Bacteria have the unfortunate disability of being much too small to be aware of gravity, thus
are unable to tell up from down. They overcome this disadvantage with magnetotaxis. They
use it to navigate to the water depth with optimum oxygen levels, usually the OATZ (oxicanoxic
transition zone), where highly oxygenated surface water meets oxygen-depleted water.
The orientation of the Earth’s magnetic field means that magnetotactic bacteria in the
southern hemisphere swim along the magnetic field lines to go up, bacteria in the north swim
against the field lines to go up, and indeed if you switch northern and southern bacteria, they
swim in the wrong direction.
This is very useful for bacteria, but how does this apply to larger organisms like birds, who are
famously good navigators? They’re born with this ability, so it must be some biological
process, but magnetotaxis seems unlikely; a bird filled with enough iron to have a noticeable
magnetic force may have some trouble getting off the ground. So scientists looked for a more
sophisticated mechanism.
They discovered that
birds became
disoriented in red light.
The necessity for a
certain wavelength of
light (about 500nm)
points towards a
photochemical reaction
which involves radicals;
these are atoms or
molecules with
unpaired electrons and
therefore the electron
‘spin’ creates a
relatively large amount
of magnetic torque
when it is not paired
with an electron of
opposite spin in an
orbital.
Researchers Anja Günther et al. looked at fluctuations
in a class of protein called cryptochromes present in
birds, because they are the only protein in vertebrates
that forms radical pairs in the presence of blue light.
They found that cryptochrome 4 (Cry4) maintained
fairly steady levels in the birds’ retinas, while Cry1, 2
and 3 fluctuated throughout the day due to the
proteins’ use in circadian rhythms (natural sense of the
passage of time, or what time of day it is). Further
evidence that this protein is involved in magnetic
navigation came from comparing Cry4 levels in the
robins’ retinas compared to non-migratory chickens.
Sure enough, Cry4 levels significantly increase during
migratory seasons in robins but not in chickens. While
the exact mechanism for the reaction is not fully
understood, it’s clear that it at least involves this
protein and its radicals. Also, the fact that this reaction
is produced from light excitation and takes place in the
retina means that
birds don’t just sense or feel the
magnetic pull, they literally see it.