Associations between Mesodinium rubrum and cryptophyte algae in the Columbia River estuary

Recurring blooms of the photosynthetic ciliate Mesodinium rubrum (= Myrionecta rubra) are observed each summer in the Columbia River estuary. Although cultured isolates of M. rubrum have been shown to consume cryptophyte prey during growth, the feeding behavior of M. rubrum in the field is poorly known. In the present study, a 3 mo time series of observations from a locale of putative bloom formation (Ilwaco harbor in Baker Bay, WA) showed that cryptophytes were present at relatively high abundance prior to and during M. rubrum blooms and declined with M. rubrum abundance. During 3 years of observation (summers of 2009, 2010, and 2011), we observed M. rubrum cells bearing numerous cryptophytes attached to the cirri throughout the estuary, especially during the bloom initiation phase and particularly in the peripheral bays. We performed a laboratory investigation in 2011 in which cryptophyte prey were introduced to high-density red-water samples in aquarium tanks. Within 2 h, individual M. rubrum cells collected multiple cryptophytes on their cirri, likely as a precursor to ingestion. The cells ceased their jumping motion and instead exhibited an undulation of the cirri and drifted slowly. We hypothesize that the availability and type of cryptophyte prey is important for bloom formation and that the acquisition of several cryptophytes at once by M. rubrum may be indicative of its ability to ‘gather’ or concentrate cryptophytes from the environment.