About the Author

When I was a student at the University of Amsterdam, I was introduced to desmids by Peter Coesel, who was at that time a researcher at the Department of Biology. After my study I changed direction and started a career in information technology. Twenty years later I spoke to Peter at his farewell ceremony and he told me that he had set up a working group of desmids enthusiasts, and asked me if I would like to join this group, which I did. I subsequently bought a microscope and a digital camera and started to examine samples that I collected during a visit to the Yukon (Canada).

What struck me was the large diversity of species I encountered there, compared to the Dutch samples that I have studied at my time at the university. How is it possible that samples that originated from ponds that are frozen solid for half of the year are so much richer than samples from a more temperate region like the Netherlands? I wondered how the situation would be in other parts of the world, especially in the tropics. There is a rule in biology that says that species diversity increases from the poles to the tropics (and that biomass decreases). Would samples taken from tropical Africa compete with samples from Canada?

author.jpgIn 2001 I visited Madagascar to witness a solar eclipse. While I was there I collected some samples and back in the Netherlands I was quite disappointed about the number of species in the samples. Did I collect samples from the wrong sites or was it because Madagascar was an island? Later Peter told me that that he also had visited Madagascar some years earlier and found more species then I, but not as many as one would expect from a tropical region and, most importantly, that endemic desmid species do not inhabit this island. That is remarkable, because Madagascar is famous for its indigenous flora and fauna. The majority of plant and animal species found on this island are only found there.

It seemed that microorganisms follow different patterns concerning there geographical distribution than higher plants and animals. Evolution explains how a species in time can change into a new species, but not why there are so many plant and animal species on our planet. Why are there approx. 3.000 desmid species (just a guess) and not 30.000 or 300? At the moment the only hypothesis that explains the diversity is the one that assumes that a population of one species becomes spatial separated by (mostly geographical) barriers and that in time the different populations that are separated from each other follow separate and thus different evolutionary lines. But how about microorganisms? For them there are almost no barriers. They are so light that they are easily transported through the air or attached to the legs and feathers of migratory birds, even from one continent to another. For microorganisms you would expect that there are no barriers. Microorganisms certainly inhabit larger areas then higher plants and animals, regularly spanning more than one continent, but there are species that can only be found on one continent or in a specific climatic region.

bangweuluswamp.jpgTo unravel the mechanisms of desmid dispersion it is necessary to know how the different species are distributed in the world. Some regions in the world, like Europe and North America are explored intensively, but other places are still blind spots. Because filling in the gaps would take more than a lifetime, I started to ask myself what the most efficient way is to contribute to this problem. It had already come to my attention that there are places in the world that are so rich in desmid species that they are called hotspots. It seems that in these hotspots you can find a wide spectrum of the desmid diversity that reflects the flora of a much larger area that surrounds these hotspots. The trick was to find these hotspots. My first choice fell upon Africa and, together with Peter Coesel, I visited the Okavango Delta in Botswana and Namibia. Although we found a rich desmid flora there, it was not the hotspot that we had hoped to find. So later we decided to visit Africa again and visited the Bangweulu Marshes in Zambia. Fortunately this turned out to be the right hotspot, and at the moment we are analyzing the samples that we have collected there. Of course I have to visit other hotspots in the world. The Amazon region in South America and the Indo-Australian Archipelago are high on my list and I hope to collect there in the future.

The author taking a sample at the Kenilworth Park & Aquatic Gardens in Washington,DC (above) and a endless field of water lilies (Nymphaea) and water shields (Brasenia) in the Bangweulu Swamp in Zambia.

Publications

Coesel, P.F.M. & Van Geest, A. (2008): Taxonomic and biographical notes on Okavango desmids. Syst. Geogr. Pl. 78: 27-46.
Coesel, P.F.M. & Van Geest, A. (2009): Taxonomic and biographical notes on Okavango desmids, II: genera Cosmarium, Xanthidium and Staurastrum. Syst. Geogr. Pl. 79: 15-31.
Van Geest, A. & Coesel, P.F.M. (2012): Desmids from Lake Nabugabo (Uganda) and adjacent peat bogs. Fottea 12 (1): 95-110.
Coesel, P.F.M. & Van Geest, A. (2014): New or otherwise interesting desmid taxa from the Bangweulu region (Zambia). 1. Genera Micrasterias and Allorgeia (Desmidiales). Plant Ecology and Evolution 147 (3): 392–404.
Coesel, P.F.M. & Van Geest, A. (2016): New or otherwise interesting desmid taxa from the Bangweulu region (Zambia). 2. Genera Staurodesmus, Staurastrum and Xanthidium (Desmidiales). Plant Ecology and Evolution 149 (1): 101-111.