Can UV Exposure Alter Marine Nitrogen Fixation?

The oceans contain carbon fixation superpowers. Forests are certainly an integral part of global ecology, but the oceans? Phytoplankton (photosynthetic creatures so small they can only be seen under a microscope) are producing half of the air you breathe. That also means they are taking carbon dioxide out of the air and turn it into food. To do that, however, they need some extra vitamin boosts.

Phosphorus and nitrogen are both necessary for a cell to fix carbon. Phosphorus is a rare nutrient in the oceans, and its concentration has a lot of control over how much cell productivity can occur in the first place. Nitrogen, on the other hand, is more abundant than oxygen itself: nitrogen gas makes up 70% of our atmosphere. Unfortunately, it also exists as N₂, or two nitrogen atoms bound with the nearly unbreakable strength of three molecular bonds. Nitrogen is plentiful, but also largely inaccessible to life.

Fixing the Problem

Life goes on, however, but why? There exist organisms capable of breaking the triple bonded nitrogen molecule. These organisms are known as nitrogen fixers, and they convert nitrogen gas into ammonia. Some plants are capable of doing this, including our own very nutritious legumes and the white tailed deer’s proteinaceous equivalent, buckbrush. In these cases, however, it’s not actually the plant fixing anything: these species are symbiotic with nitrogen-fixing bacteria. Similar bacteria exist in the world’s oceans.

In the oceans, there are two major species that do this. Trichodesmium and Crocosphaera are cyanobacteria that can work with nitrogen gas. Thus, they are the major nitrogen sources for marine ecosystems around the world.

Sea and Sky

The nitrogen cycle does not happen in a vacuum. These bacteria are operating up and down within this uppermost, sunlit layer, and are subject to chemical variations and the weather. In a 2020 paper by Zhu et al., scientists consider how nitrogen cycling in these two species might change where chemical variations (in phosphorus availability) and environmental variations (in UV exposure) occur.

When it comes to marine life, sea and sky are intimately linked. Oceanography and atmospheric scientists often work together, as changes in each cause changes in the other. In fact, we aren’t really sure if El Niño is caused by atmospheric pressure changes (that trigger changes in sea surface temperature) or changes in sea surface temperature (that trigger atmospheric pressure changes). It’s an old-fashioned chicken-and-egg sort of problem.

Surface-dwelling microbes, then, are living right on this dynamic boundary. In last week’s journal club, we introduced the thermocline and halocline boundaries. The sea surface is warm and bathed in sunshine; go down to the thermocline, and you’ll hit a very sudden drop into much cooler water. That change in temperature acts as a physical boundary, dividing surface waters from the deep. This actually has a lot of implications for the environment.

The upper waters are full of cyanobacteria (and other plankton), fixing carbon and fixing nitrogen. They are eating up nutrients and that means scarce resources. The upper layers are subject to the weather, losing salinity if it rains, and temperature rising and falling with the sun. The Deep, however, is much more static. The temperature is cool, and stays that way. There is not much to change how salty it is, and with a constant influx of dead stuff floating down from the upper layers, and the earth itself offering vitamins through its heated hydrothermal vents, it is actually full of nutrients.

The thermocline is a boundary, but there are exceptions. Upwelling and downwelling and vertical ocean currents, and in different regions, downwelling sends what little nutrients there are on the surface into the abyss, while upwelling floods the starved surface with cool, rich waters.

The sharper the temperature change is, the more difficult it is for these vertical currents to share their waters. What exactly would cause a thermocline to be ‘sharper’ in this way? Recall the temperature in the deep is mostly constant; the surface waters, however, are at the whim of the capricious sea-sky boundary. When temperatures increase, the sea surface temperature warms, making the difference between surface and abyss even stronger. This results in a more intense thermocline. We typically call this intensifying thermocline stratification.

Stratification is often accompanied by shoaling, which is a shallower thermocline. When sea surface temperatures warm, the rapid change in temperature moves from to a shallower depth. That thermocline acts as a boundary, so when is shoals, approaching the surface, it squishes the microbes into a slimmer surface layer.

Marine plankton live in the upper layer of the ocean. There are benefits to being at the very top of the ocean, where you get the most sunlight. But you also get the most UV there! Most plankton crowd at the surface, and then concentrations of plankton decrease as you move down the sunlit layer, where less and less sunlight gets through. Photosynthesis is less productive, but you also get less UV exposure.

If the thermocline shoals, the upper layer rises, pushing plankton closer to the surface. Now the whole population is forced to deal with more UV exposure.

In other words, changes in the climate control the lives and livelihood of marine plankton on the ocean surface. That includes nitrogen fixers like Trichodesmium and Crocosphaera. These species are the primary sources of nitrogen for their surrounding ecosystem! Can these climate changes impact the available nutrients in the ocean?

Testing the System: UV Impacts on Nitrogen Fixation

In Zhu et al., we get a glimpse into the scientific prodding of that just that question. They wanted to look at both species, since they fix nitrogen during different times. Trichodesmium fixes nitrogen during the day, working under whatever UV it is subjected to. Crocosphaera, however, fixes nitrogen at night. Will it fare better under higher UV?

To find out, they picked out a few species from each genus and grew them in a laboratory. They exposed the bacteria to three different light wavelengths: UV-A, UV-B, and something good for photosynthesis. To measure how well the microbes are holding up, they looked at growth rate, nitrogen fixation rate, carbon fixation rate, and nutrients taken up by the cell (i.e., particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP)).

Both species produce an enzyme called nitrogenase to break down the triple bond in nitrogen. However, Trichodesmium produces it during the day; Crocosphaera produces it just before nightfall. Thus, Trichodesmium gets more UV-degradation of the enzyme. However, UV-A and UV-B were separately shown to affect growth rate. That means that there are two ways UV might negatively affect nitrogen fixation: it can directly interfere with nitrogen fixing enzymes, or it might harm the cell as a whole, reducing overall productivity.

Nitrogen fixation and carbon fixation were reduced by UV exposure, here with UV-B showing a stronger negative effect than UV-A wavelengths.

They also found that Trichodesmium increased their filamentation when exposed to stress. It would appear that they use this cell shape to collect into clumps, protecting each other from something like excessive UV exposure. Despite this, Crocosphaera fares much better than Trichodesmium under increased UV exposure. So overall, it appears that nitrogen fixation by day or night is the most important factor in a cell’s survival and productivity under these conditions.

As a whole, this research depends on the principle that increased temperatures will stratify the oceans, constraining plankton to a shallower surface layer. This will result in increased UV exposure. The research suggest that increased UV exposure decreases nitrogen fixation. Since these bacteria are the main nitrogen nutrient sources in the oceans, the implications are that this will result in nutrient limitation in the future.

The plankton in the ocean, including cyanobacteria like these ones, are keystone species. They provide nutrition for entire food webs, and their chemical fixation is a major part of global biogeochemical cycling for carbon and nitrogen. Continued research on their physiology can help us better understand their role in the global climate, how they may be influenced by climate change, and how they may influence the climate themselves.

References

Zhu, Z., Fu, F., Qu, P., Mak, E. W. K., Jiang, H., Zhang, R., Zhu, Z., Gao, K., & Hutchins, D. A. (2020). Interactions between ultraviolet radiation exposure and phosphorus limitation in the marine nitrogen-fixing cyanobacteria Trichodesmium and Crocosphaera. Limnology and Oceanography, 65(2), 363–376. https://doi.org/10.1002/lno.11304