For decades the knockout has been an indispensable tool for geneticists and molecular biologists. Removing something — and for geneticists, the something is usually a gene or the protein synthesized from it — and then watching for any resulting effects is a time-honored approach to biology, used on organisms from E. coli to mice.
Extension of the knockout idea to ecosystems instead of organisms is conceptually straightforward: instead of removing a gene from the organism, you remove an organism from the ecosystem. And then see what happens. Unfortunately for science, the the most interesting ecosystems are large-scale, complex, and can’t be replicated in a laboratory. While theoretically one could go out into the wild and remove particular species from real ecosystems just to see what happens, in practice that would be (a) technically difficult and (b) a horribly unethical for many reasons. Lucky for us, science has restrained itself from adopting this approach.
But the gedanken version of the experiment is always available, and that’s the route Jack Gilbert and Josh Neufeld take:
What if all prokaryotic microorganisms on Earth disappeared suddenly? If someone were to wave an antimicrobial wand and eliminate all bacterial and archaeal life on the planet, what would happen? The usual rhetoric is that life as we know it would end, human societies would collapse, and eukaryotic life would cease to exist. Is all of this true?
[…] Plants require fixed nitrogen, and bacteria play an essential biological role in the fixation process. […] Without help from humans, most global photosynthesis would cease within a year. Humans could potentially increase synthetic fertilizer production via the Haber-Bosch process and initiate a massive global fertilization scheme, alleviating some of the enormous losses of life. Such human intervention would be facilitated, to some extent, by the absence of bacterial denitrification and anaerobic ammonia oxidation, which would otherwise deplete fixed nitrogen. Ultimately, nitrogen would begin to accumulate in the global oceans. One possibility is that life would distribute along the coasts, where N-rich fish could be harvested and fixed nitrogen scavenged from seawater when atmospheric nitrogen depletion, due to the Haber-Bosch process, exhausted atmospheric reserves.
Unfortunately, the inevitable increase in atmospheric CO2 concentration due to animal respiration and human fossil fuel use would lead to rapid global warming through the greenhouse effect. Lipson points out that the process would require hundreds of years to eliminate life on the planet—ample time to find a carbon capture solution? In this way, some degree of agricultural food production and marine photosynthesis could continue indefinitely, supporting a subset of humans. Nonetheless, the world’s oceans and soils would likely begin a process of stagnation due to the myriad absent contributions to global biogeochemistry.
…and that’s only the scenario of intermediate horror. The rest gets worse.