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SAND CLUMPING - ANOTHER VIEW by Sam Gamble July 1997 Aquarium.Net

This month Sam Gamble gives us an alternative answer to why sand clumps, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Sand Clumping

By Sam Gamble


In the April article sand clumping was discussed from the chemical and physical properties. The Marine Geologist at CaribSea, Rick Greenfield, helped us understand that the observation generally has two pathways, 1. to supersaturate the water of the system with calcium ions, 2. the lowering of magnesium concentration allows calcite formation to become easier.

This understanding is based on physical and chemical aspects of our sand bed aquariums. It's not hard to see that when the abundance of calcium ions becomes great enough they congregate to an easily used surface (precipitation). The calcium ions and the medium in which they are suspended are positively charged. The sand bed and the grains of sand are mostly negatively charged. The precipitation onto the sand grains is a coating that satisfies charge and abundance of ions. A path of least resistance to a desirable surface.

Since it is usually observed on the surface of the sand bed, let's focus our attention there. Also, let's take a shift from physics and chemistry to biology. This is where a very dynamic part of our aquarium is cycling organic material, to gain energy for existence and reproduction. Some people are still referring to the living process as filtration. But, to truly understand it, we must visualize the cell and its environment.

Our living sand system consists of a sand bed, which provides suitable surface area for microbial cells to attach themselves. If we do it correctly there are populations of varied species in colonies large enough to use up the compounds added and produced in the closed system. We depend heavily on the natural checks and balances of the natural systems from which we have borrowed the ingredients. I equate success, with balance of living cells to use and produce energy for maintenance of their immediate environment at equilibrium. To satisfy this qualification the macro environment thrives.

The micro environment of cells on the surface of the sand can be termed, "biofilm". A biofilm consists of cells immobilized at a substratum and frequently embedded in an organic polymer matrix of microbial origin; a surface accumulation, which is not necessarily uniform in time or space; may be composed of a significant fraction of inorganic or abiotic substances held together by the biotic matrix (protein goo).

A biofilm system consists of the biofilm, the overlying liquid layer, and the substratum on which the biofilm is immobilized. A biofilm is composed of microorganisms immobilized at a support surface. This is generally in association with the organic polymer matrix. However, the biofilm may also contain degradable and/or inert particles and sometimes may include microorganisms. The biofilm system has been classified in terms of phase and compartments.

Many compartments can be defined in a biofilm system:

  1. the substratum
  2. the base film
  3. the surface film
  4. the bulk liquid

Each compartment is characterized by at least one phase (gas, liquid, or solid), using the term "phase" in the strict thermodynamic sense. Thus, each compartment can be described in terms of its thermodynamic and transport properties. This includes the transport and transformation processes that dominate within the compartment. The system encompasses all of the "compartments," the phases, the process compartments (properties and processes), and the geometry of the system (e.g., a rotating biological contactor or a sand bed).

The substratum plays a major role in biofilm processes during the early stages of biofilm accumulation and may influence the rate of cell accumulation as well as the initial cell population distribution. The substratum can sometimes also serve as the substrate, the rate limiting nutrient for growth.

Biofilms serve beneficial purposes in the natural environment or engineered biological systems. For example, biofilms are responsible for removal of dissolved and particulate contaminants from natural streams and in waste water treatment plants (fixed film biological systems such as trickling filters, rotating biological contactors, and fluidized beds). Biofilms in natural water, called mats, frequently determine water quality by influencing dissolved oxygen content and by serving as a sink for many materials. Biofilms provide opportunity for syntrophic and other community interactions between microorganisms, and a means for survival of microorganisms in natural habitats.

When microbial cells in the biofilm are reproducing and expanding their colony, perhaps the sudden appearance of calcite deposits could be inhibited by the cells when attaching their fibrils to form the matrix. An early phase in the biofilm process is the conditioning of the substrateto which they attach. With regard to precipitation of calcium, it would seem to be a controlled factor when there is biofilm formation of fibrils and matrix during the beginning stages of the colonization process.

This condition would fit the observations; sand-bed systems tend to have unstable chemistry in the early stages, and reported clumping seems to be early in the sand bed setup. The interesting thing to note is that seeding with live sand has a positive effect. Sand beds seeded with live sand do not tend to have as many reports of clumped sand. If applied to the process of conditioning the substrate surface and the recruitment of microbial populations for ecological succession, it makes sense.

But, we must caution ourselves from making too many hardened statements. The high cell densities and the (electro)chemical properties of the matrix influence transport of soluble and particulate components. Conceptually, the diffusion process can be described with present available models. However, the factors influencing detachment (erosion and sloughing) processes are not so obvious. Since detachment influences several processes, including the population distribution, more attention to causes and effects of detachment is necessary. In addition, the integration of the biofilm with the inorganic chemistry of the bulk water has not been addressed sufficiently. The exact role of calcium precipitation has not been determined.

Various environmental variables (e.g., pH, temperature, salinity) influence microbial stoichiometry and kinetics and, thus, physiological ecology, in a biofilm. This is an important consideration if application to clumping is valid. With the success of live sand seeding, it is possible to accept the importance of diversity to balance the environment. Microbial ecology is concerned with the interactions between different microorganisms and between organisms and their environment, e.g. high concentrations of calcium ions and/or decreased magnesium concentrations.

If any correlation to inhibition of clumping and the inoculation of a well functioning biofilm is valid, then the matrix and microbial conditioning of the substrate is a plausible influence. It stands to reason microbial biofilms are biotic and abiotic barriers between the ionic attraction of calcium carbonate to the substrate surface. The natural tendency for the biofilm to grow and mature would also be an inhibitive force against clumping, since it is the matrix that conditions the substrate.

To me a more basic question arises when considering the initial phases of biofilm formation. If, it is part of the biofilm's succession in growth and establishment, to condition the substrate and maintain its environment, then is a steady supply of supersaturated calcium ion necessary for health and reproduction? Does it actively achieve equilibria in the system that the biofilm is part of?

I tend to side with the idea that in sand bed systems where a diverse and healthy population of microbial cells has been supplied, buffering is adequate. pH fluctuations are for reasons other than total alkalinity. Thereby it is much less crucial to add a supersaturation of calcium and magnesium ions. The micro environment will, with its own time line, maintain balance for the macro environment. Of course the catch in the statement is "diverse and healthy". Those essential elements can be easily abused.

For questions and comments,

Sam Gamble


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Last modified 2006-11-19 01:14