And where are the bacteria..?

This is one of the most frequently asked questions we get from people about our recently installed aquaponic system opposite the Students’ Union of the University of Sheffield.


And the answer is usually something like this: “Well, you know, in the water. Probably everywhere – we think so, at least.” And that is about as much as we could say. The truth is, we don’t know much about where they are, what they are doing, how many of them there are or what exact strains we have got in the system. (In our defence, it’s not easy to tell with microscopic organisms without the necessary lab equipment!) All we know for sure is that they are there and are doing a pretty good job.

But wait a minute. If they are invisible, how can we be sure they really are there? That is a fair question – but one we have a fair answer for, too.

Despite their minute size, nitrifying bacteria are key players in aquaponics. They are responsible for converting nitrogenous waste produced by fish into readily available nutrients that can then be absorbed by plants. To be more exact, ammonia is first turned into nitrites by a certain type of bacteria, which is then further metabolised into nitrates by another, providing a form of nitrogen suitable for plant uptake. Measuring the concentrations of these compounds in the water using some basic equipment can give an idea of whether and how much conversion is taking place.


If there were no bacteria present, we would find virtually no nitrites or nitrates and an ever increasing level of ammonia from fish waste (not to mention far-from-healthy-looking plants as a result of nutrient deprivation). In contrast, in our system we get constant, low levels of ammonia, no nitrites and slightly variable, but usually fairly high concentrations of nitrates, as well as healthy, fast-growing vegetables. As far as we know, this can only be explained by simultaneous removal of ammonia and production of nitrates, something that neither fish nor plants are capable of doing, which clearly suggests the involvement of an invisible third party.

OK. So they are there, doing whatever it is that nitrifying bacteria do, supplying nutrients for our plants in the process. But how did they get there in the first place? First of all we created a welcoming environment, using expanded clay balls for our grow media:


These balls have a very high surface area, so there’s lots of space for the bacteria to take hold (if they want to, that is – they might prefer floating around, we are not sure). Then we added a little powdered fish food (before there were any fish in the system) to create some ammonia for the bacteria to feed on, and finally a little vermicompost we hoped would contain some suitable bacterial populations. Yes, we know it is not exactly what you would call a reliable way of introducing the right microorganisms to the system, but as they don’t sell nitrifying bacteria in little test tubes in garden centres (yet…), this seemed to be the most workable solution. And then all we had to do was add some fish and seedlings and wait for the bacteria to start doing their job. And… hey presto! There they were 🙂  At least we think so!

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