Ecology of nitrogen fixers in the Pacific Ocean

Limiting nutrients and abundance of nitrogen fixers in the Pacific Ocean.

Nitrogen is an essential component of all cells. It is used to make the amino acid building blocks of proteins, and is also required in the nucleic acids of DNA and RNA. Although nitrogen extremely abundant in the open ocean, it is mostly found in dissolved N₂ molecules that cannot be used by most phytoplankton, who require nitrogen in its reduced, or “fixed” forms, such as nitrate or ammonium.

Fixed nitrogen is rare across large tracts of the ocean, and phytoplankton abundance is closely tied to its availability. There are however a small but important group of marine microbes who can bypass the need for reduced nitrogen, by fixing their own.

These organisms are known as diazotrophs, a term that derives from the Greek words dis, azōos, and trophikos. The diazo- suffix is in this case drawn from the scientific shorthand for a paired nitrogen bond, but azōos and trophikos literally mean “non-living”, and “nutrition”. Diazotrophs use the inert nitrogen gas that is dissolved in the ocean to make new organic compounds, significantly increasing productivity across large areas of the ocean where other forms of nitrogen are in short supply.

Working with scientists at MIT and the University of Bristol, we have been investigating the large scale distribution of diazotrophs in the Pacific Ocean. We know that there are many thousands of different species of phytoplankton that compete and coexist together in the ocean, but by reducing the system down to a number of key equations, we have been able to show that iron is the dominant limiting factor controlling the distribution of nitrogen fixers in the Pacific.

The model equations broke down in a way that clearly explained the behaviour of a much more complex and realistic model, and we found that the Pacific Ocean can be divided up into three key provinces. Over the much of the equatorial, and Southern Pacific, phytoplankton are strongly limited by a lack of mineral iron, which is essential for growth. The diazotrophs in particular need a lot of this essential micronutrient, and where it is scarce they are outcompeted by the regular phytoplankton.

Further north, iron is added to the ocean as large amounts of mineral rich dust is carried in the winds blowing off Asia and Australia. This addition of iron to the ocean quickly relieves the iron limitation among the regular phytoplankton. This allows them to grow more, but in doing so they quickly become nitrogen limited instead.

In theory, this is where the nitrogen fixers should come in to there own. They can fix their own nitrogen, and are known to thrive where other phytoplankton are starved of of this nutrient. Our analysis reveals that while this is true, it is not quite the whole story.

The high iron content of nitrogen fixing enzymes means that diazotrophs need more iron than most other phytoplankton. Consequently, there are large patches of the Pacific where iron levels are still too low to support this group, when the other phytoplankton have abundant iron. In the figure above, the boundary between iron and nitrogen limitation for the regular phytoplankton is shown with a dotted line, but we can see that this does not mark out the regions where the diazotrophs are able to grow (shown with a solid black line). Only when the iron supply was noticeably higher where the diazotrophs able to survive.

We found that diazotroph ecology is more closely governed by the ratio of iron to nitrogen supplied to the ocean surface, rather than to the absolute concentrations. The theoretical study has helped us to better understand the factors that control the growth of these important marine microbes. By adding fixed nitrogen, diazotrophs play a potentially significant role in regulating the biological uptake of carbon by the ocean. We hope that by breaking the complex marine system down into its component parts, we will be better able to understand how it has changed in the past, and how it might respond in the future.

This study is currently under peer-review with Global Biogeochemical Cycles:

Dutkiewicz, S., Ward, B.A., Monteiro, F. and Follows, M.J. Interconnection of nitrogen fixers and iron in the Pacific Ocean: Theory and numerical simulations. (in review)