SGM Journals

Research explained · Microbiology

Did Kerala’s red rain contain DNA?

Yes. Researchers detected DNA in cells from Kerala’s red rain after removing their red pigment. In a 2013 study, the same stain gave no detectable signal in untreated cells, while the centers of treated cells fluoresced blue. [1]

Read the original paper free View the SGM research record

Same stain, different result

Pigment left in place No detectable fluorescence
Pigment removed with DMSO Blue fluorescence in the cell center
Schematic illustration of the reported result. Compare panels B and D in Figure 5 on page 110 of the original paper.

Why did the two studies get different results?

In 2006, Louis and Kumar crushed the particles and tested them with a fluorescent dye called ethidium bromide. They even repeated the experiment after grinding the cells in liquid nitrogen. The red-cell mixture gave no stronger signal than the dye alone. A yeast sample did give a stronger signal, showing that the setup could detect a signal in a different sample. [2]

In 2013, Gangappa and Hogg used a different dye, DAPI, and compared untreated cells with cells they had treated with the solvent dimethyl sulfoxide (DMSO). Under the microscope, the centers of the treated cells emitted blue light at 461 nanometers, consistent with DAPI bound to DNA. The untreated cells gave no detectable signal under the same staining conditions. [1]

The 2013 team proposed that removing the red pigment helped the stain reach the cell interior. That explains how the same stain could give two different outcomes in their experiment. The exact reason the 2006 test came back negative remains unresolved. [1] [2]

What did the evidence establish?

Did the cells contain DNA?
Yes, in the cells examined by Gangappa and Hogg. In Figure 5, panel B shows the untreated result and panel D shows the blue signal concentrated inside the treated cells. [1]
What organism was it?
The staining experiment left the cells’ identity open. In a separate study, researchers grew material from rainwater collected in Changanasserry and identified the cultured alga as belonging to the genus Trentepohlia. [1] [3]
How did the cells get into the rain?
The DNA test examined what was inside the cells. The path those cells took into the rain remained outside the scope of that experiment. [1]

Original research

  1. Gangappa, R. and Hogg, S. I. (2013). DNA unmasked in the red rain cells of Kerala. Microbiology, 159, 107–111. SGM research record · Publisher and Figure 5. DOI 10.1099/mic.0.062711-0.
  2. Louis, G. and Kumar, A. S. (2006). The Red Rain Phenomenon of Kerala and its Possible Extraterrestrial Origin. Astrophysics and Space Science, 302, 175–187. Original paper, section 9 and Figure 15.
  3. Kumar, V. S., Sampath, S., Mohanan, C. N., and Abraham, T. K. (2002). Colored rain falls in Kerala, India. Eos, 83, 335. Original culture study.