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I've often come across references of filter bacteria competing with plants for certain nutrients. This then leads folks to 'underfilter'. What exactly are these 'nutrients' that are being competed for?

Someone told me once that one of those nutrients is nitrogen, which I did not understand. Aren't the bacteria in our filters ultimately converting NH4/NH3 to NO3? If so, then why would that be a bother? Aren't plants capable of uptaking either forms?

Thank you. :-k
 

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The only competition I'm aware of is for ammonium. The extra energy that plants need to assimilate nitrate rather than ammonium may account for a small difference in their growth rate. I don't see much reason to let that stop you from using a filter on a planted tank. I kept tanks without filters for a few years and I still keep my tanks pretty lightly filtered. Generally other problems tend to outweigh any increase in growth you might get from leaving the tank unfiltered.

(Edit) I should add that there is one important resource that the bacteria are competing for -- your time. If you don't have a filter on your tank you can spend less time cleaning filters and more time gardening.


Roger Miller
 

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Bacteria can compete for nitrogen sources, but the competition is not very significant except under unusual conditions. If there is a lot of carbohydrate as a food source bacteria can use it for food, and take up nitrate or ammonia so that they can make proteins, just as plants do. Given a pure carbohydrate food source and mineral nutrients, bacteria can grow and multiply, using the mineral nutrients the same way that plants do. In fact, they need virtually the same list of nutrients as plants. However, the biomass of bacteria in your filter and other places in your aquarium is rather small, and their consumption of mineral nutrients is neglegable, compared to that of the plants.

The competition of bacteria for nutrients can be seen if you mix a large volume of dead leaves in soil. The leaves are very low in mineral nutrients, and you will see rather deficient plant growth.

When fungi are decomposing wood, they also need mineral nutrients lacking in wood, and they have all kinds of ways of getting N and P. They trap nematodes in a variety of ways and digest them just for their nutrients. They even digest bacterial colonies for the same reasons.
 

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I think I saw a reference that a typical wetland soil has around 1-5% ocked up as bacterial biomass. Take that biomass and get a N amount and that should be relatively close to the N needs. Our tanks will have less bacteria than a natural wetland soils most likely.

But....the turn over rate is very high with bacteria. How much is recycled and how much is needed for new growth is another matter. N15 studies are fairly decent for answering these questions. Submersed Aquatic Plant turnover is very very slow. Years vs days.
Dr Reddy has an enriched artificial wetland meter ^3. I'm sure there's a reference somewhere, but I don't recall it it off hand.

There is also an arguement about antibiosis, substances produced by microorganisms/plants/fungi/algae to favor some bacteria and reduce others. There is also the issue of predation by a variety of protozoa.

Some bacteria are very good at reproduction at low substrate concentrations also.

You can do a simple test as an aquarist, take a tank and measure the NO3 with plants and then without plants for NO3=> N2 gas.
This is a bit weasier than figuring out how much NH4=>NO3 by bacteria vs the plants.
Then also the same thing can be done with NH4 additions and NO2 measurement and NO3 with and without plants. There are many issues with this method also but you can get an idea.

Regards,
Tom Barr
 
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