All you want to know about sandbeds

Here's my article on Sandbeds

Many people have conflicting ideas on setting up a refugium, especially one with a sandbed. I have seen set ups with a DSB (Deep Sand Bed) of about 4" to 6" thick. And sadly, a lot of people think that just by putting in some sand and macroalgae, they have a working refugium. I firmly say NO.

In my opinion, its important to know and understand the thing you are doing before doing it. Although sandbed dynamics are still being researched, I will provide the current information available.

Bacteria and zones

The main purpose of adding a sandbed is to allow bacteria to colonise on its surface. Many people think of diseases when the word "bacteria" is mentioned. The bacteria we are talking about here are those that convert certain compounds to another form.

There are mainly 2 groups of bacteria, namely the aerobic and anaerobic. Aerobic refers to the presence of air and anaerobic refers to the absence of air.

And the confusion comes as each group will split into 2 groups. The 2 groups are the autotrophs and heterotrophs. Autotroph meaning the organism is able to synthesize its own food from inorganic substances and heterotroph meaning
an organism that cannot synthesize its own food and dependent on organic substances.

Basically, there are 3 groups of bacteria we are interested in. The aerobic autotrophs, aerobic heterotrophs and anaerobic heterotrophs. The aerobic heterotrophs will break down organic substances into inorganic compounds (eg. ammonia). These inorganic compounds are then taken up by aerobic autotrophs which will perform nitrification. The anaerobic heterotrophs will perform denitrification.

The aerobic autotrophs are usually found on the suface layer of sand since they need oxygen and light to synthesize food. The other area around the top 1/3 of the sandbed will be colonised by aerobic heterotrophs. And as oxygen levels decrease down the sandbed, anaerobic heterotrophs will colonize there.

An important point to note is that anaerobic zone actually consisted of 2 different zones. The zone that has oxygen concentrations of 0.5-2 ppm (test conducted by Sam Gamble) is named as the "anoxic" by Bob Goemans. This zone will contain facultative (bacteria that live with or without oxygen) bacteria that fully oxidize nitrate back to nitrogen gas.

The other zone that has no oxygen at all will consist of anaerobic heterotrophs with only convert nitrate into ammonium which is a better fertilizer than nitrates for algae.

This explains why I think that really deep sandbeds are not good as your ultimate goal is to achieve an anoxic zone. The recommended sandbed depth should be 3"-3.5"

Grain size and size of refugium

Normally, people try to use the smallest grain size for their sandbed. This differs between systems and I shall explain.

It is good to have small grain size as it increases the surface area for bacteria to colonise. But this only applies to systems which have the skimmer output there or any other source of oxygenated water. If the area is in lowly oxygenated water, the oxygen diffusion will be limited to the upper layers.

For those people who have lowly oxygenated water in their sandbeds, it is advised to use bigger grain sizes and a bigger sandbed. This will allow oxygen to diffuse deeper faster.

Infauna and their importance

The word "infauna" is new to many people. It actually means the fauna (animals) IN the sandbed. So why do these animals make such an importance in sandbeds? The infauna consist of mini-stars, micro-stars, worms, flatworms and a myraid of other small animals. These infauna will constantly shift the sand and their action will prevent the sandbed from reaching a completely anaerobic zone. This action also ensures that the sand does not clump together as a result of precipitation and bacterial actions.

I really hope that LFS will start to bring in recharge kits consisting of these infauna that reefers can add to their sandbed. If not, these animals will come as hitchhikers on live rock.

Quorum Gene Sensing

This is a relatively new concept to sandbed dynamics and it involves bacteria. Quorum sensing is actually a way bacteria produce this gene that senses the population of the bacteria. The researched bacteria is the Vibiro spp. and it is found that the bacteria will produce toxins that kill human cells in high populations.

It will be interesting to do research on the nitrifying bacteria and see whether they will be detrimental in high population sizes. And we could see whether we can stop the production of the Quorum gene .

This comes to the end of my article and I hope readers will find this useful.

The top information on different types of bacteria and zones are from Sand Mail, FAMA Aug 2004 by Bob Goemans

Copywright Dickson Goh 2005

Black substance in sandbeds

I am definately sure that any aquarists who have kept sandbeds before would have come across the statement that a black layer would be formed. Some say this is hydrogen sulphide, some say this is cyanobacteria. Actually, this black stuff is none other than FeS or Iron sulphide.

Iron sulphide itself is a brown precipitate and due to the intense colour, we often perceive it as black. All sulphides are toxic to organisms and thus, this chemical is not good for reefs.

This compound is formed by sulphate reducers which being anaerobic bacteria, usually thrive under mats of the cyanobacterium Lyngbya sp. or in any anaerobic region. Thus, in anaerobic regions of sandbeds, a "black" layer of FeS is formed.

FeS can be oxidised in the sediment by MnO2 or maganese oxide into FeOOH, S, Mn and H2O. It can also be oxidised into sulphuric acid by autotrophic or mixotrophic bacteria and will be neutralised by calcium carbonate into calcium sulphate.

All in all, the black substance is not much of a worry if you do not release it into your system BUT it indicates that the sandbed has a totally anaerobic zone and this should be the main worry of aquarists

Copywright Dickson Goh 2005

From Ammonium to Nitrogen

As previously stated, in a zone which is totally anaerobic, ammonium will be produced rather than nitrogen gas. This can be a problem as ammonium is a very good fertiliser for nuisance algae.

But recent research conducted in the Black Sea found that bacterium from the order Planctomycetales is able to convert ammonium to nitrogen gas together with nitrite.

This process termed Anammox stands for anaerobic ammonium oxidation. Nutrient profiles, fluorescent RNA probes, 15N tracer experiments and the distribution of specific "ladderane" membrane lipids show that the ammonium that is diffusing upwards is consumed by anammox bacteria just below the aerobic zone.

This shows that anammox actually plays a part in the Nitrogen Cycle in the sea and occurances in captive systems is a new field that is worth researching. By showing how deeper sandbeds can actually be a zone for the colonisation of anammox bacteria might just unlock a new methodology for sandbed keeping and who knows what benefits it can hold?

The above information about anammox bacteria are from nature publishing group, www.nature.com

thx for this article bro~

No problem, I actually have another article on coral colouration but I'm not sure whether you guys like the articles I write or not

of cos...i support u la!

Originally Posted by blueheaven

No problem, I actually have another article on coral colouration but I'm not sure whether you guys like the articles I write or not

Of course bro, have been noticing your posts, very pro though abit cheem for me. Thanks!

Thanks bro for such good article.