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Biological oceanography

Microbes pull nitrogen out of air to feed the warming oceans

Reviewing: Yang, N., Merkel, C. A., Lin, Y. A., Levine, N. M., Hawco, N. J., Jiang, H. B., Qu, P., DeMers, M. A., Webb, E. A., Fu, F., Hutchins, D. A. (2021). Warming iron-limited oceans enhance nitrogen fixation and drive biogeographic specialization of the globally important cyanobacterium Crocosphaera. Frontiers in Marine Science.

 

Structure of DNA, a molecule that carries genetic information in all living organisms. Nitrogen atoms are shown as blue balls. (Wikimedia Commons)

 

 

Step outside your house and you will find nitrogen everywhere, in the air you breathe, the soil you stand on, the rivers flowing by. Nitrogen is a very important nutrient to all life on Earth because it makes up DNAs and proteins, which carry the biological information to build life, form and repair bodies, and fuel energy. Marine microbes are no different from us, but while we can get nitrogen from eating plants and animals, these microbes need to get it from their surrounding environment – the ocean.

 

Unfortunately, there is not enough nitrogen in the ocean to support all marine life. While nitrogen is very abundant in the air we breathe – making up three-quarters of the air – this form of nitrogen cannot be used by ocean microbes. Instead, they need to convert nitrogen gas to different forms of nitrogen (such as nitrate) to use them as food. This process is called “nitrogen fixation”, and some microbes have acquired this ability over thousands of years so that they can survive in a nitrogen-limited environment. Trichodesmium and Crocosphaera are two marine microbes that are responsible for about half of the nitrogen fixation in the ocean. Once nitrogen in the air is fixed by these organisms, microbes can use fixed nitrogen to carry out photosynthesis, a process that uses carbon dioxide and sunlight to make food for themselves and oxygen for us to breathe. This is an important process not only for the ocean ecosystem but for us, as every other breath we take comes from photosynthesis in the ocean.

Tricodesmium (left) and Crocosphaera (right) are two common cyanobacteria that can fix nitrogen from the air (left from Flickr; right from Zehr et al. 2001)

When Trichodesmium flourish in the ocean, they create golden-brown surface mats that look like this photo (taken near the Great Barrier Reef), which gave them the nickname “sawdust of the sea”.  (Wikimedia Commons)

 

 

 

 

 

 

 

 

 

 

 

The efficiency of nitrogen fixation is controlled by many factors. For instance, different nitrogen fixing organisms grow and survive better in warmer or colder temperatures, so ocean temperature can control how much nitrogen gets fixed. The availability of iron is also important, because microbes cannot make enzymes and proteins that promotes nitrogen fixation and photosynthesis without iron. But when there’s not enough iron in the ocean, microbes can actually adjust how much iron they need in their cells, so that they can use less iron to fix nitrogen (thereby increasing the efficiency of iron use). A team of scientists had looked into the effect of temperature and iron availability on the growth of Trichodesmium and found that Trichodesmium can use iron very efficiently at tropical surface ocean temperatures (27-32°C). Then, a different group of scientists asked, “Would Crocosphaera also respond to temperature and iron availability in the same way Trichodesmium do?”.

Annual mean global sea surface temperature (1971-2000) (from NOAA/National Weather Service)

To answer this question, the scientists grew Croscosphaera at five different temperatures, ranging from 20 to 36°C, and supplied them with either high or low iron. They discovered that when not enough iron was supplied, Crocosphaera was also capable of reducing the amount of iron in their cells so that they can keep up with fixing nitrogen. However, Crocosphaera could fix nitrogen most efficiently at 22-27°C, a temperature range slightly below the ideal range for Trichodesmium. This suggests that as we continue to burn fossil fuels and warm the future ocean, most tropical oceans may be too warm for Crocosphaera to efficiently fix nitrogen and grow quickly.

 

The scientists took a step further to estimate how much nitrogen will be fixed by both microbes by the year 2100, assuming that we continue to burn as much fossil fuel as we are burning now. They calculated that the rate of global nitrogen fixation by Crocosphaera and Trichodesmium would increase up to 91% and 22% respectively. In particular, Crocosphaera can fix nitrogen more effectively in the higher latitude oceans, which is a bit too cold for them in the present day ocean, but would warm over the next few decades. While Crocosphaera cannot fix nitrogen as effectively in the future tropical ocean, the tropical ocean would still be fueled by Trichodesmium, which fix nitrogen most efficiently at high temperatures.

In summary, these projections suggest that with the ocean warming, Crocosphaera population would increase in the higher latitude oceans and Trichodesmium population would increase in the tropical oceans. This could bring major changes in fixed nitrogen availability in the future oceans, and eventually affect how much carbon dioxide gets drawn into the ocean via photosynthesis.

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