Bacteria were the first life-form on Earth and may be the last, if climate change meets worst-case scenarios. In fact, the bacteria themselves may contribute to their ultimate triumph. As the ambient temperature grows hotter, bacteria breathe faster, which releases more carbon dioxide, which can be expected to accelerate climate change, a new paper projects.
The study, “Community-level respiration of prokaryotic microbes may rise with global warming,” was published Wednesday in Nature Communications by Thomas Smith et al.
Don’t we breathe faster when it gets hotter? Yes, all organisms do. But in bacteria, that effect is on speed. They deviate upwards from the average. Bacterial respiration responds more strongly to temperature than complex organisms, including plants, fungi and animals, the paper explains.
All bacteria and all archaea (which used to be called archaebacteria) are collectively called “prokaryotes.” These differ from eukaryotes, which are every other known form of life on the planet, by having their DNA loose in the cell, not encapsulated in a cellular nucleus. Free DNA is the archaic condition. An intriguing if presently irrelevant factoid about archaea is that many are extremophiles, thriving in usually inhospitable conditions such as hypersaline lakes like the Dead Sea; the deepest trenches of the oceans; ice; boiling hot springs, and so forth.
Forecasting weather, global warming, sea level rise, etc., requires factoring in as many parameters as necessary. But these systems are horribly complex and the number of parameters practically infinite. Theoretically, the more parameters we identify and factor into the models, the more reliable the models should be. (Of course, there is always the risk that a newly noticed parameter is so weighty that it causes skewing until balanced by other newly noticed parameters.)
So, as bacteria adapt to rising heat, their respiration rate speeds up and more carbon dioxide is exhaled, the paper suggests that microbial respiration could accelerate climate change and therefore should be factored into climate change modeling.
From fire to ice
It had indeed been assumed that microbial respiration rates would respond to temperature, but now the team has proven and quantified the phenomenon, using 482 prokaryote species dwelling in a vast range of conditions: from saline Antarctic lakes existing at temperatures colder than freezing point, to thermal pools above 120 degrees Celsius (248 degrees Fahrenheit).
The team demonstrated that prokaryotes that usually live in a “medium temperature range” — below 45 degrees Celsius — responded strongly to changing temperature, increasing respiration in both the short term (days to weeks) and long term (months to years).
The increase in carbon output as a response to higher temperatures was greater than expected among most of the germs, the team writes.
Prokaryotes that typically live in higher temperatures, above 45 degrees Celsius, did not have that kind of response — and that was sort of expected. Since they live in blistering conditions to begin with, they weren’t expected to respond to a piddling increase in temperature from their bacterial perspective. But note that compared with other prokaryotes, these types are the minority.
How much of a change could lowly germs make to the whole planet?
Prokaryotes make up around half of global biomass, the total weight of all organisms on Earth, the team writes.
But to be clear, the report relates to aerobic bacteria, not anaerobic ones.
“Indeed, the climate change aspect that we’ve presented is specific to aerobic prokaryotes — most anaerobic respiration pathways aren’t producing carbon dioxide,” Smith tells Haaretz. “Anaerobes do contribute to the global carbon budget, but in different ways.”
One such way is methanogenesis, a metabolic phenomenon of some archaea. Under hypoxic conditions they don’t generate carbon dioxide, they generate methane — which is 80 times more potent as a greenhouse gas than carbon dioxide. So, anaerobic bacteria will also spur climate change, but to a lesser degree than their aerobic brethren, Smith clarifies.
That begs the question: how much of the biomass of bacteria is aerobic?
Smith qualifies that saying bacteria comprise around half of global biomass is an estimate, with wide error margins. “In reality, it’s very difficult to figure out how much that prokaryotes contribute to the total biomass of Earth, and 50 percent is at the upper end of published estimates,” he tells Haaretz. “The majority of the more productive bacteria in soils are expected to be aerobic, with anaerobic things preferring much deeper soil/sediment layers with their lives taking place at a much slower pace. The same is true for the water column. As oceans and freshwater systems tend to be well oxygenated, the majority of prokaryotes there are aerobic. I can’t put a number on the proportion of aerobes to anaerobes — but the majority in the most productive environments should be aerobic.”
Yes, bacteria already account for a significant proportion of carbon dioxide output, Smith says. And as it gets hotter, their contribution is likely to increase by more compared to other organisms in the short (hours, days, weeks) and long term (months to years).
So, are bacteria also contributing to ocean acidification? “Potentially, yes, due to the contribution of heterotrophic [non-photosynthetic] bacteria,” says Smith. “However, the abundance of photosynthetic bacteria in the oceans [cyanobacteria] may offset this somewhat by using the extra carbon dioxide produced by heterotrophs.”
Point of no return
For what it’s worth, all this applies up to a maximum temperature — from which point the bacteria will start to wilt.
“In the short term, on a scale of days to hours, individual prokaryotes will increase their metabolism and produce more carbon dioxide. However, there is still a maximum temperature at which their metabolism becomes inefficient,” explains lead author Samraat Pawar, from the Department of Life Sciences at Imperial College London.
However, Pawar projects that the bacteria — which have mad mutation rates (from our perspective) because their life cycles are so short (from our perspective) — will adapt. They will “learn” to thrive at higher temperatures. And as they do so, their carbon dioxide output will continue to climb. If bacteria can live in thermal vents…
And there we have it. The rising global temperatures will enable the germs to function more efficiently; they will live long and prosper in both the short and long term; and they will deliver an ever-increasing contribution to global carbon and global warming. Until a certain point of global warming. We absolutely cannot predict what that point will be.
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