Ciliate Community

Ciliates are single-celled predatory organisms (10-300 micrometers) covered in tiny hair-like cilia that they use for both swimming and sweeping food into their mouths. The model represents a generic freshwater bacterivorous ciliate community (like Tetrahymena, Paramecium, or Colpoda). They are the dominant bacteria-eaters in the system and form the critical link in the microbial loop: they convert bacterial biomass -- which is too small for copepods to efficiently capture -- into ciliate biomass that copepods can eat. Ciliates are highly efficient feeders with very high clearance rates, and they reproduce rapidly by binary fission (cell division). They primarily eat bacteria (preference 1.0), with important secondary feeding on nanoflagellates (preference 0.7) and fungal zoospores (preference 0.5, ~20% access -- only the small chytrid zoospores are ingestible; substrate-embedded hyphae are far too large), and minor feeding on small periphyton cells, planktonic algae, and suspended detritus. Ciliates are nearly continuously active, feeding at 95% of daytime rates even at night. They have a high metabolic rate and are sensitive to environmental stress -- they are strictly freshwater (optimal 0.5 PSU, lethal above 10 PSU) and prefer moderate temperatures (stressed below 8 or above 30 degrees Celsius). They experience starvation mortality when food is insufficient and are moderately sensitive to ammonia toxicity. Ciliates also suffer density-dependent mortality from giant viruses (Nucleocytoviricota) that infect and lyse ciliate cells. The virus-ciliate contact rate scales with ciliate density, so lysis follows a saturating curve: maximum lysis rate is 0.02 per hour (~48% per day at high density), with a half-saturation constant of 5e-6 mol C/L (about 0.06 mg C/L). At low ciliate densities, viral encounters are rare and lysis is negligible; as the population grows, lysis ramps up and acts as a stabilizing feedback that prevents unchecked blooms (Montagnes et al. 2008). This is analogous to the viral lysis that regulates nanoflagellates, but at a lower maximum rate -- ciliates have a denser pellicle that makes them somewhat harder for viruses to infect. When ciliates die, 25% of their biomass dissolves directly into the dissolved organic matter (DOM) pool -- their cells lyse on death, spilling cytoplasmic contents (amino acids, sugars, nucleotides) into the water as dissolved organics that bacteria can immediately consume. The remaining 75% becomes detritus, of which most stays suspended (70% suspended, 30% settled) given the tiny cell size. Their fecal pellets are very small (80% stays suspended). The 25% death-to-DOM fraction is lower than nanoflagellates (30%) because ciliates have a denser pellicle (outer cell covering) that holds together longer before fully dissolving, producing more particulate material (Nagata 2000). Their key ecological contribution is rapid nutrient recycling: they excrete excess nitrogen as ammonium and excess phosphorus as phosphate, making them quickly available to algae again.