Changing CO2 Levels Require Microbial Coping Strategies

July 31, 2020

Carbon sustains life. It's the basis of all of life's building blocks—the nucleic acids, proteins, carbohydrates, and lipids that make up our cells. Carbon is also at the heart of one of the most pressing issues on our planet: climate change. Atmospheric carbon dioxide and methane levels are at an all-time high, trapping heat in the atmosphere.

Microbes are another player in climate. They transform the state of carbon, by sequestering carbon from and releasing carbon into the atmosphere, oceans, and biosphere. Climate change shapes microbes and microbes shape the climate.

Microbes and the Carbon Cycle

Most of the earth's carbon lies in rocks and kerogens (from which petroleum and natural gas forms), with the rest in ocean waters, living organisms, and the atmosphere. Carbon dioxide in the atmosphere can be fixed by photosynthetic organisms such as plants. CO2 can also dissolve into the ocean where it gets incorporated into microorganisms and into the food web there.

This flow of carbon has been predictable until now (see Figure 1). With the burning of fossil fuels, we're adding another input of carbon into the atmosphere. In other words, we're releasing carbon at an alarmingly fast rate, much faster than the rate that carbon can be stored via the carbon cycle.

Figure 1. Diagram of the carbon cycle

Figure 1. Diagram of the carbon cycle.
Source: Wkimedia

The Carbon Cycle of the Oceans

Much of carbon sequestration takes place in the oceans where about t 45% of CO2 released by humans is sequestered. And microbes, despite their small size, have a lot to do with this.

When carbon dioxide from the atmosphere dissolves into the ocean, photosynthetic bacteria and eukaryotes take it up and change it into biologically useful forms. Through a process called carbon fixation, a byproduct of photosynthesis, marine microorganisms incorporate carbon into their molecular building blocks, with 2 important outcomes: (1) the carbon is introduced to the food web and (2) molecular oxygen is released as a byproduct into the ocean, and eventually the atmosphere.

Microscopic organisms called phytoplankton are thought to be responsible for creating 50-85% of the oxygen on earth through photosynthesis, with one microbe, the cyanobacteria Prochlorococcus, responsible for about 5% of all photosynthesis on earth. The name phytoplankton comes from the Greek words phyton (plant) and plankton (wanderer or drifter), since these photosynthetic, single-celled microorganisms float through the ocean. There are both prokaryotic and eukaryotic phytoplankton, such as diatoms and dinoflagellates.

Microorganism introduce the carbon into the food web by serving as food for more complex organisms. When other organisms consume these microscopic creatures, that carbon is transferred to the larger organisms, who carry the carbon in their bodies or release it into the ocean as waste or through decay after death. Most of the carbon in the food web stays within the top 100 meters of the ocean, where it can eventually return to the atmosphere.

However, a fraction of the carbon in the food web eventually sinks to deeper waters as "marine snow," tiny specks of dead animals, algae, and waste materials that escape consumption by other organisms. When this happens, the carbon is more likely to be stored in the ocean instead of being released into the atmosphere. When the carbon reaches a depth where it's unlikely to be brought back up to the surface for over hundreds of years, the carbon is considered sequestered.

How Increasing CO2 Levels Decreases Microbes' Carbon Sequestration Abilities

The increased CO2 in the atmosphere has dire consequences for the oceans' food webs via two main drivers: ocean acidification and rising ocean temperatures. Atmospheric CO2 increases lead to more CO2 dissolved in the oceans, decreasing the ocean's pH. Additionally, the heat trapped by atmospheric CO2 is absorbed by the oceans, thus increasing their average temperature.

These changes have a diverse set of effects on microorganisms many of which have the same end result: decreased carbon sequestration.

(i) Dissolving Shells

For phytoplankton that grow shells, ocean acidification is bad news. The extra CO2 drops the pH of the oceans to a point where shells on organisms can become deformed and begin to dissolve. Plus, it's harder to grow shells in the first place. Organisms build their shells using carbonate ions, which are less available with ocean acidification (see Figure 2).

Fewer phytoplankton in the ocean mean the amount of CO2 that becomes fixed in the oceans decreases, leading to lower rates of long-term carbon sequestration.

Figure 2. Phytoplankton morphology as acidification and warming increases in a cultured experiment. Credit: UAB
Figure 2. Phytoplankton morphology as acidification and warming increases in a cultured experiment. Credit: UAB.

(ii) Changing Relationships Between Microorganisms

For the photosynthetic bacterium Prochlorococcus, ocean acidification presents a different problem because higher CO2 levels affect its interactions with other microorganisms, such as its "helper" bacterium, Alteromonas. Prochlorococcus is responsible for about 5% of all photosynthesis on earth, so environmental changes that alter Prochlorococcus could have additional effects on climate.

Prochlorococcus lacks the catalase enzyme, which breaks down hydrogen peroxide, a product of many biological processes that is toxic to Prochlorococcus. Alteromonas makes plenty of this enzyme to share and breaks down the hydrogen peroxide to benefit both organisms.

With changing levels of dissolved CO2 in the water, Alteromonas takes on a different behavior. When researchers from Columbia University, University of Alabama at Birmingham, and University of Tennessee tested the Prochlorococcus - Altermonas relationship under 800 parts per million CO2 (the amount of CO2 expected to be in the atmosphere by 2100), Alteromonas became more antagonistic to Prochlororoccus. Alteromonas produced less catalase and instead began producing proteins that increase the free radicals surrounding it. Prochlorococcus cannot get rid of these toxins and Alteromonas begins to consume the dying cells.

This is a bad sign for carbon sequestration. Less Prochlorococcus in the ocean means less carbon will make it into the food web, leading to less carbon sequestration.

(iii) Increased Microbial Activity Means More CO2 Release

Marine microbes are also more active at higher temperatures. As phytoplankton sink through the ocean, zooplankton and bacteria may consume the phytoplankton before it can reach the ocean floor. Increased phytoplankton consumption means the phytoplankton carbon molecules are more likely to be released as CO2, and potentially back into the atmosphere, rather than reach the deep ocean for long-term sequestration.

In a study from the University of Tasmania, researchers harvested samples of decaying phytoplankton and measured the microbial respiration rate at over a 10°C temperature range to estimate the effect of warming temperatures on carbon sequestration. Using a projected warming of 1.9°C by 2100, they calculated that carbon sequestration could decrease by 17 ± 7%.

Using Microbes to Increase Carbon Sequestration

These examples show how microbial cycles can trigger damaging feedback loops: warmer temperatures either reduces microbial populations or reduces their ability to sequester carbon and propels a further increase in temperature. On the flip side, scientists have been seeing if microbes could increase carbon sequestration by iron fertilization: the intentional introduction of iron into iron-depleted ocean waters to stimulate phytoplankton growth. The intended outcome? To accelerate carbon sequestration from the atmosphere.

Iron is often the limiting nutrient in many areas of the ocean; evidence lies in the large phytoplankton blooms that can result from increasing iron levels. Adding just enough iron to promote marine microbial activity, without overstimulating to create a phytoplankton bloom, may help counteract higher CO2 concentrations.

Iron fertilization is not a new concept. In the 1930s, the biologist Joseph Hart speculated that areas of ocean surface that seemed rich in nutrients but could not sustain plankton activity were iron-deficient. The oceanographer John Martin later hypothesized that increasing phytoplankton photosynthesis could reduce global warming by sequestering CO2. IronEx I, the first iron-enrichment experiment near the Galapagos Islands in October 1993, found that enriched areas showed increased primary production, biomass, and photosynthetic energy conversions relative to untreated waters.

However, iron fertilization experiments have yet to demonstrate increases in carbon sequestration. Even the biological oceanographer Penny Chisholm, who discovered Prochlorococcus, has doubts. By increasing carbon flux into the sea, the food webs below may be altered in unintentional ways, as phytoplankton blooms can lead to blooms of other organisms that can re-release the carbon back into the atmosphere. Thus, there is potentially no benefit in terms of long-term carbon sequestration. And it's hard to predict the long-term, global consequences of iron fertilization with small-scale and short-term experiments like IronEx I.

This makes it difficult to find solutions to carbon storage in the oceans. We can't prevent changes in one part of the ocean from affecting another part of the ocean. The conditions in one area of the ocean may be quite different from another area or the conditions in one area may change from night to day, or day to day. This only highlights the importance of considering these parameters both spatially and temporally. We are only at the beginnings of understanding these things on a global scale.

Further Reading

Author: Jennifer Tsang, Ph.D.

Jennifer Tsang, Ph.D.
Dr. Jennifer Tsang works in science communications and marketing and writes her own microbiology blog called "The Microbial Menagerie." She completed a Ph.D. in microbiology studying bacterial motility.