Heterotrophic Bacteria

AMR, ZEAH, bacterial floc, heterotrophs, autotrophs – these terms maybe new to the ears of fish and shrimp farmers in the Philippines. AMR stands for Aerated Microbial Reuse while ZEAH stands for Zero-Exchange, Aerobic, Heterotrophic. Both refer to the same thing: a system of intensive aquaculture that has been around for at least ten years and is becoming more popular– starting in the Western hemisphere but now spreading in Southeast Asia. The basic technology was developed at the Waddell Mariculture Center in the USA in the early 1990s. AMR or ZEAH, whichever term you prefer, has been found to reduce feeding cost, makes possible operation of a farm with very little or even zero water exchange, while still producing 10 to 30 tons of shrimps per hectare and from 10 to 100 kg of tilapia per square meter per crop. It seems too good to be true. But it is for real and if you think it is still experimental and still needs to be verified in a commercial scale operation, think again!

Autotrophic vs Heterotrophic
AMR is basically a “heterotrophic” system as against the conventional culture system that fish and shrimp farmers in Asia and the Philippines which is considered “autotrophic.” To understand the difference between the two systems, one has to go into the respective roots of the two terms. Heterotrophic comes from “heterotroph” – an organism which rely on carbon in organic form (i.e. other organisms) for food. Animals, fungi, parasitic plants and most bacteria are heterotrophs. In contrast autotrophic comes from the term “autotroph” an organism capable of sustaining itself due to its ability to produce their own food (or organic carbon) from inorganic materials which are basically water, carbon dioxide and nitrogen. The food is synthesized using energy from light or photosynthesis or inorganic chemical reaction or chemosynthesis. Autotrophs include all (except parasitic) plants and some bacteria.
A pond where In where food is produced by autotrophs, mainly plant organisms, whether microscopic and in the water column such as phytoplankton or resting on the bottom such as benthic algae is considered an autotrophic system. In contrast a pond where food has to be introduced is considered a heterotrophic system. It of course does not mean the system is completely free from any phytoplankton. In fact the presence of phytoplankton is believed by some to be essential as oxygen source and reduce aeration need in the daytime.

Extensive vs Intensive
In conventional pond culture that Filipino farmers are familiar with great pains are taken to prepare the pond so that plankton, particularly diatoms flourish before stocking the shrimps or fish. Presumably the plant plankton becomes food to tiny animal plankton and the two types of plankton together becomes natural food for the newly stocked shrimp or fish fry. This is thought to be the ideal condition for the shrimps or fish because it simulates their natural habitat. Because food is produced within the pond itself the system is considered “autotrophic”.
When the stocking density is low, the food that is generated within the pond is sufficient to support the shrimp or fish stock and no feeding is required. This is what is known as extensive aquaculture. Such a system is capable of producing at most a few hundred kilograms per hectare. In an effort to increase production it is inevitable for farmers to try stocking more – to the extent that the natural food that is produced becomes insufficient to support the stock. In such case feed has to be introduced in order to supplement the nutrition coming from naturally occurring food in the pond. Such system which relies on a combination of naturally occurring food and introduced food or feeds is often referred to as “semi-intensive” aquaculture.
At high stocking density, and as the animals grow, more feed is required to the extent that the role of natural food becomes insignificant and the culture becomes “intensive” in nature. Since no more than 30 of the carbon, nitrogen and phosphorus in feeds is assimilated or converted into flesh by the fish or shrimps, more of it serves only to pollute the water in the form of uneaten feeds and excretory wastes which is high in ammonia – a substance that stresses fish and shrimps, reduce growth rate and at high levels even cause mass mortality. At low stocking density, this poses no problem since they can still be fully utilized by phytoplankton and bacteria. At high densities however, the amount of such wastes overwhelms the system and if left unchecked accumulates in the system. The conventional approach is to change the water in order to reduce the level. Considerable skill and experience is required to maintain the phytoplankton population at the right level but which due to weather variation may collapse and create havoc. Furthermore the water discharged is high in organic load – one aspect of intensive aquaculture that is at the forefront of environmentalist’s list of negative effects of aquaculture to the environment.

Shifting from Phytoplankton to Heterotrophic Bacteria
As now practiced in Belize by the Belize Aquaculture Ltd (BAL) , applying AMR technology requires deliberately converting the pond ecosystem from one that is autotrophic or phytoplankton-based to a heterotrophic system dominated by bacteria after the fry has been stocked and has established itself. This requires providing a low-protein, high carbohydrate diet with Nitrogen to Carbon or C:N ratio of 16:1 (18 protein) so that the Carbon-hungry bacterial population has adequate food to multiply even as the shrimps are also being fed with regular starter feed The culture is deliberately overfed at 200 to 250 of the shrimp biomass. The addition of wheat flour or even molasses, both of which are carbon rich, has been found also to hasten the growth of heterotrophic bacteria.