Hydrodynamics of microbial filter feeding

What physical constraints govern microbial filter feeding, and what can we derive about the microbial filter feeder as a trait?

Choanoflagellate model with a flagellar vane (A), observed average velocity field for D. grandis (B), and velocity field from CFD model including a 5-μm-wide flagellar vane (C).

Figure: Choanoflagellate model with a flagellar vane (A), observed average velocity field for D. grandis (B), and velocity field from CFD model including a 5-μm-wide flagellar vane (C).

Unicellular microbial filter feeders are an important group of grazers, significant in the microbial loop and aquatic food webs and for biogeochemical cycling.

In this study we quantify the feeding flow of the filter feeding choanoflagellate Diaphanoeca grandis. Choanoflagellates are interesting beyond their role in pelagic food webs because they are believed to be the ancestral form of multicellular life, and the principal cell type can be found in all higher life, including the human kidney that filters our blood.  Choanoflagellates generate a flow of water through a filter by the beating of a single flagellum, and the filter retains sub-micron sized prey particles. We show, however, that the flow rate of water through the filter as estimated from both computational fluid dynamics (CFD) and analytical estimates is more than an order of magnitude lower than the observed filtration rate: The beating flagellum is simply unable to draw enough water through the fine filter. We find similar discrepancies for other choanoflagellate species, highlighting an apparent paradox.

We suggest a radically different pumping mechanism that requires a flagellar vane (sheet), something notoriously difficult to visualize but sporadically observed in the related choanocytes (sponges). With a vane, our model estimates agree well with observed clearance rates, both for our study species and for most other choanoflagellates. We also predict how optimum filter mesh size increases with cell size in microbial filter feeders, a prediction that accords very well with observations.

Read the paper 'Hydrodynamics of microbial filter feeding

Contact

Lasse Tor Nielsen
Researcher
DTU Aqua
+45 35 88 34 11

Contact

Thomas Kiørboe
Professor
DTU Aqua
+45 35 88 34 01

Contact

Anders Peter Andersen
Associate Professor
DTU Physics
+45 45 25 33 12