Petrichor smell & the role of bacteria in our tanks

  • #1
heya guys! so today while i was feeding Benji and Ulysses, I got a good whiff of the tanks espesially near the filters and it smelled very earthy. Kinda like Petrichor (how it smells after it rains). Its actually not a bad smell at all, it makes my room smell a bit like a nice pond. I assume this is a sign that the bacterial colonies within my tank are thriving, as looking through other forums indicates that people take it as a sign of the health of their aquarium. Others also said to watch out for cyanobacteria, as they can produce this scent too. You dont want your tank to reek like a swamp, but it should smell 'alive', like a well maintained pond, the censeus seems to be.

I did a little research and found that this scent is also known as 'Geosmin', and the human nose is sensitive to this scent, ' ranging from 0.006 to 0.01 micrograms per liter in water.' As freshwater fishkeepers, we often only focus on nitrobacter and nitrosomonas, but I'm interested in all the other microorganisms that make up our home aquariums and keep them healthy too!

Wikipedia states: "When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols. Such aerosols carry the scent, as well as bacteria and viruses from the soil. Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why petrichor is more common after light rains. Members of the Actinomycetes, gram-positive bacteria, are responsible for producing these aerosols."

I assume the waterfall effect from my filters is helping to carry these aerosols into my room allowing me to smell it when I get close to the tanks. Plus my room is quite small lol. This kinda sent me down a bit of a rabbit hole on wikipedia.

according to Wikipedia (1, 2,3)

"The scent is primarily produced by Geosmin is produced by various blue-green algae (cyanobacteria) and filamentous bacteria in the class Actinomyces, and also some other prokaryotes and eukaryotes. The main genera in the cyanobacteria that have been shown to produce geosmin include Anabaena, Phormidium, and Planktothrix, while the main genus in the Actinomyces that produces geosmin is Streptomyces."

In fact, it seems that these actinomyces play an active role in our aquariums, as they obviously play a role in other freshwater ecosystems:

"Actinomycetales can be found mostly in soil and decaying organic matter, as well as in living organisms such as humans and animals. They form symbiotic nitrogen fixing associations with over 200 species of plants, and can also serve as growth promoting or biocontrol agents, or cause disease in some species of plants. Actinomycetales can be found in the human urogenital tract as well as in the digestive system including the mouth, throat, and gastrointestinal tract in the form of Helicobacter without causing disease in the host. They also have wide medicinal and botanical applications, and are used as a source of many antibiotics and pesticides."

"Many species of Actinomycetes produce antimicrobial compounds under certain conditions and growth media. Streptomycin, actinomycin, and streptothricin are all medically important antibiotics isolated from Actinomycetes bacteria. Almost two-thirds of the natural antimicrobial drug compounds used currently are produced by different species of Actinomycetes."

"The human nose is sensitive to geosmin and is able to detect it at concentrations as low as 0.4 parts per billion. Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival."

"Geosmin is responsible for the muddy smell in many commercially important freshwater fish such as carp and catfish. Geosmin combines with 2-methylisoborneol, which concentrates in the fatty skin and dark muscle tissues. It breaks down in acid conditions; hence, vinegar and other acidic ingredients are used in fish recipes to reduce the muddy flavor."

"This compound is reported to be an issue for saltwater fish farmed in recirculating aquaculture systems, such as Atlantic salmon, but there are also studies that show that the presence in seawater is significantly lower than that found in freshwater which is why many people consider freshwater fish to taste muddy compared to marine fish. These systems rely on biological filtration using cultured microbial communities to process the nitrogenous waste from the fish (ammonia) into less harmful compounds (nitrite and nitrate) that can be tolerated at higher concentrations."

I thought you guys would find this as interesting as I did, and I wonder if any of you have any insight into how these geosmin-producing bacteria play a role in our aquariums. Any experts in water chemistry or microbiology here wanna chime in? I find this very cool and want to learn more, but most of the subject matter on the internet relating to this specific subject has to do with treating our drinking water, more than anything relating to aquatic life and aquariums specifically.

  • #2
There's plenty of info available. In aquatics, bacteria tend to reside in biofilm structures. This is what you need to search for if you like to learn more about bacteria and microbiota in general.

A biofilm structure consists of all sorts of different bacteria, fungi, algae and protists. The exact configuration depends on conditions.

The biofilm on plants is different from biofilm found in a mechanical filter. The biofilm on substrate is different from biofilm found on the water surface.

If you learn more about this, it will change your thoughts on e.g. aquarium filtration for sure.

  • Thread Starter
  • #3
There's plenty of info available. In aquatics, bacteria tend to reside in biofilm structures. This is what you need to search for if you like to learn more about bacteria and microbiota.

A biofilm structure consists of all sorts of different bacteria, fungi, algae and protists. The exact configuration depends on conditions.

The biofilm on plants is different than in found in a mechanical filter. the biofilm on a substrate is different that found at the water surface.

If you learn more about this, it will change your thoughts on e.g. aquarium filtration for sure.
thank you so much! Sometimes searching on google is as simple as getting the right keywords :)

I find that stuff so interesting. Actually seeing all the little creatures you got in your tank inspired me to look it up more hehe. Also, would it be possible to get a look at some of these microorganisms. do you know any sort of microscope would be suitable? I would like to take a look at whats swimming around in my tank water.
  • #4
When humans talk about bacteria, we often are referring to pathogenic bacteria that cause disease. However, only a very small percentage of bacteria are pathogenic. Most bacteria are beneficial. Without beneficial bacteria, life as we know it would not exist in our bodies, in the soil, in the air, or in water.

While cyanobacteria (aka blue green algae) can be the scourge of aquariums, these bacteria exist in nearly every body of water, both salt and fresh. These bacteria are responsible for production of a significant portion of the oxygen in our atmosphere. So without them, we would not have access to the oxygen we need to live.

Without bacteria, there would be no nitrogen cycle.Nitrogen fixing bacteria in the soil convert atmospheric nitrogen to inorganic compounds which are converted by plants into amino acids/proteins. These plant proteins are consumed by both land and water animals. The waste product is urea which is converted by other bacteria into ammonia. Then nitrifying bacteria convert the ammonia into nitrites and then into nitrates. Finally, denitrifying bacteria can convert the nitrates into nitrogen gas to start the cycle all over again.

Aquarium water always contains a variety of bacteria and other microorganisms. The fishkeeper only needs to be concerned with pathogenic bacteria. If your fish are healthy and the water has an "earthy" smell, then all is well. If you start to detect putrid odors, the something is going on that should not be. Dead fish and snails will give off putrid odors if they are not removed promptly.
  • #5
Aquatic microecosystems are very vast and contain thousands of different species of bacteria, with some species not even being identified yet. Like others said, they survive mostly in biofilms which also contain many different species of bacteria usually working together to promote growth. Bacteria have sophisticated methods of maintaining population density. Biofilms facilitate gene transfer and plasmid transfer, thereby helping the health of the entire film microecosystem. Biofilms are incredible and highly intelligent. I’ve taken many microbiology courses throughout my career and I think you would be very interested in environmental microbiology which deals with soil and aquatic bacterial ecosystems, along with their interactions with archaea and other protozoans. The reason you find a lot of papers relating these concepts to drinking water is because that is the implication of understanding how the bacterial ecosystems work, so even if the study is about drinking water you can still learn a lot. You may also be trying to search using the term “aquarium” when you should be using “body of water” “River” “stream” etc. hopefully I can link the paper I found that would be interesting to you below about the Mediterranean river basin-
Driving Factors of Geosmin Appearance in a Mediterranean River Basin: The Ter River Case

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