未知の微生物は気候転換点の早期警告信号である可能性がある

世界中の湖や川に生息するこのような単細胞生物 ゾウリムシ 食べることもできるし、光合成もできる。 このような微生物は、動物のようなライフスタイルを採用するか植物のようなライフスタイルを採用するかに応じて、気候変動において二重の役割を果たし、温暖化の主な要因である熱を閉じ込める温室効果ガスである二酸化炭素を放出または吸収します。 クレジット: Daniel J Wiczynski、デューク大学

熱レベルの上昇により、海洋プランクトンやその他の単細胞生物が炭素閾値に向かって押し上げられ、地球温暖化が悪化する可能性があります。 しかし、最近の研究では、これらの微生物がその臨界点に達する前に、早期の警告兆候を特定できる可能性があることが示唆されています。

広範囲に存在するが、しばしば見落とされがちな微生物の研究を行っている科学者のグループが、地球温暖化を促進する可能性のある気候のフィードバックループを発見した。 ただし、この発見には利点もあります。つまり、早期の警告信号である可能性もあります。

デューク大学とカリフォルニア大学サンタバーバラ校の研究者らは、コンピューターシミュレーションを使用して、湖、泥炭地、その他の生態系に生息する多くの単細胞生物とともに、地球上の海洋プランクトンの大部分が危機に瀕している可能性があることを示した。点。 ここでは、二酸化炭素を吸収する代わりに、その逆のことを始めます。 この変化は、温暖化に対する代謝の反応の結果です。

二酸化炭素は温室効果ガスであるため、これにより気温が上昇する可能性があります。これは、少量の温暖化が大きな影響を与える、急速な変化につながる可能性のある正のフィードバック ループです。

しかし、その存在量を注意深く監視することで、転換点が訪れる前に予測できるかもしれない、と研究者らは6月1日にジャーナル・ネイチャーに掲載された研究で報告している。 機能生態学。

新しい研究で研究者らは、混合栄養生物と呼ばれる微生物の一群に焦点を当てた。混合栄養生物は、条件に応じて、植物のように光合成をしたり、動物のように食物を狩ったりするという、2つの代謝モードを組み合わせることからこの名前が付けられた。

「彼らはまるで[{” attribute=””>Venus fly traps of the microbial world,” said first author Daniel Wieczynski, a postdoctoral associate at Duke.

During photosynthesis, they soak up carbon dioxide, a heat-trapping greenhouse gas. And when they eat, they release carbon dioxide. These versatile organisms aren’t considered in most models of global warming, yet they play an important role in regulating climate, said senior author Jean P. Gibert of Duke.

Most of the plankton in the ocean — things like diatoms, dinoflagellates — are mixotrophs. They’re also common in lakes, peatlands, in damp soils, and beneath fallen leaves.

“If you were to go to the nearest pond or lake and scoop a cup of water and put it under a microscope, you’d likely find thousands or even millions of mixotrophic microbes swimming around,” Wieczynski said.

“Because mixotrophs can both capture and emit carbon dioxide, they’re like ‘switches’ that could either help reduce climate change or make it worse,” said co-author Holly Moeller, an assistant professor at the University of California, Santa Barbara.

To understand how these impacts might scale up, the researchers developed a mathematical model to predict how mixotrophs might shift between different modes of metabolism as the climate continues to warm.

The researchers ran their models using a 4-degree span of temperatures, from 19 to 23 degrees Celsius (66-73 degrees Fahrenheit). Global temperatures are likely to surge 1.5 degrees Celsius above pre-industrial levels within the next five years, and are on pace to breach 2 to 4 degrees before the end of this century.

The analysis showed that the warmer it gets, the more mixotrophs rely on eating food rather than making their own via photosynthesis. As they do, they shift the balance between carbon in and carbon out.

The models suggest that, eventually, we could see these microbes reach a tipping point — a threshold beyond which they suddenly flip from carbon sink to carbon source, having a net warming effect instead of a cooling one.

This tipping point is hard to undo. Once they cross that threshold, it would take significant cooling — more than one degree Celsius — to restore their cooling effects, the findings suggest.

But it’s not all bad news, the researchers said. Their results also suggest that it may be possible to spot these shifts in advance, if we watch out for changes in mixotroph abundance over time.

“Right before a tipping point, their abundances suddenly start to fluctuate wildly,” Wieczynski said. “If you went out in nature and you saw a sudden change from relatively steady abundances to rapid fluctuations, you would know it’s coming.”

Whether the early warning signal is detectable, however, may depend on another key factor revealed by the study: nutrient pollution.

Discharges from wastewater treatment facilities and runoff from farms and lawns laced with chemical fertilizers and animal waste can send nutrients like nitrate and phosphate into lakes and streams and coastal waters.

When Wieczynski and his colleagues included higher amounts of such nutrients in their models, they found that the range of temperatures over which the telltale fluctuations occur starts to shrink until eventually the signal disappears and the tipping point arrives with no apparent warning.

The predictions of the model still need to be verified with real-world observations, but they “highlight the value of investing in early detection,” Moeller said.

“Tipping points can be short-lived, and thus hard to catch,” Gibert said. “This paper provides us with a search image, something to look out for, and makes those tipping points — as fleeting as they may be — more likely to be found.”

Reference: “Mixotrophic microbes create carbon tipping points under warming” by Daniel J. Wieczynski, Holly V. Moeller and Jean P. Gibert, 31 May 2023, Functional Ecology.
DOI: 10.1111/1365-2435.14350

The study was funded by the Simons Foundation, the National Science Foundation, and the U.S. Department of Energy.