A new study, “An unexpected role for mixotrophs in the response of peatland carbon cycling to climate warming” by Vincent Jassey and others, just came out in Scientific Reports. The study is fairly preliminary, but fascinating, and unfortunately may signal that yet another effect of global warming that would result in more global warming.
What makes this study interesting is that it examines the detailed ecological relationships between several different kinds of organisms in both field and lab settings, in order to get a handle on what they do when conditions warm. Mixotrophs are organisms that mix up their role in the trophic web, shifting between being producers (using sunlight to make carbohydrates) or consumers (eating either producers or consumers). Several different mixotrophs that photosynthesize can be found in aquatic and semi aquatic ecosystems, but in this study only mixotrophic amoebae were considered. But before revealing what these shape shifting strategy shifters are up to, a word about the carbon cycle.
About 770 gigatons of Carbon Dioxide (note: Not Carbon, but the gas CO2) are added to the atmosphere every year from natural sources. About 780 gigatons are taken in by those same systems. This is today; At various times in the past this has been different. Anyway, about 10 gigatons are removed, on average, per year over time. Human activities, including burning fossil fuel and other factors, add about 30 gigatons a year, for an imbalance of +20 gigatons. This has caused the concentration of the greenhouse gas CO2 in the atmosphere to go from somewhere south of 280ppm (parts per million) to just north of 400ppm since the beginning of the Industrial Era. This has caused global warming.
Adding CO2, and thus retaining more heat on the surface of the planet, could increase biological activity in such a way that natural ecosystems absorb more Carbon, thus offsetting the human contribution. Also, CO2 is plant food. So, with more CO2, and a longer growing season, plants could bulk up and take in more CO2 than normally, offsetting the human-caused imbalance. Unfortunately, this does not happen.
Well, plants may well take in a bit more CO2 and convert it to plant tissues, but other things happen as well. For example, expanding the growing season also means melting permafrost in northern climates. There, huge amounts of Carbon sequestered long term is released (as CO2 or methane, which is also a greenhouse gas and eventually converts to CO2). So warming caused by adding CO2 to the atmosphere has an amplifying effect, causing even more warming, and more amplification, and so on. We refer to this as a positive feedback cycle, but it is not positive at all from the perspective of planetary health.
See this for more information about a negative feedback involving plants that turns out to not be a negative feedback.
Of all that CO2 moving in and out of the natural system, about 890 gigatons is interchanging on the land (soils, vegetation, freshwater ecosystems, etc.) while about 670 gigatons interact with the ocean. This means that a good amount more than half of the carbon cycle happens over land. Of that, several percent (estimates vary) happens in relation to peat bogs. Here the numbers can get a bit misleading. The normal amount of Carbon that goes into, or comes out of, the world’s peat bogs over a period of time under normal conditions may be very small compared to the total amount of carbon stored in those bogs, which might be rapidly released as CO2 or methane if certain things happen. For example, peat is a fuel source, and has been widely mined for many years. In other areas, bogs are drained or covered over. Here in the Twin Cities, vast bogs are now Urban Saint Paul (covered over) or farms growing turf or corn (drained) so whatever they were doing in the early 19th century as part of the natural system, they are not doing that any more. The peat that is burned for fuel or used in smelting operations, etc., adds most of its Carbon to the atmosphere all at once, and thereafter contributes nothing to sequestering carbon.
For all these reasons, what happens in peat bogs does not stay in peat bogs. Changes to peat bogs that cause changes in their role in the Carbon cycle may be very important under global warming.
Now, think about this for a second. If you have an organism that can either sequester carbon by acting like a plant, or release carbon (as CO2) by acting as an animal (though it is neither), then what that animal is doing matters to the Carbon cycle. Also, if the organism can either grow and reproduce using mainly sunlight, or consume other organisms at a higher rate, that strategy shifting may influence the entire ecosystem. The new research project looks at all of this, and seems to show that on balance, warming up the ecosystem significantly changes the amounts of Carbon released vs. retained in many peat bogs.
From the Abstract:
…little is known about the responses of peatland mixotrophs to climate change and the potential consequences for the peatland C cycle. With a combination of field and microcosm experiments, we show that mixotrophs in the Sphagnum bryosphere play an important role in modulating peatland C cycle responses to experimental warming. We found that five years of consecutive summer warming with peaks of +2 to +8°C led to a 50% reduction in the biomass of the dominant mixotrophs, the mixotrophic testate amoebae (MTA). The biomass of other microbial groups (including decomposers) did not change, suggesting MTA to be particularly sensitive to temperature. In a microcosm experiment under controlled conditions, we then manipulated the abundance of MTA, and showed that the reported 50% reduction of MTA biomass in the field was linked to a significant reduction of net C uptake (–13%) of the entire Sphagnum bryosphere. Our findings suggest that reduced abundance of MTA with climate warming could lead to reduced peatland C fixation.
The way in which MTA biomass reduction reduces peatland Carbon fixation is not entirely clear. I asked Vincent Jassey, the study’s lead author, how the reduction in biomass of the dominant mixotrophs in the Sphagnum bryosphere reduces overall photosynthesis. He told me that what is measured is the overall rate of photosynthesis of the entire ecosystem, and that further research would be needed to assess exactly what is happening. “MTA are living within/between Sphagnum leaves. So, when we measure photosynthesis on Sphagnum, it takes into account the photosynthesis made by MTA as well,” he said. “This is the first time a paper outlines the potential role of MTA in overall Carbon fixation of bryosphere, showing its potential importance for peatlands…we showed that a decrease of MTA abundance/biomass is linked to a decrease of Sphagnum bryosphere photosynthesis. [So far] we made indirect measurements and this needs to be verified in future research.”
The next step, then, is to how MTA benefits the moss. So far, “…these links are largely unknown. This is something I’m working on. We want to quantify more precisely the contribution of MTA photosynthesis to bryosphere photosynthesis in the field in our future research and see if its response to warming will be significant in term of C loss in peatlands.”
I think this research is important for several reasons. One is that this is a step towards understanding a complex ecological system that makes up a significant percentage of terrestrial ecosystems, which may involve inter-species symbiosis or other important interaction. The other is that this system appears to represent yet another case of amplifying feedback in global warming, where more warming ultimately leads to more warming. Decades ago, many scientists hoped or assumed that the anthropogenic greenhouse effect would be at least partly attenuated by negative feedback systems, where more Carbon is sequestered as a result of warming, but we now know that while this does happen, it is more common to find amplifying feedbacks. This, of course, relates to the question of climate sensitivity. Many factors will determine where the global surface temperatures will equilibrate with a doubling of pre-Industrial CO2 levels in the atmosphere, and how organisms or ecological systems respond to warming is part of that.
I will speculate further and suggest that this is important in relation to the question of carbon sequestering through geo-engineering. It has been suggested that preserving or expanding peat bogs, like growing more trees or similar measures, would help sequester more carbon. But the carbon-sequestering value one places per unit area on various kinds of peat bogs or other wetlands has to be correctly measured and understood. If these values are going to change in a warming world, we need to know this.