Iron and phytoplankton

All phytoplankton need light and nutrients (nitrate and phosphate) to grow
and do photosynthesis. In almost all oceans and seas the growth of
phytoplankton is limited by light (e.g. in winter, when days are short), or
nitrate. Thus, if you take a bit of seawater from just about anywhere you
won’t find much nitrate in there. Except for the Southern Ocean, South of
about 55S. For the longest time oceanographers had no idea why there
weren’t more phytoplankton growing there, as there should be plenty of
light and nitrate available.

It was known that phytoplankton also need a little bit of iron to grow, but
the iron concentrations that researchers measured on their research ships
were so high that that had to be plenty for the phytoplankton. Then, in the
’80s, some smart researchers thought about potential contamination from
their iron research vessels. So they came up with various tricks to be able
to measure iron in water that had not been in contact with the ships hull.
They would take a zodiac out to get water away from the ship, and take this
back to measure the iron concentrations. Or they would have a pump out on
the side of the ship so they could get water away from the wake of the
ship. From these samples it became apparent that the iron concentrations in
the Southern Ocean were much lower than previously thought. And with every
subsequent trick the iron chemists came up, the concentrations became even
lower!

The next step was to prove that phytoplankton would grow better with more
iron. So phytoplankton biologists took bottles with water and phytoplankton
from the Southern Ocean, added iron, and…. Rolling drums… the
phytoplankton began to grow. In addition, researchers would dump a large
amount of iron solution in the water, hang around on their ship, and
measure if more phytoplankton would grow. In a number of cases
phytoplankton did respond to the iron, but in a number of other cases not
much happened.. It seems like in these spots phytoplankton did not have
enough light to use the extra iron/

The relation between iron and phytoplankton is very interesting, since
there are indications that during the last ice ages (when it was colder,
and there was less CO2 in the atmosphere), there was more iron in the
water. If there would be a relation, a few smarty pansies thought, we could
dump a lot of iron in the ocean, which would make the phytoplankton grow
and that would get the CO2 out of the atmosphere. Voila, greenhouse problem
solved! Unfortunately, there are a number of problems with this plan. It
turned out that phytoplankton did not always start to grow after iron was
added, so light was also a factor. And even if phytoplankton start to grow,
this doesn’t mean they will take extra CO2 out of the atmosphere. If
there are a lot of bacteria, they will convert the phytoplankton straight
back to CO2. Finally, researchers found toxic phytoplankton after iron
additions. Not a great idea for birds and whales. On top of this, you have
to drive a ship very far out on the ocean to dump your iron in the water.
This costs a lot of fuel, and thus, CO2. So even if the phytoplankton would
get the CO2 out of the atmosphere, there are countless cheaper
alternatives.

The phytoplankton that grow around Antarctica naturally do have a positive
(=cooling) effect on the climate, and form the basis of the food web for
whales and penguins. Thus, researching the effect of iron and light on
phytoplankton growth is very relevant. Especially as both may change with
global warming. Therefore, our team of Gert, Kate, and myself will
investigate the effect of iron and light on Antarctic phytoplankton.