Changing CO2 Levels Require Microbial Coping Strategies

April 18, 2019

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.
 
To monitor climate change and identify ways to mitigate the increasing amounts of carbon in the atmosphere, scientists have been studying carbon sequestration as a way to pull carbon down from the atmosphere and into the biosphere. To optimize carbon sequestration, we first have to understand the global carbon cycle.
 

The Basics of the Carbon Cycle

 
The carbon cycle describes how carbon is moved through the atmosphere, the oceans, and the biosphere. Carbon is constantly moving between, or cycling, through these reservoirs, hence being called 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

The Carbon Cycle of the Oceans

The oceans have a carbon cycle of their own. The oceans currently absorb about 45% of carbon dioxide (CO2) released by humans. 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.

Changing Oceans with Increasing CO2

How does the additional CO2 being released to the atmosphere affect this oceanic carbon cycle? Increased atmospheric CO2 levels have dire consequences for the oceans’ food webs via 2 main drivers: ocean acidification and rising ocean temperatures. Atmospheric CO2 increases lead to more CO2 dissolved in the oceans, decreasing the ocean pH. Additionally, the heat trapped by atmospheric CO2 is absorbed by the oceans, thus increasing their average temperature.
 
Many microorganisms are affected by these changes, including:
 
(i) Shelled Phytoplankton
 
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

(ii) Prochlorococcus
 
For the photosynthetic bacterium Prochlorococcus, ocean acidification presents a different problem because higher CO2 levels affect the relationships between Prochlorococcus and other microorganisms, such as its “helper” bacterium, Alteromonas.
 
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.
 
What could this mean for carbon sequestration? Less Prochlorococcus in the ocean means less carbon will make it into the food web, again leading to less carbon sequestration.
 
(iii) Microbial respiration rates
 
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. In warmer temperatures, the respiration rate increased. Using a projected warming of 1.9°C by 2100, they calculated that carbon sequestration could decrease by 17 ± 7%.

Can Iron Fertilization Increase Carbon Sequestration?

All of the effects on the microorganisms above have the same outcome: less long-term carbon sequestration. To bring phytoplankton production back and subsequently increase carbon sequestration in the oceans, scientists have been toying with the idea of iron fertilization. Iron fertilization is the intentional introduction of iron into iron-depleted ocean waters to stimulate phytoplankton growth. The intended outcome? To accelerate CO2 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

Jennifer Tsang
Dr. Jennifer Tsang is the science communications and marketing coordinator at Addgene and a freelance science writer.