The ciliate Mesodinium rubrum and its cryptophyte prey in Antarctic aquatic environments

For growth, the mixotrophic ciliate Mesodinium rubrum sequesters organelles from ingested prey. Feeding in the field is poorly known; however, isolates of an Antarctic M. rubrum strain are maintained in culture on a diet of Teleaulex/Plagioselmis/Geminigera-like cryptophytes, suggesting that cryptophyte presence may be an important factor controlling M. rubrum distribution and abundance in nature. We augmented field surveys with published findings to establish the distribution of both M. rubrum and a Geminigera-like cryptophyte in a range of East Antarctic aquatic environments. We also studied their overwinter abundances in Ace Lake to determine whether a predator–prey relationship existed between the two species in this natural enclosed ecosystem. The ciliate and the cryptophyte co-occurred in the majority of sites sampled which ranged widely in salinity (2.2–150 ‰) and temperature (−3.0–16 °C). We observed a cyclic relationship in species abundances in Ace Lake, implying population growth for M. rubrum responded to changed cryptophyte abundance. Mesodinium rubrum likely exerts top-down pressure on cryptophyte abundances in natural enclosed ecosystems especially under low-light winter conditions. The ciliate can be considered an apex predator in highly truncated food webs.