperature threshold below which C. aestivalis infection is suppressed. Our results demonstrate the utility of integrating DNA meta‐barcoding and plankton imaging to observe the dynamics of diatom–parasite interactions in marine systems and shed light on the diversity of infection dynamics in diatom–parasite systems and the forcings governing competition among diatom parasites for a single host.