Microbes: A Quest for Iron
Did you know that you can find a whole ecosystem in a single drop of seawater? This photo from David Littschwager shows a suite of marine “microbes” – any organisms that are too small for our naked eyes. If this photo was magnified further, you may have also spotted a million bacteria, ten million viruses and even more. Microbes are amazingly diverse (as you can see from the photo!) and vital to fueling the ocean. They are responsible for roughly half of photosynthesis on Earth, become food to other larger organisms and eventually support the fisheries, and break down materials released by other living things. But as any living things, they also require a number of nutrients to live, one of which is iron.
Now you may wonder how microbes in the ocean are getting iron in the ocean, especially if you assumed correctly that iron doesn’t really dissolve well in natural seawater. In fact, there is as much iron in seawater as a drop of ink in 10,000 gallons of water. How are the microbes getting all the iron they need for their growth? Why did the microbes even choose iron as a necessary nutrient during their evolution?
Actually, using iron as a nutrient was probably a natural choice for the microbes when they first appeared in the ancient ocean 3.5 billion years ago. The early ocean was very different from today’s ocean, as it was much more acidic (as much as a glass of water with a teaspoon of lemon juice) and anoxic (very little oxygen for any modern-day fish to survive). But iron actually dissolves very well in acidic and anoxic water, so it would have been very easy for the ancient microbes to use iron in the surrounding water. However, about 2.5 billion years ago, organisms that can produce oxygen by photosynthesis started to appear, and oxygen content in seawater started to increase. Dissolved iron in seawater started to react with oxygen, precipitate into solids and sink out to the bottom of the ocean, and these solid forms of iron could not be used by microbes anymore. The microbes needed to develop a way to survive with decreasing amounts of iron in the ocean.
Fortunately, microbes have found a way to deal with iron limitation in the ocean by producing materials called “siderophores” (“iron carrier” in Greek), which grab onto iron very strongly. When microbes release siderophores into the surrounding seawater, siderophores will stick to iron before it reacts with oxygen and becomes unavailable to microbes so that microbes can pick up siderophore-bound iron. You can think of this as finding a needle in the haystack, but instead of searching through the haystack with your bare hands, you’re using a powerful magnet so that you can simply pick up the magnet and the needle attached to it. There are hundreds of known siderophores so far, and while some microbes can only take up iron bound to the specific siderophores they produced, other sneaky microbes can also take up iron bound to siderophores produced by other than themselves.
Since siderophores were discovered, scientists found that they can have important applications in different fields like medicine and agriculture. Siderophores are being used to develop antibiotics and treat diseases related to iron deficiency or overload, or to clean up soil contaminated by heavy metals like iron. What microbes successfully developed as a survival strategy is holding great promises for a variety of fields!
** Visit http://dive-shield.us/infonewspages/Underthemicroscopejustasplashofseawater.html if you want to learn more about what you’re seeing in the photo.
Jiwoon Park is a PhD student in chemical oceanography. She studies organic molecules that are produced by marine microbes to efficiently take up metals (e.g. iron, cobalt) for growth, to understand how the production or consumption of organic molecules may affect metal availability and eventually the microbial population in the North Pacific Ocean.