Do raindrops contain bacteria?

Submitted by chalamo1993 t3_xvyomg in askscience

I know there's life in pretty much every body of water in varying quantities, but since raindrops form in the atmosphere, are there already lifeforms in them at their formation? What about during the raindrop's flight?

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iayork t1_ir56nc0 wrote

Bacteria are definitely found in rain, though not at high concentrations:

> Rain samples were collected in sterilized 3-L beakers equipped with sterilized homemade stainless steel rain collectors. … All of the materials used for sampling were sterilized by autoclaving, and a sterile mask and gloves were worn during sample collection to avoid any potential contamination. … DNA analysis by Sanger sequencing was carried out for these particles >220 nm in the 9 rainwater samples. No isolated strain culture test was carried out. A total of 908 bacterial clones and 470 fungal clones were generated across all samples. … Pseudomonas, composed of numerous species of highly IN active bacteria, was detected in four samples (July 2013, July 2012, August 2012 and August 2011), accounting for 25%, 2%, 6%, and 7% of each sample sequence, respectively.

—Evidence for a missing source of efficient ice nuclei

It’s been proposed that bacteria are important for ice nucleation in the atmosphere, and may have an important role in forming clouding and triggering precipitation.

> It has been known for several decades that some bioaerosols, such as ice-nucleation-active (INA) bacteria, especially Pseudomonas syringae strains, may play a critical potential role in the formation of clouds and precipitation.

—Evidence for a missing source of efficient ice nuclei

But it still seems unclear how important they are, if at all.

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regular_modern_girl t1_ir6k8r2 wrote

This is a single-celled green algae rather than bacteria, but it still fits the spirit of OP’s question as it’s an example of high concentrations of a microbe in rain; the rare but officially-recorded phenomenon of red rain or “blood rain” seems to very likely be the result of an unusual bloom of the microalgae Trentepohlia in storm clouds, or at least that seems most likely from studies of recent instances of the phenomenon in Southern India. Exactly why this occurs is not understood, although it has been recorded multiple times in the same geographic areas.

There are, of course, anecdotal reports of “blood rains” in various parts of the world going back to antiquity, which could conceivably be due to the same algae or a similar one, but since those instances generally fall more into the realm of folklore and mythology at a certain point, I’ll save speculation on them.

EDIT: since the above hypertext is just a news article, here’s an actual paper on the subject. Just wanted to make sure I had something truly scientific in here, since this a subject that got taken up by ufologists and conspiracy theorists when it was first being reported in the ‘00s, and there was all sorts of wild speculation about the cells in the rain being extraterrestrial in origin (with no real basis, of course), so I figured detailed published research on the Trentepohlia theory from a reputable journal was needed. Also, it seems that most likely it isn’t an algal bloom in storm clouds so much as the cells being swept up in storms from aquatic environments on the surface.

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JoshN1986 t1_ir87r55 wrote

> Natural raindrops contain bacteria at a concentration of 1.06 × 10 4 (/cm 3 ), including plant pathogens such as Pseudomonas syringae , Xanthomonas campestris, and Pantoea ananatis . Likewise, raindrops contain fungi such as Alternaria sp., Fusarium sp., Cladosporium sp., Phoma sp., Rhizopus sp., and Botrytis cinerea. …”

From: https://www.nature.com/articles/s41467-022-28813-8

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