Spore Ballistics

Post by Tianyou Xu

To be crowned as "the fastest thing alive" has been a title long sought after by competitors across our planet. No, not the fastest man alive nor the fastest mammal alive - the fastest thing alive. But how fast is fast?

It turns out that if you can see it with your eyes, it's not nearly quick enough. Indeed, it has only been with the recent development of high speed imaging technologies that scientists have been able to peek into this invisible world and discover what is now believed to be some of the fastest known flights in nature.

Fungi - a kingdom within the domain Eukaryota - is comprised of an enormous population of micro-organisms such as yeast, mold and mushrooms. Owing to its vast genetic pool,
fungi of different species exhibit different behaviors and characteristics. For example: fungal organisms respond differently to separate environments, hence thrive in their own unique ecosystems. Moreover, different species may operate under different modes of reproduction and employ varying methods for spore dispersal.

The phyla examined in this study were Ascomycota and Zygomycota ^{(Note 1)}. Species within these phyla, commonly known as hat throwers, employ a unique catapulting technique (as their name may suggest) for dispersing their spores. Spores are reproductive structures disseminated from and by the parent fungus. Adapted almost solely for the purposes of dispersal, these spore structures are capable of surviving extended periods of time in unfavorable conditions.

The act of ‘hat throwing' runs on a simple mechanism: it includes (1) a spore, or projectile, which is placed atop (2) a supporting fluid-filled stalk pressurized by osmosis. See picture. As the turgor pressure within the stalk reaches a certain threshold, it triggers the discharge of the spore. The liberation of pressure from the stalk is so rapid and converged (only at the tip) that it allows for an almost instantaneous release of energy ^{(Note 1)}.

High-speed cameras running at maximum frame rates of 250,000 frames/sec were used to capture the entire launch sequence. Launch speeds of the spores ranged from 2 to 25 m s-1 with corresponding accelerations of 20,000 to 180,000 G ^{(Note 1)}! To put that into perspective consider the following: space shuttles upon liftoff generates ~3 G and supersonic fighter jets may reach a maximum of 9 G ^{(Note 2)}! For humans, any sustained G-force above 10 G may be fatal ^{(Note 3)}. Hence, in terms of acceleration, these spore propulsions are the fastest recorded flights known in nature ^{(Note 1)}.

The question that naturally follows is then: why. Why is it necessary for these fungi - their spores, specifically - to have such enormous accelerations? The reasons are two-fold. The first is biological. The life cycle of these particular species of fungi begins with spores discharged onto grass (or some plant substrate). Herbivores, upon consuming the grass, take in the spores as well. However, the spores survive the passage through the hosts' gastrointestinal tracts and emerge on the other side in the excrement. There, in dung, these spores flourish and grow into matured fungi, only to germinate and produce more spores. Therefore, in order to propagate, these fungi must be able to propel their spores away from the dung (beyond the zone of repugnance if you will) and onto - preferably - fresh grass, where they will wait to be ingested by a passing herbivore.

But achieving the necessary propulsion away from the ‘zone of repugnance' is no simple task. The spores are incredibly small - only 5 to10 microns in diameter. Conversely, the air from the spore's perspective is immensely thick and poses tremendous drag (imagine a coin falling through a jar of honey). Against these physical constraints, large launch speeds becomes a necessity should the spore hope to cover any considerable distance in such viscous a medium. Indeed, studies showed that the maximum distance achieved of any launch was merely 2.5 meters (barely enough for the spore to escape the dung and to find fresh grass).

The diagram to the left illustrates the effect of the spore's small size (or relatively speaking the large viscosity of air) on its projectile motion. B.r., P.a., P.k., A.i. represents the different species studied: Basidiobolus ranarum, Podospora anserine, Pilobolus kleinii (closely related to Pilobolus crystallinus, pictured above) and Ascobolus immerses, respectively. Note that the decelerations for these spores are almost instantaneous, due to the large viscosity of air and the minute size of the projectile. Imagine what it would be for us to jump into a pool of honey!

Ultimately, how do we stack up? Usain Bolt, who currently holds the world record time in the 100 and 200 meter sprint and is crowned as the fastest human being on Earth, has a maximum acceleration of less than 1 G ^{(Note 4)}! ONE G!

Alas, it would seem that in the game of out-besting Nature, we will forever be one small step behind.

References

1. Yafetto et al. The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi. PLoS One 3(9): e3237. (2008).

2. The Pull of HyperGravity. NASA Science News (February 2003).

3. Creer et al. Centrifuge Study of Pilot Tolerance to Acceleration and the Effects of Acceleration on Pilot Performance. NASA Technical Note D-337. (1960).

4. Eriksen et al. Velocity Dispersions in a Cluster of Stars: How Fast Could Usain Bolt Have Run? American Journal of Physiology 77, 224 (2009).