Plankton Hydrodynamics
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?
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.
Lasse Tor Nielsen, Seyed Saeed Asadzadeh, Julia Dölger, Jens H. Walther, Thomas Kiørboe and Anders Andersen. Hydrodynamics of microbial filter feeding. Proceedings of the National Academy of Sciences USA 114, 9373–9378 (2017) (pdf)
An analytical model of flagellate hydrodynamics
We present an analytical model framework with which we can predict near-cell flows and trajectories of freely swimming unicellular organisms that swim with different numbers, arrangements, and beat patterns of their flagellar appendages.
Flagellates are unicellular organisms that use long, slender appendages (flagella) to create flows that propel them, support their nutrient and prey uptake, but also expose them to flow-sensing predators. We present an analytical hydrodynamics model that represents the cell as a solid sphere and the flagellar beat as a number of steady or time-varying forces on the water. We show that in a breast-stroke swimming mode with two symmetrically arranged flagella, not only the backwards pointing power stroke, but also the concomitant transversal strokes contribute to propulsion. For an organism with both a longitudinal and a transversal flagellum (figure) we find the trajectory to be helical, the shape of which can be tuned by adjusting the forces produced by the two flagella relative to each other. The transversal flagellum can lead to strong feeding currents to localized capture sites on the cell surface. The model is especially useful for the investigation of prey and nutrient uptake on the cell for different flagellar arrangements and beat patterns due to its generality and adequate representation of the flow close to the cell surface.
Individual of the flagellate species Heterosigma akashiwo (left), helical trajectory of a model flagellate (middle), and velocity field for the flagellate model of H. akashiwo (right).
Julia Dölger, Tomas Bohr and Anders Andersen. An analytical model of flagellate hydrodynamics. Physica Scripta 92, 044003 (2017) (pdf)
Swimming and feeding of mixotrophic biflagellates
Imagine you are a microscopic cell with two thin “arms” and want to survive in the ocean. How should you arrange and move those appendages to swim fast and efficiently, to avoid predators, and to get enough food?
Unicellular planktonic organisms form an essential part of the marine ecosystem. Many of them possess flagella for swimming and to create currents improving the encounter with prey and nutrients. This movement however also leads to a higher predation risk due to flow-sensing predators. In this study we use observations on freely swimming individual flagellates and theoretical modeling to study the flagellar arrangement and beat pattern as a key trait in biflagellates with two symmetrically arranged flagella. We find that equatorial breast-stroke arrangements are advantageous for fast and stealthy swimming, but not for creating favorable currents for prey capture. The observed organisms are able to perform photosynthesis and dissolved nutrient uptake additionally to prey capture. We find this so-called mixotrophic strategy to be necessary for survival, since prey capture alone seems not to fulfil the energy needs of the organisms.
Julia Dölger, Lasse Tor Nielsen, Thomas Kiørboe and Anders Andersen, Swimming and feeding of mixotrophic biflagellates,Scientific Reports 7, 39892 (2017) (pdf)
Quiet swimming at low Reynolds number
Plankton are microscopic organisms that inhabit the water masses of the oceans. They are faced with a dilemma: They need to swim to find food and mates, but by swimming they inevitably create flow disturbances that attract predators. We have shown that planktonic swimmers can reduce the flow disturbances that they create in the water around them, simply by appropriately arranging their propulsion apparatus. In particular we explored a simple mathematical model of the water flow due to small breast stroke swimmers. We found that breast stroke swimming is a quiet swimming mode that causes significantly reduced flow disturbances in comparison with other types of swimming. Breast stroke swimming may thus be advantageous, and this might explain why it is very common in the world of the plankton.
Anders Andersen, Navish Wadhwa, and Thomas Kiørboe, Quiet swimming at low Reynolds number, Physical Review E 91, 042712 (2015) (pdf)
Hydrodynamics and energetics of jumping copepod nauplii and copepodids
Many zooplankton inhabit an intermediate Reynolds number regime, which is poorly understood in its mechanics and ecological implications. In a recent study using particle image velocimetry, we have quantified the flows around a tiny but abundant aquatic organism - the copepod. A copepod goes through dramatic changes in shape, size, and swimming gait, as it goes through the different stages of its life cycle. The anatomical changes are bound to result in physical changes, affecting the hydrodynamics of swimming and feeding. By measuring the fluid flow around a swimming copepod, we have quantified these changes.
Navish Wadhwa, Anders Andersen, and Thomas Kiørboe, Hydrodynamics and energetics of jumping copepod nauplii and copepodids, The Journal of experimental biology 217(17), 3085-3094 (2014) (pdf)
Hydrodynamics of ambush-feeding zooplankton
Copepods are millimeter size crustaceans which constitute the dominating group of marine mesozooplankton, and copepods are therefore of great importance in the marine food web. The paper presents for the first time how the surprisingly fast ambush attacks take place, and it explains how copepods can carry out attacks without pushing away their prey. It turns out that the hydrodynamic constraints and the power requirements of the fast attacks restrict ambush-feeding to a few, but very successful, larger muscular forms of zooplankton.
T. Kiørboe, A. Andersen, V. J. Langlois, H. H. Jakobsen, and T. Bohr, Mechanisms and feasibility of prey capture in ambush-feeding zooplankton, Proceedings of the National Academy of Sciences USA 106, 12394-12399 (2009) (pdf)
Nutrient uptake in swimming flagellates
We have worked on two simple models of the flow generated by a self-propelled flagellate: a sphere propelled by a cylindrical flagellum and one propelled by an external point force. We have used the models to examine the role of advection in enhancing feeding rates in different situations. The models showed that a more correct representation of the flow field than that predicted by a naive sinking sphere model leads to substantially higher clearance rates for interception-feeding flagellates since the streamlines are drawn closer to the cell surface. We also found that a short flagellum is favourable for interception feeding, but at the cost of an increase in the drag on the body of the swimming unicell. We finally demonstrated that prey motility significantly enhances prey encounter rates in interception-feeding flagellates and is, in fact, often much more important for food acquisition than the feeding current.
V. J. Langlois, A. Andersen, T. Bohr, A. W. Visser, and T. Kiørboe, Significance of swimming and feeding currents for nutrient uptake in osmotrophic and interception feeding flagellates, Aquatic Microbial Ecology 54, 35-44 (2009) (pdf)