SGM Journals

Research explained · Environmental microbiology

What caused Kerala’s red rain in 2001?

Researchers who cultured particles from rain collected in Changanasserry identified the resulting alga as Trentepohlia. That is direct evidence about that sample. How those particles became airborne, and whether the same explanation fits every colored shower reported across Kerala, are separate questions. [1][2]

See original studies View the SGM research record

Three questions, three kinds of evidence

What was in the rainwater?
Researchers grew an alga from a Changanasserry sample and identified it as Trentepohlia.
The culture is evidence about the sampled material. It cannot identify every particle or every shower. [1]
What was already in the air?
A laser instrument near Trivandrum detected an atmospheric dust layer the previous day. Air-mass modeling pointed to possible long-range transport.
This supports a dust contribution around Trivandrum. The atmospheric measurement did not identify the cells cultured from the Changanasserry rainwater. [1][2]
Did the red cells contain DNA?
Gangappa and Hogg detected a DNA signal after treating red rain cells with a pigment-removing solvent.
The stain established DNA in the tested cells. It did not establish their species or route into the clouds. [3]

Why did researchers suspect a local source?

The original investigation also cultured material from lichen collected near the affected area. The cultures resembled the algae grown from the rainwater. That supports a local source for the sampled algal material, while leaving the mechanism that carried it into the rain open. [1][5]

Where did the meteor claim come from?

Louis and Kumar reported a negative dye test and proposed that a meteor might have delivered unusual cells. The later detection of DNA under different test conditions undercut the claim that the particles were DNA-free. The original dye test provided no direct evidence of where the particles came from. [3][4]

Could algae and dust both be part of the story?

They are findings from different places and different measurements. A culture from collected rainwater helps identify what grew from that sample. A nearby atmospheric dust profile and wind model help trace what was in the air. To show how much each material contributed to a specific shower, researchers would need to connect those measurements to the same rain sample. [1][2]

Original research

  1. Kumar et al. (2002), Eos. Colored rain falls in Kerala, India. Reports the Changanasserry rainwater culture and identification.
  2. Satyanarayana et al. (2004), Aerosol Science and Technology. Colored Rain on the West Coastal Region of India: Was it Due to a Dust Storm?. Describes the dust layer detected near Trivandrum and the air-mass trajectories.
  3. Gangappa and Hogg (2013), Microbiology. DNA unmasked in the red rain cells of Kerala. Figure 5 compares the fluorescent signal in treated and untreated cells.
  4. Louis and Kumar (2006), Astrophysics and Space Science. The Red Rain Phenomenon of Kerala and its Possible Extraterrestrial Origin. Section 9 and Figure 15 document the earlier dye test.
  5. Sampath et al. (2001), CESS and TBGRI. Coloured Rain: A Report on the Phenomenon. The original investigation compares the rainwater cultures with cultures from local lichen.