The Floating Dead: Zombies, Viruses and Life on Earth

But like zombies from early horror stories and films they are usually feared as bringing doom via sickness and death. This is certainly true for the viruses that infect us humans. I’m sure we’ve all felt like a zombie ourselves when suffering from a cold, flu or pox virus inflicted illness. But most of the time we recover, partly in thanks to our amazing multicellular body and its complex defence systems.

But have you ever considered what it’s like to be a microbial cell subjected to viral infection?

It’s a lonely place being a single cell when a virus comes knocking on your cell membrane. You either live and survive another day, or you die after being turned into a brutal microscopic virus factory. ‘Undead’ infected microbial cells really are like zombies: they are mercilessly churning out newly replicated viruses and, when the cell runs out of the resources to continue, it simply disintegrates and becomes nutrients for the organisms around it. When precisely the cell turns from being alive to dead is an interesting philosophical debate. Is it the moment the virus infects? Possibly not, because not all infections are successful. Even single celled organisms have defence mechanisms. Is it the moment a virus genome is replicated? Possibly not, because some viruses integrate into genomes and lie dormant until conditions are better. In that transition period between life and impending death, the cell can gruesomely be considered as undead! Can the virus itself be considered alive? Maybe so, when it is within a cell and replicating. This is why some have argued that they represent the 4th domain of life (alongside prokaryotyes, eukaryotes and archaea). But the virus particle itself (where the virus spends most of its actual ‘life’ cycle) is just a metabolically inert piece of nucleic acid, usually wrapped in a simple protein case waiting for suitable living host to hijack and coerce into doing its metabolic bidding.

So why is this talk of undead zombie cells interesting? Well, consider how microbial cells live: they are small and divide incredibly quickly. Whilst we often marvel at the charismatic macrofauna around us, we actually live in a world dominated by microbes. It is relentless microbial activity that keeps us alive. For example, half of the oxygen we breathe is produced by single cell microalgae in our oceans. Just think about it: the oxygen in every other breath you take is powered by microbes. There are more microbial cells in your body than human cells. Dividing on an hourly or daily basis (or sometimes weekly/monthly for some of the slower ones. They’re dead interesting those slower ones…), their lives are short, and they are always on the go, always dividing, always growing, always improving…. and always dying. It’s this relentless death that provides the nutrients for others to grow, and more importantly, it drives the entire planet’s ecosystem. Stable populations are nowhere near as productive as growing populations, but to keep growing you need a constant supply of nutrients. It’s catch 22: the more successful you are, the quicker you’ll run out of nutrients and die. Fortunately, viruses keep the system running. At any given time, if you’re a microscopic cell the chances are you’re under attack from viral infection. It’s a relentless battle that drives biogeochemical cycles.

It’s why there are so many viruses in the world: a teaspoon of seawater contains more than a million viruses. The world’s oceans contain 1031 viruses. Write that number down in its long form and you’ll understand the scale of virus activity. A staggering 1021 infections are predicted to occur EVERY SECOND in our oceans. Up to half of marine microbial cells are predicted to die on a daily basis as a consequence of viral infection. The scale of zombification in the microbial world is clearly staggering, especially in our oceans, forget The Walking Dead, it’s The Floating Dead out there. People always say the oceans are teeming with life. They are wrong. The oceans are teeming with death. It’s a brutal and relentless massacre out there, dominated by zombie-like undead cells. But unlike the human zombies in The Walking Dead, the microbial zombies aren’t all acting the same and they are making their hosts do some rather clever things to maximise their propagation chances. The microbes themselves are incredibly diverse, but so too are the viruses. Infected cells don’t all simply become subdued shadows of their previous selves. Just like we have seen zombie dramas emerge which have attempted to alter our perception of zombies from inanimate slouches to energetic beings (World War Z), often displaying heightened abilities (iZombie) and new functions (Z-Nation).

Whilst the viruses continue to lust after their microbial hosts, infected cells have been shown to display enhanced nutrient uptake, photosynthetic activity and additional metabolic capacities mimicking what we’ve seen in recent years on our TV screens at the microscopic level, but crucially in the real world. The zombie apocalypse is real and it is happening all around right now. But don’t worry, their series has been running continually for at least 2.9 billion years and without it we’d all be dead. Vive la virus!