Geology Reference
In-Depth Information
Figure 28. The DMSP molecule, with positively and negatively charged ions (circled) clustering
around it.
When the algal cell dies, or is eaten by predators, the DMSP rapidly degrades into DMS
gas, so in this scenario DMS is no more than a mere by-product.
It turns out that there might be another benefit of making DMSP— protection from
predators. The algae make an enzyme which breaks down DMSP into DMS and acrylic
acid, a rather acerbic, foul-tasting molecule which is very good at deterring small fish
and planktonic unicellular carnivores, such as the silica-secreting radiolarians that love
to eat the algae; but here, once again, DMS is again no more than a mere by-product.
Could there be a direct benefit from producing DMS? After food and sex, dispersal is
of major importance for living things, including our tiny oceanic algae. Ocean currents
have stirred up sediments from the bottom of the ocean, and all sorts of algae have taken
advantage of the nutrient bonanza coming up from the depths, until the nutrients are al-
most depleted. Most DMS-emitting algae, such as our coccolithophores, bloom in vast
numbers when the nutrients have almost disappeared, perhaps because they experience
less competition from other species in nutrient poor waters. But it isn't too long before
nutrient levels become so low that the spectre of starvation looms large. From the per-
spective of a single coccolithophore cell, the situation seems hopeless. The cell hasn't
got the physical capability to swim to a new nutrient-rich patch of ocean, and if it's sum-
mer time and a lid of warm water has formed on the ocean surface, there is no possibility
that currents will bring up fresh nutrients from the sediments below. But the algae seem
to have developed a cunning way of averting disaster.
We are back inside our coccolithophore, which has sensed the fact that nutrient levels
in the surrounding sea water are becoming dangerously low. Suddenly we see huge num-
bers of DMSP-digesting enzymes appearing as if from nowhere to begin the conversion
en masse
of DMSP to DMS gas and acrylic acid. The DMS gas rapidly leaves the cell
through the cell membrane, journeying into the air via the sea, and the acrylic acid helps
to deter passing predatory beings. As billions of tiny algal cells respond in this way to
the nutrient crisis, the air above the bloom receives a great pulse of DMS that triggers
the condensation of massive, dense white clouds above the ocean. As the clouds form, a
huge quantity of energy is released as heat that makes the clouds rise. The newly form-
ing clouds literally hoist themselves aloft, sucking air in behind them, much as air is
drawn into a bicycle pump when the plunger is pulled outwards. The coccolithophores
begin to experience wave action as the updrafts of air stir the surface of the ocean, and if
our cell happens to be close to the water surface during a strong upward gust, it stands a
good chance of being sucked up into the air and into a rapidly ascending cloud. What an
experience it must be for a tiny sub-microscopic speck of marine life, which until now
