Historically, the oceans have done much of the planet’s heavy lifting when it comes to sequestering carbon dioxide from the atmosphere. Microscopic organisms known collectively as phytoplankton, which grow throughout the sunlit surface oceans and absorb carbon dioxide through photosynthesis, are a key player.
To help stem escalating carbon dioxide emissions produced by the burning of fossil fuels, some scientists have proposed seeding the oceans with iron an essential ingredient that can stimulate phytoplankton growth. Such iron fertilization would cultivate vast new fields of phytoplankton, particularly in areas normally bereft of marine life.
A new MIT study suggests that iron ferilization may not have a significant impact on phytoplankton growth, at least on a global scale.
The researchers studied the interactions between phytoplankton, iron, and other nutrients in the ocean that help phytoplankton grow. Their simulations suggest that on a global scale, marine life has tuned ocean chemistry through these interactions, evolving to maintain a level of ocean iron that supports a delicate balance of nutrients in various regions of the world.
According to our framework, iron fertilization cannot have a significant overall effect on the amount of carbon in the ocean because the total amount of iron that microbes need is already just right,” says lead author Jonathan Lauderdale, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
The paper’s co-authors are Rogier Braakman, Gael Forget, Stephanie Dutkiewicz, and Mick Follows at MIT.
The iron that phytoplankton depend on to grow comes largely from dust that sweeps over the continents and eventually settles in ocean waters. While huge quantities of iron can be deposited in this way, the majority of this iron quickly sinks, unused, to the seafloor.
The fundamental problem is, marine microbes require iron to grow, but iron doesn’t hang around. Its concentration in the ocean is so miniscule that it’s a treasured resource, Lauderdale says.
Hence, scientists have put forth iron fertilization as a way to introduce more iron into the system. But iron availability to phytoplankton is much higher if it is bound up with certain organic compounds that keep iron in the surface ocean and are themselves produced by phytoplankton. These compounds, known as ligands, constitute what Lauderdale describes as a soup of ingredients that typically come from organic waste products, dead cells, or siderophores molecules that the microbes have evolved to bind specifically with iron.
Not much is known about these iron-trapping ligands at the ecosystem scale, and the team wondered what role the molecules play in regulating the ocean’s capacity to promote the growth of phytoplankton and ultimately absorb carbon dioxide.
People have understood how ligands bind iron, but not what are the emergent properties of such a system at the global scale, and what that means for the biosphere as a whole, Braakman says. That’s what we’ve tried to model here.
Massachusetts Institute of Technology