Colorful niches of microorganisms explained by vibrating water molecules

12 July 2007

The lakes and oceans on our planet are teeming with aquatic microorganisms that absorb light for photosynthesis and thereby provide the basis of the food web while fixing significant amounts of CO2. They come in a variety of colors which allows them to utilize different parts of the solar light spectrum. However, why are some colors used less than others? In the August 2007 issue of the ISME Journal, a new Nature journal, Drs. Maayke Stomp, Prof.dr. Jef Huisman, Prof.dr. Lucas J. Stal and Dr. Hans C.P. Matthijs of the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the Universiteit van Amsterdam give a surprising new answer.

Water molecules are never at rest. They rotate and vibrate by stretching and bending of the bonds within the H2O molecule (see the figure below). These molecular vibrations require energy, which water molecules obtain by absorption of specific colors of the solar light spectrum. As a result, when light traverses through the water column, specific colors of light are attenuated, while other colors pass through unhindered. In the end, the turbidity of the water determines what color of light penetrates the deepest. In clear oceans blue light prevails, in coastal waters it is green light, while in murky peat lakes red light dominates as illiustrated in the photo above. However, all these waters show ‘gaps’ in their underwater light spectra, caused by vibrations of the water molecules.

Many aquatic microorganisms (like phytoplankton, cyanobacteria, and other phototrophic bacteria) rely on solar irradiance for their photosynthesis and growth. Similar to higher plants, these aquatic microorganisms are equipped with pigments to absorb light. However, while higher plants mainly use green pigments for photosynthesis, the pigments of microorganisms cover a much wider variety of colors, giving rise to for instance green, blue, pink, orange and purple species as illustrated in the picture to the right. The different pigments of these species allow them to absorb different parts of the solar light spectrum. Some of these microorganisms can even absorb infrared light for photosynthesis. Yet, the pigments do not cover all parts of the light spectrum.

By means of model simulations, Drs. Maayke Stomp and Prof. dr. Jef Huisman of the Aquatic Microbiology group of the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the Universiteit van Amsterdam calculated the underwater light colors of lakes, seas and oceans of our planet. The model calculations revealed a series of distinct niches in the underwater light spectrum. Each niche covers a different color, and the niches are separated from each other by ‘gaps’ in the underwater light spectra caused by vibrations of the water molecules. They compared these colorful niches with the colors preferentially absorbed by the various pigments of aquatic microorganisms. Strikingly, this revealed that the pigments of these microorganisms exactly match the niches in the underwater light spectrum. It is an excellent adaptation. In this way, aquatic microorganisms make optimal use of the light not utilized by vibrating water molecules. Vibrations of the water molecule thus play a major role in the ecology and evolution of the photosynthetic microorganisms on planet Earth.

The research was not only published in ISME journal of the Nature group, but was also highlighted in News & Views in the journal Nature itself.

Published by  IBED