Just looking for clarification...your article seems to imply that most SW aquariums are using live rock that isn't really beneficial as a biological filter because of its lack of animal life. What, then, is serving as the biological filter in these aquariums? Many of them seem to be successful in terms of keeping their chosen species alive and healthy, so something has to be going on. And if the live rock isn't the bio filter, then why spend so much money on it? Also, doesn't the bacteria living in the surface of the rock aid in biological filtration?
recent article - live rock as bio filter
- Thread starter msn711
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Hi,
The bacteria that are doing the filtration live on surfaces and I suspect most surfaces in an aquarium are covered with them. However, that really is not enough of a bacterial bed to be effective. In tanks with a sand bed, of course, the sand bed is being the filter.
As to why spend so much money for it? Well.... some of it looks pretty.
If the live rock has a good animal population in it pumping water through it, and some does, it may be acting as a filter.
However, the bottom line is that I don't think that the standard paradigm of a "biological" filter really is valid. I think in most cases in our systems, the algae in the tank are what is removing the nitrogenous compounds.
The bacteria that are doing the filtration live on surfaces and I suspect most surfaces in an aquarium are covered with them. However, that really is not enough of a bacterial bed to be effective. In tanks with a sand bed, of course, the sand bed is being the filter.
As to why spend so much money for it? Well.... some of it looks pretty.
If the live rock has a good animal population in it pumping water through it, and some does, it may be acting as a filter.
However, the bottom line is that I don't think that the standard paradigm of a "biological" filter really is valid. I think in most cases in our systems, the algae in the tank are what is removing the nitrogenous compounds.
Ron,
I can accept your argument that diffusion through a rock is probably low and water will not really travel an appreciable distance, however, in terms of bacteria, it doesnt really have to. One of the things to consider is the formation of biofolms, and mixed layer biofilms. I think that the surface of all inert structures are coated with a two layer biofilm, the inner is comprised of anaerobes, breaking nitrates to N2 and O2 while the outer is aerobes. The aerobes protect the anaerobes and provide the nitrates. Such proximity of growth leads to a remarkably efficient system. This system is negated when we add a wet-dry type filter. In this case the anaerobes predominantly grow on the filter. Since the oxygen is extremely high at the filter they are unable to mask the anaerobes and we get a an uncoupled nitrogen cycle. The nitrates build up.
My point with this is it may not be the porosity of the rock but more the extremely large surface structure of an organically derived material. Microscopically, the surface of coral skeletons and live rock dereived from them is incredibly ridged and provides the massive surface area to permit such growth of large bacterial populations.
Just my thoughts,
Regards,
I can accept your argument that diffusion through a rock is probably low and water will not really travel an appreciable distance, however, in terms of bacteria, it doesnt really have to. One of the things to consider is the formation of biofolms, and mixed layer biofilms. I think that the surface of all inert structures are coated with a two layer biofilm, the inner is comprised of anaerobes, breaking nitrates to N2 and O2 while the outer is aerobes. The aerobes protect the anaerobes and provide the nitrates. Such proximity of growth leads to a remarkably efficient system. This system is negated when we add a wet-dry type filter. In this case the anaerobes predominantly grow on the filter. Since the oxygen is extremely high at the filter they are unable to mask the anaerobes and we get a an uncoupled nitrogen cycle. The nitrates build up.
My point with this is it may not be the porosity of the rock but more the extremely large surface structure of an organically derived material. Microscopically, the surface of coral skeletons and live rock dereived from them is incredibly ridged and provides the massive surface area to permit such growth of large bacterial populations.
Just my thoughts,
Regards,
Hi Paul,
I agree that might well be the place where some filtration occurs.
But, I don't think there is enough surface area there or "filtration" of the wastes produced by our systems. There is also some good evidence that the whole concept of anaerobes being the source of nitrogen reduction is incorrect, at least on real reefs; see this article, I believe, for more information: Capone, D. G. et al., 1992. Microbial nitrogen transformation in unconsolidated reef sediments, Marine Ecology Progress Series. 80:75-82; also: Stimson and Larned, 2000. Nitrogen efflux from the sediments of a subtropical bay and the potential contribution to macroalgal nutrient requirements Journal of Experimental Marine Biology and Ecology. 2522)159-180.
However, one of the (maybe) more minor points of the article is that there is no hard data about any of this. We can postulate all sorts of ways for filtration to occur, but until we do some collection of data we won't know about any of it.
I am convinced that the standard idea of the rock providing filtration is simply wrong, and the purpose of the article is get people thinking about it.
I agree that might well be the place where some filtration occurs.
But, I don't think there is enough surface area there or "filtration" of the wastes produced by our systems. There is also some good evidence that the whole concept of anaerobes being the source of nitrogen reduction is incorrect, at least on real reefs; see this article, I believe, for more information: Capone, D. G. et al., 1992. Microbial nitrogen transformation in unconsolidated reef sediments, Marine Ecology Progress Series. 80:75-82; also: Stimson and Larned, 2000. Nitrogen efflux from the sediments of a subtropical bay and the potential contribution to macroalgal nutrient requirements Journal of Experimental Marine Biology and Ecology. 252
2)159-180.However, one of the (maybe) more minor points of the article is that there is no hard data about any of this. We can postulate all sorts of ways for filtration to occur, but until we do some collection of data we won't know about any of it.
I am convinced that the standard idea of the rock providing filtration is simply wrong, and the purpose of the article is get people thinking about it.
I think one of the ways to go about this would really be to estimate what the bacterial requirement would be to reduce a set concentration of ammonia to N2 and O2. Bacteria are extremely efficient, as im sure you are aware. Given biomass data we could figure the depth of bacteria required over porous surfaces.
I think at the end of the day the variables are too great to really get a good handle on this, but I do think anecdotal evidence as the wet dry filter I mention above may provide insights.
I know a number of tanks where the wet filter (usually a biowheel) once removed and allowed to be submersed experience a drop in nitrates over a relatively short period. It may be informative to creat such a set up, a clean tank with a biowheel and an ammonia source. Let it run and then immerse the wheel. Periodic sampling of the wheel matrix will indicate if nitrate decrease corresponds to increase in anaerobes.
Having established if bacteria are the players we can then move on.
Either way..........its an interesting and stimulating topic.
I think at the end of the day the variables are too great to really get a good handle on this, but I do think anecdotal evidence as the wet dry filter I mention above may provide insights.
I know a number of tanks where the wet filter (usually a biowheel) once removed and allowed to be submersed experience a drop in nitrates over a relatively short period. It may be informative to creat such a set up, a clean tank with a biowheel and an ammonia source. Let it run and then immerse the wheel. Periodic sampling of the wheel matrix will indicate if nitrate decrease corresponds to increase in anaerobes.
Having established if bacteria are the players we can then move on.
Either way..........its an interesting and stimulating topic.
Northside Reef
Premium Member
LOL, sure drop a bomb like ââ"šÂ¬Ã…"œLive rock hit or mythââ"šÂ¬Ã‚ then run for cover for a few days.
We see how it is.
Frankââ"šÂ¬Ã‚¦
We see how it is.
Frankââ"šÂ¬Ã‚¦
I actually spent a few hours yesterday looking into natural and industrial (still bacterial) rates of denitrification. For industrial processes the rates can be as high as kilograms nitrogen in the form of ammonia denitrified per meter squared of biofilm surface area. Of course these sytems are highly tuned. In natural sediments the rates are much lower but are still in the the region of mg Ammonia nitogen processed per meter squared. So........the surface area of live rock is more than adequate for denitrification. I think one of the issues I have with the article is the explanation of the requirement for "Porosity". By taking this to mean that the solutes in the water have to penetrate the rock is most probably wrong. Biofilms grow over the surface of the rock, and every other structure. They are not particularly deep, maybe a few microns in depth. Thus they are readily able to snag the ammonia and nitrites they are denitrifying. The rock is porous, and thus the surface of the rock is very pitted and broken. Thus we probably have something along the orders of 10-100 sq M of surface area in most of our tanks. This surface area is constantly bathed in water flowing in our tank.
I hope no one that reads this article pulls their live rock out. This article is not based on an understanding of microbiology. Instead it brings into play inverts as the main players in denitrification by providing water flow. The assumption that this water flow is needed at all is most probably erroneous.
Paul.
I hope no one that reads this article pulls their live rock out. This article is not based on an understanding of microbiology. Instead it brings into play inverts as the main players in denitrification by providing water flow. The assumption that this water flow is needed at all is most probably erroneous.
Paul.
birdman204
New member
I am curious as to the brand of test kits being used, or that the author would request aquarists use. I would like to help out with the study mentioned at the end of the article. I have a few tanks up and running for varying amounts of time. 1 here at work contains 100% cultured rock. Thanks
-Ryan-
-Ryan-
sdmike
New member
I've been in the aquarium hobby since the early 80s. In that time, I've seen LOTS of things come and go, but the basics still remain.
I've just started a reef and have been curious how a rock "filters" water. Seems hard to believe...
I'm not 100% convinced that LR/LS is the ONLY answer and that an additional form of filtration isn't a bad thing. We'll just have to see what biologists come up with in the next few years as we figure more and more out.
Thanks for clarifying,
M
I've just started a reef and have been curious how a rock "filters" water. Seems hard to believe...
I'm not 100% convinced that LR/LS is the ONLY answer and that an additional form of filtration isn't a bad thing. We'll just have to see what biologists come up with in the next few years as we figure more and more out.
Thanks for clarifying,
M
The standard explanation given throughout the reef literature is that the porosity of the rock allows for the growth of bacteria in an internal environment that is condusive for the bacterial growth. The point of the article is that the rock isn't likely porous enough for that, and that water will not move into it in any case. And as with a lot of other myths in the hobby, there are no data supporting the dogma.
The rock surface may well be acting as a base for the appropriate bacteria, however, there is no evidence or data from aquaria to support that either.
I would love to see any actual data you have that support your claims.
The rock surface may well be acting as a base for the appropriate bacteria, however, there is no evidence or data from aquaria to support that either.
I would love to see any actual data you have that support your claims.
Dr. Shimek,
Thanks for a great article and a healthy dose of skepticism in general.
I must point out that at from a geologic perspective, which is what we're dealing with once the oragnisms become the limestone we call "live rock", porosity is indirectly related to the ability for water to flow through a rock.
For a full description you must also consider the permeability of the rock, or how well interconnected the pores of the rock are. You can have a very porous rock that is completely impermeable and would therefore not allow any "flow" what so ever.
It would be interesting if you could have a geologist analyze various live rock samples for both their porosity and permeability. I'd be interested to see results from rock of various origins and ages.
Thanks for being a scientific voice for those of us looking beyond the marketing hype.
Danny Dorsey
Thanks for a great article and a healthy dose of skepticism in general.
I must point out that at from a geologic perspective, which is what we're dealing with once the oragnisms become the limestone we call "live rock", porosity is indirectly related to the ability for water to flow through a rock.
For a full description you must also consider the permeability of the rock, or how well interconnected the pores of the rock are. You can have a very porous rock that is completely impermeable and would therefore not allow any "flow" what so ever.
It would be interesting if you could have a geologist analyze various live rock samples for both their porosity and permeability. I'd be interested to see results from rock of various origins and ages.
Thanks for being a scientific voice for those of us looking beyond the marketing hype.
Danny Dorsey
BobB
New member
rshimek said:
Hi Dr. Shimek,
What a great, thought-provoking article! I'm a marine microbiologist and particularly interested in biofilms. I also have a scientific and hobbyist interest in coral reefs. In reading and thinking about liverock, it occurs to me that we need to keep a couple of points in mind. First, the use of the work 'porosity'. I think many reef aquarists envisage pores as in swiss cheese, but this may be wrong, or at least inaccurate from the point of view of bacterial cells. At 0.5-2.0 um in size, bacterial cells look at a world where porosity is more appropriately measured in micrometers, maybe even nanometers. So, a piece of calcareous liverock may be extremely porous at the micrometer scale, while being seemingly solid at the macroscale that we see. I think its entirely conceivable that most well-aged liverock, having come from old reefs, is extremely porous at the level of the bacterium.
The second point is the concept of bacteria being on the 'surface'. Again, I think we (as humans) have an ill-defined idea of what a surface is to a bacterium. The available evidence suggests that living bacteria are found in 'dense' stone several mile below the surface of Earth, as well as in halide crystals (with no macroscopically perceptible pores). Thus, I would suggest that it is entirely possible, indeed likely, that bacteria are capable of colonizing the entire liverock, from exterior surface to the inner core (multiple inches below the surface). It would be fascinating to section liverock and examine it with modern molecular tools for both the presence and the metabolic activities of potential bacterial colonizers. I wager that bacterial cells would be found far below the surface of what appears to us as solid, nonporous liverock.
This said, I bet we can all agree that macroscopic pores are also extremely necessary for the best carbon and nitrogen utilization and decomposition in liverock bacterial communities. As your article says these provide a wonderful means of circulation that aids in transport of the wastes through the liverock, as well as providing additional niches for a host of invertebrates, each (by the way) with its own thriving community of bacterial symbionts and commensals. Good quality liverock is truly alive at both macro- and microscopic levels.
Thanks once again for a stimulating article.
BobB
Hi Folks,
My [thanks] to all the responders here.
The idea of the article was to challenge the dogma and that appears to have been successful.
I would love to see some of you folks that have the equipment and specialized knowhow to examine some of the questions raised here to spend a bit of your hard-to-come by spare time and get some more information about what has been hitherto treated so simplistically.
My [thanks] to all the responders here.
The idea of the article was to challenge the dogma and that appears to have been successful.
I would love to see some of you folks that have the equipment and specialized knowhow to examine some of the questions raised here to spend a bit of your hard-to-come by spare time and get some more information about what has been hitherto treated so simplistically.
Lazyreefer
Premium Member
Research
Research
I disagree Dr. Ron.
With all the PHD"s and MD's on this thread perhaps you guys could cure Parkinson's disease instead.
Research
I disagree Dr. Ron.
With all the PHD"s and MD's on this thread perhaps you guys could cure Parkinson's disease instead.
G-money
Premium Member
rshimek said:
I agree. But alas, I lack the equipment and the larger portion of the know-how.
I would especially like to see bacteria more incorporated into the equation. Since they are no doubt covering every square micrometer of every accessible surface of the rock, I'd go out on a limb and guess they may be more responsible for movements in and out of the rock than all other factors (biologic and other) combined.
Hi Graham,
Bacteria do a lot of things, but they don't move water much, particularly in comparison to burrowing animals. Without water movement in and out of the rock, there isn't going to be much exchange with the external water.
Now, maybe they don't have to move it, if the processes are occurring on the rock surface or in biofilms. In these cases, we don't need "live" rock at all. And, of course, in many cases what we have is definedly "dead rock."
Bacteria do a lot of things, but they don't move water much, particularly in comparison to burrowing animals. Without water movement in and out of the rock, there isn't going to be much exchange with the external water.
Now, maybe they don't have to move it, if the processes are occurring on the rock surface or in biofilms. In these cases, we don't need "live" rock at all. And, of course, in many cases what we have is definedly "dead rock."
The persistance that water has to enter the rock to any significant degree is, in all likely hood, false. You have to understand bacteriology, and in particular, the incredible ability of biofilms to utilize nutrients in a water column. You ask for scientific evidence Ron.... Well I am sure you will agree that denitrification is predominantly a two part process catalyzed by bacteria. This process is utilized commercially and industrially by companies around the world to remove pollution from waste waters. That it works and is highly succesful is a fact. f you want scientific evidence that is does work I can post paper after paper. Biofilms grow on surfaces. Any surface. All they need is a solid base to grow. The surface of rock (irrespective of whether we call it live or dead) is highly pitted. This is referred to as porous. I enclose the definition of porous.
--------------------
[adj] full of pores or vessels or holes
[adj] allowing passage in and out; "our unfenced and largely unpoliced border inevitably has been very porous"
[adj] able to absorb fluids; "the partly porous walls of our digestive system"; "compacting the soil to make it less porous"
-----------------------
Since it is full of holes, both marcroscopic (those we can see) and microscopic (those we can not see) the surface structure of the rock is extremely high per unit weight. Coupled with the sand beds in our tanks, this is more than sufficient to suffice. To say that water has to be actively transported through the rock or else we don't get denitrification is incorrect. In industrial filters there are no invertebrates.
I think what it comes down is the long term use of the word porosity. We know that rock can not absorb water. It can take it into cavities, that is all. However, the two layer biofilm is probably able to satisfy the requirements of the aerobes and anaerobes.
As BOBb sums it up, porosity and diffusion mean different things at the microscopic level.
With refernce to the rates of diffusion that you quote in your article. You say that the only motive force capable of moving water is invertebrate action. What is missing from this argument is the total volume of water moving at any one time. If it is a few cubic millimeters of water diffusing then it is insufficient, however this is a totally dynamic process. The entire nutrient content of the water volume in the rock is moving. This provides a very scenario to the one you suggest. In fact it probably accounts for a greater nutrient flow than is capable by invertebrates.
Finally,
There are flaws in your proposed experiment. If you hypothesize that movement of animals through rock exchanges water, then insertion of a hypodermic needle will certainly create that movement. If the user is to avoid drawing up water from outside the rock by negative pressure then they can not draw water from the rock faster than diffusion can replace/balance the nutrients. From your figures this would be 3.6mm per hour. Thus, to take approxiamtely 5 mls to water sample would require several days of drawing a sample. Thus the data is invalid.
The only way to measure inernal Oxygen is by use of a microprobe installed in situ.
Also, depending on the tank, the invertebrate life is going to be quite diverse in each rock. If your statements are correct, then it will be impossible to interpret the data in any cogent fashion.
I am not saying this to rain on your article, however, as a person who works with bacteria daily and designs experiments that have to be extremely rigorous and reproducible, I feel that you should adress the design you propose.
I do think it would be great to look at various rocks and determine the cavitation of the structure (lets avoid porosity as a term for a while). If anyone has access to electron microscopes it would be neat to look at a number of fresh live rock specimens from various places.
Regards,
Paul.
--------------------
[adj] full of pores or vessels or holes
[adj] allowing passage in and out; "our unfenced and largely unpoliced border inevitably has been very porous"
[adj] able to absorb fluids; "the partly porous walls of our digestive system"; "compacting the soil to make it less porous"
-----------------------
Since it is full of holes, both marcroscopic (those we can see) and microscopic (those we can not see) the surface structure of the rock is extremely high per unit weight. Coupled with the sand beds in our tanks, this is more than sufficient to suffice. To say that water has to be actively transported through the rock or else we don't get denitrification is incorrect. In industrial filters there are no invertebrates.
I think what it comes down is the long term use of the word porosity. We know that rock can not absorb water. It can take it into cavities, that is all. However, the two layer biofilm is probably able to satisfy the requirements of the aerobes and anaerobes.
As BOBb sums it up, porosity and diffusion mean different things at the microscopic level.
With refernce to the rates of diffusion that you quote in your article. You say that the only motive force capable of moving water is invertebrate action. What is missing from this argument is the total volume of water moving at any one time. If it is a few cubic millimeters of water diffusing then it is insufficient, however this is a totally dynamic process. The entire nutrient content of the water volume in the rock is moving. This provides a very scenario to the one you suggest. In fact it probably accounts for a greater nutrient flow than is capable by invertebrates.
Finally,
There are flaws in your proposed experiment. If you hypothesize that movement of animals through rock exchanges water, then insertion of a hypodermic needle will certainly create that movement. If the user is to avoid drawing up water from outside the rock by negative pressure then they can not draw water from the rock faster than diffusion can replace/balance the nutrients. From your figures this would be 3.6mm per hour. Thus, to take approxiamtely 5 mls to water sample would require several days of drawing a sample. Thus the data is invalid.
The only way to measure inernal Oxygen is by use of a microprobe installed in situ.
Also, depending on the tank, the invertebrate life is going to be quite diverse in each rock. If your statements are correct, then it will be impossible to interpret the data in any cogent fashion.
I am not saying this to rain on your article, however, as a person who works with bacteria daily and designs experiments that have to be extremely rigorous and reproducible, I feel that you should adress the design you propose.
I do think it would be great to look at various rocks and determine the cavitation of the structure (lets avoid porosity as a term for a while). If anyone has access to electron microscopes it would be neat to look at a number of fresh live rock specimens from various places.
Regards,
Paul.
Hi,
The persistance that water has to enter the rock to any significant degree is, in all likely hood, false.
Perhaps, but there no data supporting the alternative suggestion.
Well I am sure you will agree that denitrification is predominantly a two part process catalyzed by bacteria.
While denitrification is definitely done by bacteria, it is incompletely understood and there are significant problems with the standard models cited in the so-called reef literature.
To say that water has to be actively transported through the rock or else we don't get denitrification is incorrect. In industrial filters there are no invertebrates.
The rock may or may not be porous. If it is, water is not going to be passively moved through it. So the contribution of any internal bacteria to any process in the tank water is nil. If there sufficient bacteria on the surface of the rock for denitrification, that is great! I'd love to see it. It would mean we wouldn' t need the so-called live rock at $$$ per pound and could do with any substrate in the tank.
You may well be right. I have no argument with your hypothesis. However, you don't have any data to show that occurs in an aquarium. Industrial processes, as I understand them use significant water movement over the biofilms. Given the boundary effects of rock like this I doubt there is sufficient water movement at the rock/water interface to provide the exchange necessary. I will freely admit I have no data to back this up, but I also see no data to contradict my statement.
... In fact it probably accounts for a greater nutrient flow than is capable by invertebrates.
I don't agree. So.. how would you show it?
There are flaws in your proposed experiment..... Thus the data is invalid.
Not invalid - but inconclusive. However, it would give some indications of water conditions within the rock. For example, if we got significant indications of sulfides, etc. I doubt anybody could get a good sample of pore water from inside the rock; I have spent a lot of time working with environmental chemists getting interstitial water from within sediments and that is hard enough. However, if we can get some data to play with.
The only way to measure inernal Oxygen is by use of a microprobe installed in situ.
Yep, and since you can't get one into the rock, you can't do it that way. So... try for some indirect measurements.
Also, depending on the tank, the invertebrate life is going to be quite diverse in each rock.
Actually, I think there won't be much invertebrate life within the rock at all. And that will be fairly conclusive.
If your statements are correct, then it will be impossible to interpret the data in any cogent fashion.
I disagree.
Dr Ron,
I copied this posting from another thread; some of these points have already been discussed.
Some observations/questions:
If we consider the ââ"šÂ¬Ã…"œammonia cycleââ"šÂ¬Ã‚ subset of biological filtration, if it is not occurring in/on the live rock, where is it occurring? Perhaps this is also ââ"šÂ¬Ã…"œurban reef legendââ"šÂ¬Ã‚ but I was always under the impression that nitrogen cycle bacteria require a surface to ââ"šÂ¬Ã…"œcolonizeââ"šÂ¬Ã‚ rather that being free floating. Maybe this was more of an oxygen gradient/concentration issue that an ââ"šÂ¬Ã…"œattachmentââ"šÂ¬Ã‚ issue.
Even in a reef tank devoid of a ââ"šÂ¬Ã…"œfish populationââ"šÂ¬Ã‚ one can add a given quantity of ammonia (as ammonium chloride) has with the appropriate test kits, observe the decline (conversion/) of ammonia to nitrite to nitrate. Not that I could quantify this but this occurred in tanks with nothing but ââ"šÂ¬Ã…"œwater and fishââ"šÂ¬Ã‚ and perhaps a limited amount of a course gravel substrate
In my experience, it is the conversion of nitrate to nitrogen gas (at a rate that kept nitrate levels at <10ppm) that required a given ration of live rock, a DSB, or some kind of purpose built de-nitrification device, such as a ââ"šÂ¬Ã…"œcoil denitraterââ"šÂ¬Ã‚ In other words, a sufficiently sized ââ"šÂ¬Ã…"œzoneââ"šÂ¬Ã‚ with the appropriate (~1ppm) concentration of oxygen was required to ââ"šÂ¬Ã…"œhostââ"šÂ¬Ã‚ the bacteria responsible for converting nitrate to nitrogen gas.
If it is indeed the critters that make the ââ"šÂ¬Ã…"œexchangeââ"šÂ¬Ã‚ work on live rock, this would also be a positive indicator for DSBs as the relative ââ"šÂ¬Ã…"œcritterââ"šÂ¬Ã‚ population densities should be much higher in a sand bed that in a given ââ"šÂ¬Ã…"œmassââ"šÂ¬Ã‚ of live rock. Also as a sand substrate is continually being broken up/moved around by the sand critters, the porosity/permeability would also be much higher.
In any case, good luck with your investigation.
Regards,
Scott
I copied this posting from another thread; some of these points have already been discussed.
Some observations/questions:
If we consider the ââ"šÂ¬Ã…"œammonia cycleââ"šÂ¬Ã‚ subset of biological filtration, if it is not occurring in/on the live rock, where is it occurring? Perhaps this is also ââ"šÂ¬Ã…"œurban reef legendââ"šÂ¬Ã‚ but I was always under the impression that nitrogen cycle bacteria require a surface to ââ"šÂ¬Ã…"œcolonizeââ"šÂ¬Ã‚ rather that being free floating. Maybe this was more of an oxygen gradient/concentration issue that an ââ"šÂ¬Ã…"œattachmentââ"šÂ¬Ã‚ issue.
Even in a reef tank devoid of a ââ"šÂ¬Ã…"œfish populationââ"šÂ¬Ã‚ one can add a given quantity of ammonia (as ammonium chloride) has with the appropriate test kits, observe the decline (conversion/) of ammonia to nitrite to nitrate. Not that I could quantify this but this occurred in tanks with nothing but ââ"šÂ¬Ã…"œwater and fishââ"šÂ¬Ã‚ and perhaps a limited amount of a course gravel substrate
In my experience, it is the conversion of nitrate to nitrogen gas (at a rate that kept nitrate levels at <10ppm) that required a given ration of live rock, a DSB, or some kind of purpose built de-nitrification device, such as a ââ"šÂ¬Ã…"œcoil denitraterââ"šÂ¬Ã‚ In other words, a sufficiently sized ââ"šÂ¬Ã…"œzoneââ"šÂ¬Ã‚ with the appropriate (~1ppm) concentration of oxygen was required to ââ"šÂ¬Ã…"œhostââ"šÂ¬Ã‚ the bacteria responsible for converting nitrate to nitrogen gas.
If it is indeed the critters that make the ââ"šÂ¬Ã…"œexchangeââ"šÂ¬Ã‚ work on live rock, this would also be a positive indicator for DSBs as the relative ââ"šÂ¬Ã…"œcritterââ"šÂ¬Ã‚ population densities should be much higher in a sand bed that in a given ââ"šÂ¬Ã…"œmassââ"šÂ¬Ã‚ of live rock. Also as a sand substrate is continually being broken up/moved around by the sand critters, the porosity/permeability would also be much higher.
In any case, good luck with your investigation.
Regards,
Scott
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