Heterotrophic Nanoflagellate (HNF)

Heterotrophic nanoflagellates (HNF) are small protists (2-20 micrometers) that are the dominant consumers of bacteria in freshwater ecosystems, consuming 30-50% of bacterial production. They use flagella for both locomotion and feeding, and reproduce rapidly by binary fission. HNF are the critical first link in the microbial loop, sitting between bacteria and larger grazers (ciliates, copepods) in the food web. They convert bacterial biomass into prey accessible to ciliates and larger zooplankton. Their primary food is bacteria (preference 1.0, 55% assimilation, ~75% access), with secondary feeding on fungal zoospores (preference 0.4, ~12% access -- only chytrid zoospores at 2-10 um are ingestible; all hyphomycete biomass is far too large), and minor feeding on periphyton (preference 0.15, very limited access since most periphyton cells are too large for HNF to ingest), planktonic algae (preference 0.15), and suspended detritus (preference 0.2). They cannot eat settled detritus. HNF have higher mass-specific ingestion and respiration rates than ciliates (allometric scaling from smaller body size), with a maximum ingestion rate of about 2.4 times body carbon per day. They are important nutrient recyclers, excreting excess NH4 and PO4 from bacterial prey. HNF are freshwater organisms (optimal 0.5 PSU, lethal above 10 PSU) with moderate temperature tolerance (stressed below 8 or above 30 degrees Celsius). They generally require oxygen but tolerate brief hypoxia. HNF populations are regulated by density-dependent viral lysis from giant viruses (Nucleocytoviricota, also called NCLDVs). These viruses infect and lyse HNF cells at a rate that scales with HNF density: maximum lysis rate is 0.03 per hour (~72% per day at high density), with a half-saturation constant of 5e-6 mol C/L (about 0.06 mg C/L). At low densities viral encounters are rare, but as HNF bloom the lysis rate climbs steeply, preventing unchecked population growth that would otherwise suppress bacteria below viable levels. In natural freshwater systems, viral lysis accounts for 10-60% of total HNF mortality (Massana et al. 2007; Montagnes et al. 2008). The HNF lysis rate is higher than for ciliates (0.03 versus 0.02 per hour) because HNF are naked flagellates with no protective cell wall, making them more vulnerable to viral attachment and penetration. When HNF die, 30% of their biomass dissolves directly into the dissolved organic matter (DOM) pool -- HNF are naked flagellates with no cell wall, so they lyse almost immediately on death, releasing cytoplasmic contents (amino acids, sugars, nucleotides) as dissolved organics that bacteria can consume at once. This is the highest death-to-DOM fraction of any consumer, higher than ciliates (25%) which have a denser pellicle that holds together longer (Nagata 2000). The remaining 70% becomes detritus, of which 85% stays suspended given the tiny cell size. Their fecal material is also very fine, with 90% staying suspended. HNF are themselves prey for ciliates (preference 0.7), rotifers (preference 0.5), and Copepods (preference 0.4).