Getting Attached: A Heterotrophic Nanoflagellate Mingling with Centric Diatoms

Heterotrophic nanoflagellates (HNANs) are central components of the microbial loop, transferring carbon from bacteria to higher trophic levels and facilitating nutrient recycling. While many HNANs are free-swimming, some exhibit enhanced feeding efficiency when attached to surfaces, including diatom frustules. Here, we describe the attachment behavior of a novel interception-feeding HNAN affiliated with the order Bicosoecida to centric diatoms common in North Carolina coastal waters. Using growth experiments, live observations, and time-lapse microscopy, we quantified attachment frequency and assessed its influence on diatom growth for three diatom species: Coscinodiscus sp., Odontella sp., and Rhizosolenia sp. HNAN attachment differed significantly among diatom taxa: Coscinodiscus sp. hosted the highest and most sustained numbers per frustule, whereas after normalizing for surface area, Rhizosolenia sp. exhibited the highest attachment efficiency. Diatom peak growth was 1.2 to 2.1-fold higher and occurred earlier in HNAN co-cultures than in controls, indicating microbial recycling by the HNAN stimulated growth. These findings highlight the nuanced ecological role attached HNANs might play as they exploit diatom-associated boundary layers to enhance bacterial encounter rates. The growth trajectories in our lab experiments suggests that attachment behavior in situ can play a role in driving diatom bloom dynamics and, therefore, play an important role for carbon cycling.