Climate Change Plus Irreversible Evolution Will Force Key Ocean Bacteria into Overdrive

I've got a press release from the University of Southern California that seems important, but I don't have time today to read the study. So, you can look at the press release and tell me what you think of it.

Climate Change Will Irreversibly Force Key Ocean Bacteria into Overdrive

Scientists demonstrate that a key organism in the ocean’s foodweb will start reproducing at high speed as carbon dioxide levels rise, with no way to stop when nutrients become scarce

Imagine being in a car with the gas pedal stuck to the floor, heading toward a cliff’s edge. Metaphorically speaking, that’s what climate change will do to the key group of ocean bacteria known as Trichodesmium, scientists have discovered.

Trichodesmium (called “Tricho” for short by researchers) is one of the few organisms in the ocean that can “fix” atmospheric nitrogen gas, making it available to other organisms. It is crucial because all life — from algae to whales — needs nitrogen to grow.

A new study from USC and the Massachusetts-based Woods Hole Oceanographic Institution (WHOI) shows that changing conditions due to climate change could send Tricho into overdrive with no way to stop — reproducing faster and generating lots more nitrogen. Without the ability to slow down, however, Tricho has the potential to gobble up all its available resources, which could trigger die-offs of the microorganism and the higher organisms that depend on it.

By breeding hundreds of generations of the bacteria over the course of nearly five years in high-carbon dioxide ocean conditions predicted for the year 2100, researchers found that increased ocean acidification evolved Tricho to work harder, producing 50 percent more nitrogen, and grow faster.

The problem is that these amped-up bacteria can’t turn it off even when they are placed in conditions with less carbon dioxide. Further, the adaptation can’t be reversed over time — something not seen before by evolutionary biologists, and worrisome to marine biologists, according to David Hutchins, lead author of the study.

“Losing the ability to regulate your growth rate is not a healthy thing,” said Hutchins, professor at the USC Dornsife College of Letters, Arts and Sciences. “The last thing you want is to be stuck with these high growth rates when there aren’t enough nutrients to go around. It’s a losing strategy in the struggle to survive.”

Tricho needs phosphorous and iron, which also exist in the ocean in limited supply. With no way to regulate its growth, the turbo-boosted Tricho could burn through all of its available nutrients too quickly and abruptly die off, which would be catastrophic for all other life forms in the ocean that need the nitrogen it would have produced to survive.

Some models predict that increasing ocean acidification will exacerbate the problem of nutrient scarcity by increasing stratification of the ocean — locking key nutrients away from the organisms that need them to survive.

Hutchins is collaborating with Eric Webb of USC Dornsife and Mak Saito of WHOI to gain a better understanding of what the future ocean will look like, as it continues to be shaped by climate change. They were shocked by the discovery of an evolutionary change that appears to be permanent — something Hutchins described as “unprecedented.”

“Tricho has been studied for ages. Nobody expected that it could do something so bizarre,” he said. “The evolutionary biologists are interested in it just to study this as a basic evolutionary principle.”

The team is now studying the DNA of Tricho to try to find out how and why the irreversible evolution occurs. Earlier this year, research led by Webb found that Tricho’s DNA inexplicably contains elements that are usually only seen in higher life forms.

“Our results in this and the aforementioned study are truly surprising. Furthermore, they are giving us an improved, view of how global climate change will impact Trichodesmium and the vital supplies of new nitrogen it provides to the rest of the marine food web in the future.” Webb said.

The research appears in Nature Communications on September 1. It can be found online at: http://www.nature.com/ncomms/2015/150901/ncomms9155/full/ncomms9155.html

The abstract of the study is here:

Nitrogen fixation rates of the globally distributed, biogeochemically important marine cyanobacterium Trichodesmium increase under high carbon dioxide (CO2) levels in short-term studies due to physiological plasticity. However, its long-term adaptive responses to ongoing anthropogenic CO2 increases are unknown. Here we show that experimental evolution under extended selection at projected future elevated CO2 levels results in irreversible, large increases in nitrogen fixation and growth rates, even after being moved back to lower present day CO2 levels for hundreds of generations. This represents an unprecedented microbial evolutionary response, as reproductive fitness increases acquired in the selection environment are maintained after returning to the ancestral environment. Constitutive rate increases are accompanied by irreversible shifts in diel nitrogen fixation patterns, and increased activity of a potentially regulatory DNA methyltransferase enzyme. High CO2-selected cell lines also exhibit increased phosphorus-limited growth rates, suggesting a potential advantage for this keystone organism in a more nutrient-limited, acidified future ocean.

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On the face of it, this adaptation creates more nutrient nitrogen in the sea but I suppose Liebig's law of the minimum applies at sea as well as on land. This could be an "Oh Shit! " discovery

By Nick Palmer (not verified) on 01 Sep 2015 #permalink

https://en.wikipedia.org/wiki/Liebig%27s_law_of_the_minimum

'Liebig's law of the minimum, often simply called Liebig's law or the law of the minimum, is a principle developed in agricultural science by Carl Sprengel (1828) and later popularized by Justus von Liebig. It states that growth is controlled not by the total amount of resources available, but by the scarcest resource (limiting factor).'

By mike tomecek (not verified) on 01 Sep 2015 #permalink

The press release seems to leave out a key word from the second paragraph. The media often tends to miss the word;'COULD'.
I'd like to be able to read more about the growth environment used for the experiment. What were the limiting nutrients?

We already see something similar with algae blooms in Lake Erie.

By Dan Chamney (not verified) on 02 Sep 2015 #permalink

There are a number of issues here. I'll list a few that occur at first pass.

With respect to Trichodesmium erythraeum itself, there's the question of whether the response in the real world would mirror that observed in culture in terms of selection for the cultured irreversible phenotype, whether any such selection would completely wipe out non-selected phenotypes in the wild, and whether reversion would be impossible in the natural environment across the span of oceans and time should all non-selected variants be wiped out.

And do the observed effects (and caveats in the preceding paragraph) apply to all species of Trichodesmium?

And if Trichodesmium is wiped out by a post-enhanced carbon crash, would its niche(s) be filled by other nitrogen fixers? Would such replacements be able to provide the same ecosystem functions?

In the mean time, the ramping up of nitrogen fixation that was observed is not necessarily a good thing. One only needs to consider the profoundly negative consequences of much eutrophication that has occurred over the last century or so: in many ways this is simply a eutrophic process. A wholesale ramping of Trichodesmium could have profound ecological effects in (and beyond) the oceans, and given their crucial place in the marine trophic web they could even affect biotic influences on climate.

There are many provisos that arise from this work, but for the cautious biologist this is a disturbing study.

By Bernard J. (not verified) on 02 Sep 2015 #permalink

...post enhanced-carbon...

By Bernard J. (not verified) on 02 Sep 2015 #permalink