Would oxygen levels be stable without photosynthesis?

Would oxygen levels be stable without photosynthesis?

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I learned how the CO$_2$ cycle works in several biology classes, high school, elementary school, geology, and physics. Then today there was an article published in the NY Times claiming:

… it is a myth that photosynthesis controls the amount of oxygen in the atmosphere. Even if all photosynthesis on the planet were shut down, the atmosphere's oxygen content would change by less than 1 percent.

I'm really confused. If the earth naturally has 20% oxygen in the absence of photosynthisis, why did it spike durring Great Oxygenation Event? I reread that several times. There's no citation; no further context that can explain this. If this were coming from the climate change denier camp, I'd just shrug it off, but the credentials would say otherwise. It's in the NY Times and Nadine Unger is an atmospheric chemist at Yale.

My guess is that this is simply a misleading paragraph with a grain of truth. The question is, where is that grain? How long would the atmospheric oxygen content remain at within 1% of the current level if we turned off photosynthesis?

The answer to your question is in the first graph of the Wikipedia article you linked to: Great Oxygenation Event. In that graph, where time is measures in billions of years, you see that O2, as it was produced, was absorbed in oceans, seabed rock, land surfaces, and finally, about .85 billion years ago, O2 sinks filled and the gas accumulated in the atmosphere.[1] The spike you speak of was probably due to

the evolution of the large vascular land plants that brought about increased O2 production and increased O2 levels due to the enhanced global burial of microbially resistant, lignin-rich organic matter during the Permo-Carboniferous (Berner 2004).

More simply put, there hadn't been equilibrium established yet between the amount of oxygen being produced, the stabilization of the ocean layers and the 'burial of excess carbon' in the deepest ocean layers. Most of the O2 on earth was produced many millions of years ago; it is a myth that the O2 we breathe today is the same O2 made by, say, ocean phytoplankton yesterday.

Nadene Unger states in the NYT article that

[I]t is a myth that photosynthesis controls the amount of oxygen in the atmosphere. Even if all photosynthesis on the planet were shut down, the atmosphere's oxygen content would change by less than 1 percent.

This is because there are tremendous oxygen sinks that store oxygen gas, such as the surface layers of the ocean. As O2 is used, the ocean would give up free O2. Because of these sinks, some estimate that

if photosynthesis were to cease, atmospheric oxygen would be depleted in 5000 years due to respiration, weathering, and combustion.[2]

while others state that

If photosynthesis were for some reason to stop, oxidation of the entire organic carbon reservoir would consume less than 1% of O2 presently in the atmosphere and there would be no further O2 loss (since there would be no organic carbon left to be oxidized). Conversely, if respiration and decay were to stop, conversion of all atmospheric CO2 to O2 by photosynthesis would increase O2 levels by only 0.2%.[3]

It is very hard to get a comprehensive answer of what would happen exactly. Do we assume all vegetation/phytoplankton suddenly disappears, or does it die (thus increasing to the carbon drain on O2)? Do we assume quick quick death of animals based on lack of vegetation/plankton, etc. as food, or would it be slower? As I said, a comprehensive answer is difficult to find. In any case, the atmospheric O2 would not fluctuate wildly in any immediate fashion.

[1] The oxygenation of the atmosphere and oceans Heinrich D Holland, Phil. Trans. R. Soc., 29 June 2006
[2] The Global Oxygen Cycle
[3] Introduction to Atmospheric Chemistry


By the time you complete this unit, you should be able to explain the core concepts on this page and in the corresponding sections of Chapter 10 (Photosynthesis) in Campbell:

  • How the light reactions capture light energy and convert it to chemical energy.
  • How the Calvin cycle makes use of the outputs of the light reactions.
  • Similarities and differences between photosynthesis and cellular respiration.

For more detailed objectives, see the review section at the bottom of this page.

The photosynthesis coverage in Campbell is excellent I strongly recommend reading it. On this page, I'll give a brief account of some of the key photosynthesis ideas for Bio 6B, intended to supplement the chapter and give you a clear idea of what to expect for the quiz and midterm.

Before you start, I recommend that you watch this video: Photosynthesis | HHMI BioInteractive Video. It's an excellent video that goes from simple overview to detailed mechanisms. This might be all you need to know, in 11 minutes!

Dissolved Oxygen and Aquatic Life

Dissolved oxygen is important to many forms of aquatic life.

Dissolved oxygen is necessary to many forms of life including fish, invertebrates, bacteria and plants. These organisms use oxygen in respiration, similar to organisms on land. Fish and crustaceans obtain oxygen for respiration through their gills, while plant life and phytoplankton require dissolved oxygen for respiration when there is no light for photosynthesis 4 . The amount of dissolved oxygen needed varies from creature to creature. Bottom feeders, crabs, oysters and worms need minimal amounts of oxygen (1-6 mg/L), while shallow water fish need higher levels (4-15 mg/L)⁵.

Microbes such as bacteria and fungi also require dissolved oxygen. These organisms use DO to decompose organic material at the bottom of a body of water. Microbial decomposition is an important contributor to nutrient recycling. However, if there is an excess of decaying organic material (from dying algae and other organisms), in a body of water with infrequent or no turnover (also known as stratification), the oxygen at lower water levels will get used up quicker ⁶.

Plants contain a vast range of chemicals which are extracted and used in the production of medicines. Aspirin the drug used as a pain reliever and to reduce blood clotting in heart patients is derived from salicylic acid, a chemical extracted from the bark of the willow tree. Much stronger pain killers (analgesic drugs) such as morphine and codeine are produced from opium, the dried sap derived from the seeds of the poppy plant.

To date the number of plants tested for medicinal properties number only in their thousands. There are still a vast number yet to be tested including many of the species sourced from the tropical rainforests. The unknown medicinal properties of these plant species adds to the importance of protecting natural habitats such as rainforests.

Wood for use as a building material, a fuel for combustion and in the manufacture of paper is sourced from trees.

What are the different methods of measuring the rate of photosynthesis?

There are a few key methods to calculate the rate of photosynthesis. These include:

1) Measuring the uptake of CO2

2) Measuring the production of O2

3) Measuring the production of carbohydrates

4) Measuring the increase in dry mass

As the equation for respiration is almost the reverse of the one for photosynthesis, you will need to think whether these methods measure photosynthesis alone or whether they are measuring the balance between photosynthesis and respiration.

Measuring photosynthesis via the uptake of carbon dioxide

Using 'immobilised algae' - It's easy and accurate to measure the rate of photosynthesis and respiration using immobilised algae in hydrogen carbonate indicator solution - known as the 'algal balls' technique. Read the full protocol on using immobilsed algae to measure photosynthesis.

Using an IRGA - Uptake of CO2 can be measured with the means of an IRGA (Infra-Red Gas Analyser) which can compare the CO2 concentration in gas passing into a chamber surrounding a leaf/plant and the CO2 leaving the chamber.

Using a CO2 monitor - More simply, you could put a plant in a plastic bag and monitor the CO2 concentration in the bag using a CO2 monitor. Naturally, the soil and roots must NOT be in the bag (as they respire). Alternatively, you could place some Bicarbonate Indicator Solution in the bag with the plant and watch the colour change. This would best be done with a reference colour chart to try to make the end-point less subjective. This could give a comparison between several plants. There are difficulties with this method, as I'm sure you can appreciate. The leaf area of the plants should be measured so you can compensate for plant size. Atmospheric air is only 400ppm CO2, so there is not much CO2 to monitor and the plant will soon run out of CO2 to fix.

Measuring photosynthesis via the production of oxygen

Oxygen can be measured by counting bubbles evolved from pondweed, or by using the Audus apparatus to measure the amount of gas evolved over a period of time. To do this, place Cabomba pondweed in an upside down syringe in a water bath connected to a capillary tube (you can also use Elodea, but we find Cabomba more reliable). Put the weed in a solution of NaHCO3 solution. You can then investigate the amount of gas produced at different distances from a lamp. Read a full protocol on how to investigate photosynthesis using pondweed.

Measuring photosynthesis via the production of carbohydrates

There is a crude method where a disc is cut out of one side of a leaf (using a cork borer against a rubber bung) and weighed after drying. Some days (or even weeks later), a disk is cut out of the other half of the leaf, dried and weighed. Increase in mass of the disc is an indication of the extra mass that has been stored in the leaf. This is very simple to do and enables you to investigate plants growing in the wild. However, you can probably think of several inaccuracies in this method.

Measuring photosynthesis via the increase in dry mass

Dry mass is often monitored by the technique of 'serial harvests' where several plants are harvested, dried to constant weight and weighed - this is repeated over the duration of the experiment. If you harvest several plants and record how much mass they have accumulated you will have an accurate measure of the surplus photosynthesis over and above the respiration that has taken place. As with most methods, you need several plants so you have replicate measurements and you can find an average and a standard deviation if necessary.

Investigating the light-dependent reaction in photosynthesis

The rate of decolourisation of DCPIP in the Hill Reaction is a measure of the rate of the light-requiring stages of photosynthesis

Ancient rocks record first evidence for photosynthesis that made oxygen

A new study shows that iron-bearing rocks that formed at the ocean floor 3.2 billion years ago carry unmistakable evidence of oxygen. The only logical source for that oxygen is the earliest known example of photosynthesis by living organisms, say University of Wisconsin–Madison geoscientists.

“Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen,” says Clark Johnson, professor of geoscience at UW–Madison and a member of the NASA Astrobiology Institute. “Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old, and quite well preserved, and we believe they show definite signs for oxygen in the oceans much earlier than previous discoveries.”

The most reasonable candidate for liberating the oxygen found in the iron oxide is cyanobacteria, primitive photosynthetic organisms that lived in the ancient ocean. The earliest evidence for life now dates back 3.5 billion years, so oxygenic photosynthesis could have evolved relatively soon after life itself.

Until recently, the conventional wisdom in geology held that oxygen was rare until the “great oxygenation event,” 2.4 to 2.2 billion years ago.

The rocks under study, called jasper, made of iron oxide and quartz, show regular striations caused by composition changes in the sediment that formed them. To detect oxygen, the UW–Madison scientists measured iron isotopes with a sophisticated mass spectrometer, hoping to determine how much oxygen was needed to form the iron oxides.

Aaron Satkoski holds a sample sawn from a 3.23-billion-year-old rock core sample found in South Africa. The bands show different types of sediment falling to the ocean floor and solidifying into rock.

“Iron oxides contained in the fine-grained, deep sediment that formed below the level of wave disturbance formed in the water with very little oxygen,” says first author Aaron Satkoski, an assistant scientist in the Geoscience Department. But the grainier rock that formed from shallow, wave-stirred sediment looks rusty, and contains iron oxide that required much more oxygen to form.

The visual evidence was supported by measurements of iron isotopes, Satkoski said.

The study was funded by NASA and published in Earth and Planetary Science Letters.

The samples, provided by University of Johannesburg collaborator Nicolas Beukes, were native to a geologically stable region in eastern South Africa.

Because the samples came from a single drill core, the scientists cannot prove that photosynthesis was widespread at the time, but once it evolved, it probably spread. “There was evolutionary pressure to develop oxygenic photosynthesis,” says Johnson. “Once you make cellular machinery that is complicated enough to do that, your energy supply is inexhaustible. You only need sun, water and carbon dioxide to live.”

Other organisms developed forms of photosynthesis that did not liberate oxygen, but they relied on minerals dissolved in hot groundwater — a far less abundant source than ocean water, Johnson adds. And although oxygen was definitely present in the shallow ocean 3.2 billion years ago, the concentration was only estimated at about 0.1 percent of that found in today’s oceans.

Aaron Satkoski is pictured with the high-resolution mass spectrometer used to analyze rock samples from South Africa that push back the date for the first oxygenic photosynthesis to 3.23 billion years ago.

Confirmation of the iron results came from studies of uranium and its decay products in the samples, says co-author Brian Beard, a senior scientist at UW–Madison. “Uranium is only soluble in the oxidized form, so the uranium in the sediment had to contain oxygen when the rock solidified.”

Measurements of lead formed from the radioactive decay of uranium showed that the uranium entered the rock sample 3.2 billion years ago. “This was an independent check that the uranium wasn’t added recently. It’s as old as the rock it’s original material,” Beard says.

“We are trying to define the age when oxygenic photosynthesis by bacteria started happening,” he says. “Cyanobacteria could live in shallow water, doing photosynthesis, generating oxygen, but oxygen was not necessarily in the atmosphere or the deep ocean.”

However, photosynthesis was a nifty trick, and sooner or later it started to spread, Johnson says. “Once life gets oxygenic photosynthesis, the sky is the limit. There is no reason to expect that it would not go everywhere.”

How oxygen-producing cyanobacteria facilitated complex life

Achim Herrmann is researching the spread of early cyanobacteria. Credit: Koziel/TUK

The "Great Oxygenation Event" (GOE), the process whereby the Earth's atmosphere was continuously enriched with oxygen, a waste product of photosynthesis, began

2.43 billion years ago. The source, according to science, was photosynthesizing cyanobacteria. But why did this all-important turnaround occur so late? Cyanobacterial life existed, as rock samples show, at least 300 million years before the GOE. Achim Herrmann, who is researching the spread of early cyanobacteria in his doctoral thesis at TU Kaiserslautern, is hot on the trail for answers. His current research paper has now been published in the journal Nature Communications.

"There are many scientific theories that intertwine to explain why the proliferation of cyanobacteria required for the GOE was delayed," explains Herrmann, who is working on his doctorate with Michelle Gehringer in Geomicrobiology. "For example, they may have originated in fresh water, which covered then, as now, only a fraction of Earth's surface. It wasn't until they adapted to saltier waters and finally inhabited the open ocean that they were able to form sufficient amounts of biomass to cause a global change in Earth's atmosphere." Another theory is that the iron-rich ocean water may have initially been toxic to the photosynthesizing bacteria. Iron had accumulated in the marine environment predominantly in the form of highly soluble, reduced iron(II) ions during the Earth's then oxygen free "Archean" age.

In his research Herrmann built upon the iron toxin hypothesis. "We wanted to check whether iron(II) inhibits not only modern Cyanobacteria but also more primitive, marine strains, specifically Pseudanabaena sp. PCC7367 and Synechococcus sp. PCC7336, in their growth and photosynthetic activity," said the biologist.

It quickly became apparent how crucial the experimental setup is. In already established systems where the bacteria are cultivated in closed glass bottles without oxygen, they demonstrated almost no growth: "The biological activity was very low in both strains, and almost completely suppressed in Synechococcus," Herrmann says. The solution: "A custom-built anaerobic workstation from the TUK metal workshop, in whose chambers the composition of the atmosphere can be regulated fully and automatically," he says. "Using this setup, we cultivated the cyanobacteria in large laboratory bottles with gas-permeable lids to allow gas exchange. The oxygen they produced was regularly removed from the system, and carbon dioxide was kept constant at proposed Archean atmospheric levels. Thus, we were able to realize a shallow marine oxygen oasis as implied in Archean rock samples."

As expected, the cyanobacteria "felt more comfortable" in the more authentic environment. But what happened when iron was injected in increasing concentrations? The bacteria from the Pseudanabaena strain grew consistently well, but more slowly than in the control system. In contrast, the Synechococcus strain clearly decreased its rate of cell division as iron increased. The oxygen produced primarily oxidized the dissolved Fe(II) ions instead of escaping into the atmosphere. And the oxygen production rate for both strains reached significantly higher values in the anoxically adjusted experimental environment than in the control setup with an oxygenated atmosphere, like that which surrounds us today. This would suggest that modern day atmospheric oxygen levels impair photosynthesis when compared to the anoxic atmosphere of Earth's past.

In addition, the formation of green rust, a mix of Fe(II) and oxidized iron Fe(III), was shown only in the culture system developed by Herrmann. The formation of green rust was accompanied by a strong decrease of biological activity, probably caused by iron oxides encrusting the bacterial cells. During the Archean, the formation of such green rust may have contributed decisively to banded iron formations, the most important source of iron ore today.

Finally, Herrmann changed the experimental scenario once again and simulated iron conditions for a tidal zone. Iron was added at night, when oxygen concentrations dropped towards zero due to no photosynthetic activity. The result: growth slowed significantly in both strains, but never stopped completely. This indicates that an Archean oxygen oasis could also have tolerated the influx of iron-rich water during the night. Here, too, the formation of green rust occurred, but could be further oxidized quickly and thus did not bring growth to a standstill.

All in all, Herrmann's research has filled in more gaps in the puzzle of Earth's history. He was able to illustrate for both cyanobacterial strains how the iron cycle might have proceeded in an Archean oxygen oasis, and that smaller colonized areas would probably have been sufficient for the start of the GOE due to the higher oxygen production rates. He has also developed a concept for growing cyanobacteria that better represents Archaean living conditions.

"I hope that with my research paper, I can help us better understand how our oxygen rich atmosphere was able to evolve in the first place," Herrmann says.

Photosynthesis in Prokaryotes

The two parts of photosynthesis&mdashthe light-dependent reactions and the Calvin cycle&mdashhave been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles. Prokaryotic photosynthetic autotrophic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis (Figure (PageIndex<4>)). It is here that organisms like cyanobacteria can carry out photosynthesis.

Figure (PageIndex<4>): A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis. (credit: scale-bar data from Matt Russell)

Dissolved Oxygen and Water

Dissolved oxygen (DO) is a measure of how much oxygen is dissolved in the water - the amount of oxygen available to living aquatic organisms. The amount of dissolved oxygen in a stream or lake can tell us a lot about its water quality.

USGS scientist is measuring various water-quality conditions in Holes Creek at Huffman Park in Kettering, Ohio.

The USGS has been measuring water for decades. Some measurements, such as temperature, pH, and specific conductance are taken almost every time water is sampled and investigated, no matter where in the U.S. the water is being studied. Another common measurement often taken is dissolved oxygen (DO), which is a measure of how much oxygen is dissolved in the water - DO can tell us a lot about water quality.

Dissolved Oxygen and Water

Although water molecules contain an oxygen atom, this oxygen is not what is needed by aquatic organisms living in natural waters. A small amount of oxygen, up to about ten molecules of oxygen per million of water, is actually dissolved in water. Oxygen enters a stream mainly from the atmosphere and, in areas where groundwater discharge into streams is a large portion of streamflow, from groundwater discharge. This dissolved oxygen is breathed by fish and zooplankton and is needed by them to survive.

Dissolved oxygen and water quality

A eutrophic lake where dissolved-oxygen concentrations are low. Algal blooms can occur under such conditions.

Rapidly moving water, such as in a mountain stream or large river, tends to contain a lot of dissolved oxygen, whereas stagnant water contains less. Bacteria in water can consume oxygen as organic matter decays. Thus, excess organic material in lakes and rivers can cause eutrophic conditions, which is an oxygen-deficient situation that can cause a water body to "die." Aquatic life can have a hard time in stagnant water that has a lot of rotting, organic material in it, especially in summer (the concentration of dissolved oxygen is inversely related to water temperature), when dissolved-oxygen levels are at a seasonal low. Water near the surface of the lake– the epilimnion– is too warm for them, while water near the bottom–the hypolimnion– has too little oxygen. Conditions may become especially serious during a period of hot, calm weather, resulting in the loss of many fish. You may have heard about summertime fish kills in local lakes that likely result from this problem.

Dissolved oxygen, temperature, and aquatic life

Water temperture affects dissolved-oxygen concentrations in a river or water body.

As the chart shows, the concentration of dissolved oxygen in surface water is affected by temperature and has both a seasonal and a daily cycle. Cold water can hold more dissolved oxygen than warm water. In winter and early spring, when the water temperature is low, the dissolved oxygen concentration is high. In summer and fall, when the water temperature is high, the dissolved-oxygen concentration is often lower.

Dissolved oxygen in surface water is used by all forms of aquatic life therefore, this constituent typically is measured to assess the "health" of lakes and streams. Oxygen enters a stream from the atmosphere and from groundwater discharge. The contribution of oxygen from groundwater discharge is significant, however, only in areas where groundwater is a large component of streamflow, such as in areas of glacial deposits. Photosynthesis is the primary process affecting the dissolved-oxygen/temperature relation water clarity and strength and duration of sunlight, in turn, affect the rate of photosynthesis.

Hypoxia and "Dead zones"

You may have heard about a Gulf of Mexico "dead zone" in areas of the Gulf south of Louisiana, where the Mississippi and Atchafalaya Rivers discharge. A dead zone forms seasonally in the northern Gulf of Mexico when subsurface waters become depleted in dissolved oxygen and cannot support most life. The zone forms west of the Mississippi Delta over the continental shelf off Louisiana and sometimes extends off Texas. The oxygen depletion begins in late spring, increases in summer, and ends in the fall.

Dissolved oxygen in bottom waters, measured from June 8 through July 17, 2009, during the annual summer Gulf of Mexico Southeast Area Monitoring and Assessment Program ( SEAMAP ) cruise in the northern Gulf of Mexico. Orange and red colors indicate lower dissolved oxygen concentrations.

The formation of oxygen-depleted subsurface waters has been associated with nutrient-rich (nitrogen and phosphorus) discharge from the Mississippi and Atchafalaya Rivers. Bio-available nutrients in the discharge can stimulate algal blooms, which die and are eaten by bacteria, depleting the oxygen in the subsurface water. The oxygen content of surface waters of normal salinity in the summer is typically more than 8 milligrams per liter (8 mg/L) when oxygen concentrations are less than 2 mg/L, the water is defined as hypoxic (CENR, 2000). The hypoxia kills many organisms that cannot escape, and thus the hypoxic zone is informally known as the “dead zone.”

The hypoxic zone in the northern Gulf of Mexico is in the center of a productive and valuable fishery. The increased frequency and expansion of hypoxic zones have become an important economic and environmental issue to commercial and recreational users of the fishery.

Measuring dissolved oxygen

Multi-parameter monitor used to record water-quality measurements.

Field and lab meters to measure dissolved oxygen have been around for a long time. As this picture shows, modern meters are small and highly electronic. They still use a probe, which is located at the end of the cable. Dissolved oxygen is dependent on temperature (an inverse relation), so the meter must be calibrated properly before each use.

Do you want to test your local water quality?

Water test kits are available from World Water Monitoring Challenge (WWMC), an international education and outreach program that builds public awareness and involvement in protecting water resources around the world. Teachers and water-science enthusiasts: Do you want to be able to perform basic water-quality tests on local waters? WWMC offers inexpensive test kits so you can perform your own tests for temperature, pH, turbidity, and dissolved oxygen.

Do you think you know a lot about water properties?
Take our interactive water-properties true/false quiz and test your water knowledge.

Importance of photosynthesis

Photosynthesis is important because it produces food for plants and oxygen. Without photosynthesis, it would not be possible to consume many fruits and vegetables necessary for the human diet. Also, many animals that consume humans could not survive without feeding on plants.

On the other hand, the oxygen that plants produce is necessary so that all life on Earth, including humans, can survive. Photosynthesis is also responsible for maintaining stable levels of oxygen and carbon dioxide in the atmosphere. Without photosynthesis, life on Earth would not be possible.


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