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I thought I'd try adding regular yeast to water taken from my 75 gallon pond, with the idea that the yeast would be a good "bottom of the food chain" kick start to feed the little beasties, along with warmth and light, to allow the creatures to multiply faster than they would in the pond, to be added back the pond as feed. But to my surprise, the yeast won't grow in the pond water, at least, not like when you make bread with it. I've tried adding sugar, molasses, and "proofing" the yeast, but each time, the yeast does not thrive. Is something eating it as soon as it starts to grow, or is something using a nutrient it needs, or what? The pond is established, with lots of plants, only a few fish, and ammonia levels are 0 ppm. It is an (apparently) thriving ecosystem. My question is, why won't yeast grow and froth up in that water?

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    $\begingroup$ How much sugar are you adding to the 'growth medium'? $\endgroup$ – Alan Boyd Jun 20 '17 at 11:32
  • $\begingroup$ I'm adding about a teaspoon of table sugar to a half gallon of water, same amount for molasses. With tap water, the water/sugar/yeast mixture foams and froths within minutes. Using pond water, it does not. I even got a mixture of tap water, one cup water, one teaspoon sugar and a tablespoon of yeast. After ten minutes, this mixture was foaming and frothing, when added to the pond water, yeast activity stops, as near as I can tell. $\endgroup$ – Tad Jones Jun 20 '17 at 14:16
  • $\begingroup$ For any microbiologists reading this, the supplementation is approximately 0.1% sugar. $\endgroup$ – Alan Boyd Jun 20 '17 at 14:26
  • $\begingroup$ What exactly is 'yeast' in your experiment? $\endgroup$ – Alan Boyd Jun 20 '17 at 14:26
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    $\begingroup$ I am not a specialist of fish, but yeast infection is a thing. For the sake of your thriving ecosystem, I would advise you to ask on pets.SE or somewhere similar if having yeast in a pond is a good idea at all. $\endgroup$ – skymningen Jun 21 '17 at 7:48
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A few random thoughts.

I think that one problem here is that by using dried yeast you aren't really measuring growth or survival. Over a short time period much of the fermentation that you see probably has very little to do with viable cells. Dead cells in dried yeast act as permeable packages of enzymes and can promote fermentation.

Following on from that, it may be that components of the pond water can poison the fermentation carried out by the dead cells.

Thirdly, if you are using frothing as a measure of CO₂ production it is possible that there are components of the pond water that collapse that foam by removing stabilisers (probably protein).

Next, Saccharomyces isn't an obvious choice for use as an aquatic micro-organism since it is adapted/selected for growth in high sugar concentrations.

If I was trying to do this I think I would try to culture from the dried yeast under conditions where you do see activity, add lots more sugar (as if you were brewing), and look for actual production of yeast cells. Yeast settles out of suspension very easily so you might be able to pour off most of the liquid and add the cells to your pond. But I have no idea how well such cells will do in a pond.

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  • $\begingroup$ Thank you for your thoughts. I think my question is actually related to your second paragraph, "it may be that components of the pond water can poison the fermentation carried out by the dead cells". I'm trying to understand what that would be. $\endgroup$ – Tad Jones Jun 21 '17 at 19:14
  • $\begingroup$ It was never my intention that the yeast survive in the pond. I was using yeast to feed the "little beasties", to promote their growth, which could then be added to the pond. I just find it interesting that with the substitution of pond water for tap water the yeast does not become active. I WAS using frothing as an indication of carbon dioxide production. I put the lid on the bottle and no (or very little) carbon dioxide production. I got the idea because yeast is used to feed brine shimp and daphnia. $\endgroup$ – Tad Jones Jun 21 '17 at 19:38
  • $\begingroup$ Now I think the simplest explanation would be that something is eating up the sugar before the yeast has time to rehydrate and become active. $\endgroup$ – Tad Jones Jun 21 '17 at 23:09

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