Ditemukan di danau dan sungai di seluruh dunia, makhluk bersel tunggal seperti ini Bursaria Paramecium Ia bisa makan dan berfotosintesis. Mikroba seperti ini memainkan peran ganda dalam perubahan iklim, melepaskan atau menyerap karbon dioksida—gas rumah kaca yang memerangkap panas yang merupakan pendorong utama pemanasan—bergantung pada apakah mereka mengadopsi gaya hidup seperti binatang atau tumbuhan. Kredit: Daniel J Wiczynski, Universitas Duke

Meningkatnya tingkat panas dapat mendorong plankton samudera dan organisme bersel satu lainnya menuju ambang karbon, yang dapat memperburuk pemanasan global. Namun, penelitian terbaru menunjukkan bahwa mungkin untuk mengidentifikasi tanda-tanda peringatan dini sebelum organisme ini mencapai titik kritis tersebut.

Sekelompok ilmuwan yang melakukan penelitian pada kelas mikroba yang tersebar luas, tetapi sering diabaikan, telah menemukan umpan balik iklim yang dapat mengintensifkan pemanasan global. Namun, temuan ini memiliki sisi positif: Ini juga bisa menjadi sinyal peringatan dini.

Dengan menggunakan simulasi komputer, para peneliti dari Duke University dan University of California, Santa Barbara, telah menunjukkan bahwa sebagian besar plankton samudra global, bersama dengan banyak organisme bersel tunggal yang menghuni danau, lahan gambut, dan ekosistem lainnya, mungkin mencapai titik kritis. titik. Di sini, alih-alih menyerap karbon dioksida, mereka mulai melakukan yang sebaliknya. Perubahan ini adalah hasil dari cara metabolisme Anda merespons pemanasan.

Karena karbon dioksida adalah gas rumah kaca, hal ini pada gilirannya dapat menaikkan suhu—putaran umpan balik positif yang dapat menyebabkan perubahan cepat, di mana sedikit pemanasan berdampak besar.

Tetapi dengan memantau kelimpahan mereka secara hati-hati, kita mungkin dapat mengantisipasi titik kritis sebelum tiba di sini, lapor para peneliti dalam sebuah penelitian yang diterbitkan 1 Juni di jurnal Nature. ekologi fungsional.

Dalam studi baru, para peneliti berfokus pada sekelompok mikroorganisme yang disebut mixotrophs, dinamakan demikian karena mereka menggabungkan dua mode metabolisme: Mereka dapat berfotosintesis seperti tumbuhan atau berburu makanan seperti binatang, tergantung kondisinya.

“Mereka seperti[{” 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.