Sea Sawdust - For Better or Worse

For the last two months, the offshore waters of Sanibel have experienced a prolific growth of a phytoplankton known as sea-sawdust. This phytoplankton is a type of bacteria – a cyanobacteria, previously known as a blue-green algae.

Within the past two weeks some of these clouds of dying phytoplankton have moved onshore and into San Carlos Bay.

This particular type of cyanobacteria occurs throughout the world in tropical and subtropical waters; it is in the genus Trichodesmium and there are about 8 known species. It was first recorded by Captain Cook in the 18th Century off the Australian coast. It was so extensive he mistook it for a sandbar and was afraid he was going aground. It is light brown in color and in calm conditions colonies of this bacteria (each colony looks like a microscopic brown rice grain) are caught in the surface tension of the water and float. It does look very much like a sandbar and still fools captains today.

Like so many things, it has good qualities and some not-so-good qualities. First , the good.

Estimates suggest that Trichodesmium accounts for perhaps 18% of the total photosynthesis on the planet – which makes it a very important component to life on this planet as we know it.
It is also fascinating because it can absorb nitrogen from air. Typically ocean waters are fairly nitrogen-poor and Trichodesmium is probably an important avenue through which ocean water gets badly needed nitrogen to support marine food webs. It gets this nitrogen from thin air (which is actually 70% nitrogen) especially well in waters that have high phosphorous concentrations; which the ocean waters near SW Florida naturally have.

It is also fascinating in that it undergoes daily migration patterns. Up to close to the surface during the day to harvest sunlight and nitrogen and then it sinks down deeper in the ocean during the nighttime. Pretty sophisticated behavior patterns for lowly bacteria. But, when there is no wind, it gets caught in the surface tension and makes big rafts on top of the water.
Now for some of the not-so-good stuff. It makes compounds that are toxic. In Brazil, in 1962 a human illness was described and linked to Trichodesmium; it is called Tamandare’ Fever (for the Bay in which it was described) or Trichodesmium Fever. Its symptoms include throat and respiratory irritations, extreme fatigue, joint and muscle soreness and post-orbital soreness (the backs of one’s eyes feeling sore). Many of these symptoms are shared with common colds, so the extent of this disease is rather unknown. It also appears that the reaction is rather short-lived – so that symptoms may go away rather quickly. Very little in know about this ailment and research is being done to know whether the species we have also can be responsible for this disease or other health issues.

The other not-so-good thing is that is has a particular odor and when colonies are decaying they smell bad.

So, next time you inhale, thank Trichodesmium for providing 18% of the oxygen you are putting in your blood, try to ignore the smell of their colonies dying, hope we don’t have a species that is dangerous to humans, and remember that among the wonders of living on our Sanctuary Island is the constant reminder of how we are a part of our natural, marine world.