Would you rather fight a trillion bacteriophage-sized alligators, or an alligator-sized bacteriophage?

Our new paper “Contrasting controls on microzooplankton grazing and viral infection of microbial prey” is now out in Frontiers in Marine Science. I am a joint coauthor on this work and am proud of how it has turned out, and especially proud as it includes research undertaken by an undergraduate I mentored. It’s been a long journey putting this paper together, since summer 2016, and along the way the following tweet came out that I think is a really fun way to think about the focal topic of our paper:

I recently put this same question as a vote to the followers of biotweeps – the vote was split 60-40 suggesting the answer is non-trivial. So let’s talk about that!

Diffusion vs. active motility

Bacteriophages (viruses of bacteria) are not metabolically active and rely on passive diffusion through the environment to encounter their microbial prey. On the other hand, alligators are actively motile and exert effort to catch their prey. As it turns out these two styles of motility – active and passive – are scale-dependent. Biophysical theory suggests that diffusion of a particle is a much more effective transportation method when really small, but this effect reduces for larger particles. For active motion, on the other hand, larger organisms can typically* move faster than smaller organisms. At small sizes, diffusion is much more effective than active motility; but as size increases, active motility becomes more effective than diffusion. Diffusion is not very effective at large sizes — an alligator-sized bacteriophage isn’t going anywhere! Converse to this, bacteriophage-sized alligators are in a regime in which diffusion dominates – attempts to direct their motion will be mostly in vain #helplessgators🐊!

* notable exceptions would be the Bdellovibrio and like organisms which are very small, but can also move very fast! At larger sizes, acceleration decreases with increasing biomass which is why the largest organisms are not the fastest.

Encounters between viral and grazer predators and their microbial prey

In our paper, we collated data of measured adsorption rates between bacteriophages and bacteria, and clearance rates by micrograzers. We found some of these rates are comparable in magnitude. We asked if we could explain and predict this data from a mechanistic perspective — using biophysical models of diffusion and swimming based encounters. In general, the empirical data supports the idea that diffusion is effective at small sizes, whilst motile encounter is more effective at large sizes.

Screenshot from 2019-04-16 14-02-35

Encounter rates of viral (red) and grazer (blue) microbial predators. Points represent empirical measurements, whilst envelopes represent predictions from biophysical models assuming predators moility is diffusive (red) or swimming (blue).

However, we also found differences between the predictions of maxmium encounter rates from theory and measured encounter rates. In part, this might be explained by uncertainty around how swimming speed scales with size. But, this cannot explain the range of encounter efficiencies found for viruses – which is in part due to encounter rate not being equivalent to adsorption rate. This might be due to the availability and compatibility of host surface receptors or other external host resistance mechanisms. Alternatively, there may be unaccounted sources of viral decay in adsorption assays.

Overall, we show that size and motile strategy are key ecological traits that drive encounters between microbial predators and their prey; and show simple biophysical models that can provide a mechanistic way to understand controls on microbial encounter. Read more in our new paper:

Talmy D., Beckett S.J., Zhang A.B., Taniguchi D.A.A., Weitz J.S., Follows M.J. 2019. Contrasting controls on microzooplankton grazing and viral infection of microbial prey. Frontiers in Marine Science 6:182.

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