Warming climate could turn ocean plankton microbes into carbon emitters

A recent study published in Functional Ecology reveals that the warming climate could have a profound impact on microbial communities worldwide. These communities, which play a crucial role in carbon sequestration, may transition from being carbon sinks to carbon emitters, potentially triggering tipping points in climate change.

The study focused on mixotrophic microbes, which possess the ability to switch between photosynthesis (absorbing carbon dioxide) and heterotrophy (releasing carbon dioxide). These organisms are abundant globally, particularly in freshwater and marine environments, and constitute a significant portion of marine plankton.

Researchers from Duke University and the University of California Santa Barbara developed a computer simulation to examine how mixotrophic microbes acquire energy in response to rising temperatures. Their findings indicated that under warming conditions, these microbes shift from being carbon sinks to becoming carbon emitters.

This discovery suggests that as temperatures continue to rise, these highly prevalent microbial communities may transition from having a cooling effect on the planet to contributing to global warming.

Dr. Daniel Wieczynski, the lead author of the study from Duke University, commented, “Our findings reveal that mixotrophic microbes are far more influential in ecosystem responses to climate change than previously recognized. By converting microbial communities into net carbon dioxide sources due to warming, mixotrophs could potentially exacerbate the warming effect, establishing a positive feedback loop between the biosphere and the atmosphere.”

Dr. Holly Moeller, a co-author of the study from the University of California Santa Barbara, added, “Mixotrophs possess the ability to both capture and emit carbon dioxide, making them pivotal ‘switches’ that can either mitigate or worsen climate change. Despite their small size, these organisms can have significant impacts on a larger scale. Models like the one used in our study are essential for understanding these processes.”

Dr. Jean-Philippe Gibert, another co-author from Duke University, emphasized the need for improved understanding, stating, “Current state-of-the-art predictive models of long-term climate change inadequately account for microbial activities, often oversimplifying or misrepresenting their role. Consequently, research of this nature is crucial in enhancing our comprehension of the biotic controls on Earth’s atmospheric processes.”

The mixotrophic protist Paramecium bursaria can eat bacteria or use photosynthesis to get energy and carbon. Photosynthesis occurs inside the endosymbiotic Chlorella algae (green spheres) that live inside P bursaria cells. Credit: Daniel Wieczynski, CC BY

An early warning system

The researchers’ model also demonstrated that just before mixotrophic microbe communities transition to emitting carbon dioxide, their abundance starts fluctuating dramatically. This finding suggests that by monitoring the abundance of these microbes, we may be able to detect early warning signs of climate change tipping points.

Dr. Wieczynski highlighted the significance of these findings, stating, “These microbes could serve as early indicators of the devastating impacts of rapid climate change, particularly in ecosystems that currently act as major carbon sinks, such as peatlands, where mixotrophs are highly prevalent.”

However, the researchers also discovered that the early warning signals can be dampened by an increase in nutrient levels, such as nitrogen, in the environment. This rise in nutrients is often caused by agricultural runoff and wastewater treatment facilities.

When the simulations included higher nutrient concentrations, the range of temperatures at which the distinctive fluctuations occur began to shrink. Eventually, the signal vanished altogether, leaving no apparent warning before the tipping point was reached.

Dr. Moeller expressed concerns about detecting these warning signs, stating, “Identifying these signals is going to be challenging, especially if they become more subtle due to nutrient pollution. However, the consequences of overlooking them are significant. Ecosystems could end up in a far less desirable state, emitting greenhouse gases into the atmosphere instead of sequestering them.”

During the study, the researchers conducted simulations spanning a temperature range of 4 degrees Celsius, from 19 to 23 degrees Celsius. It is projected that global temperatures will rise by 1.5 degrees Celsius above pre-industrial levels within the next five years and could reach 2 to 4 degrees Celsius by the end of this century.

The researchers caution that their mathematical modeling relies on limited empirical evidence to explore the impact of warming on microbial communities. Dr. Wieczynski emphasized the need for further experimental and observational testing to validate their results, stating, “While models are powerful tools, theoretical findings ultimately need to be empirically tested. We strongly advocate for additional experiments and observations to verify our conclusions.”

Source: British Ecological Society

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