The Grass of the Sea

Usually we think of seaweed as the 'grass' of the sea.  Actually, diatoms, and more generally, phytoplankton, are the equivalent of sea grasses.  There are diatoms in soil and other places, but diatoms represent 45% of oceanic primary production.  In the ocean, the microscopic organisms convert sunlight and nutrients through photosynthesis into the bottom of the food pyramid. 

Satellites can detect the amount of chlorophyll in the oceans.  Figure 1 below shows the average concentrations of chlorophyll (representing phytoplankton populations) from October 1997 to October 2002.  I find it interesting that with the exception of a small super-productive region at the mouth of the Amazon River off the coast of Brazil, most of the ocean's chlorophyll is the colder water of the poles, not the equator.  That is worrisome as the ocean waters heat up - will we lose a significant contributor to the food pyramid?

 
Figure 1.  Average Chlorophyll Concentration  Credit: SeaWiFS Image Gallery

Diatoms are not plants - they are single celled algae.  They are nearly invisible - hundreds of them can fit on the point of a pin.  You have probably seen the effects of a soaking rain upon a parched landscape - for a short while afterward, everything is in bloom.  The same is true with diatoms, sometimes they reproduce rapidly, causing the blooms noticed along the coasts, or large floating masses observed from space. 

Diatoms are important in our carbon cycle, trapping accumulations of carbon dioxide and turning it into food.  The only external element in the diatom's cycle is silica - the opaline shell of the diatom consists of silica, which protects the diatom's protoplasm.  This of course, contains the chloroplasts, oil globules, and cell nucleus.  Because they require sunlight to perform photosynthesis, they reside either on the surface or sea bottoms no deeper than 600 feet.  The floating diatoms are called planktonic, whereas the bottom dwellers are called benthic.   The floaters are so light that in calm water, it can take hundreds of years for dead cells to reach the bottom of the sea.

Atlantic krill feed giant whales tells that an order of crustaceans related to shrimps and prawns feed upon the diatoms, and that one of the largest species of animals on the earth feed upon the krill.  It is this food chain that traps carbon and returns it to the bottom of the sea where millions of years later we will have new fossil fuels.  We have a marvelous earth, don't we?

For a fascinating tutorial of diatoms, see Diatoms, produced by the Scripps Institution of Oceanography.  Factoid of the day - The Amazon River does not have a bridge crossing it.