A straightforward exploration of the remarkable, albeit somewhat historic, discovery of bacteria that inhabit the cells in the gills of freshwater fish that assist the purification of waste and thus lessen its accumulation in aquaria. But first, essential knowhow for the absolute beginner.
Proteins are indispensable for all life, so much so, that storing of the information necessary to recreate them, is the only role of DNA. Responsible for most of the physical attributes of higher organisms, they are made up of nitrogen-containing building blocks called amino acids of which there are generally 20 kinds, a few of which animals cannot make and must therefore be eaten. However, they are impossible to store unless manifested in living structures. Once these building blocks are digested by stomach and intestinal enzymes, their remains are absorbed into the bloodstream and used to create others which are transported around the body to where they are needed and used, while surplus collect in the liver. Whilst there, they are once again broken down (catabolized) to generate urea in humans and mostly ammonia in finfish [5].
Human kidneys filter waste from blood. We therefore pass urine comprising urea, salts, water, and other unwanted byproducts, whereas the solitary kidney of bony fish (teleosts) isolates a modicum of urea and ammonia and ammonium, yet the vast majority evades the kidney as it is excreted into water through the gills [3].
Dissolved ammonia gas is very toxic where its accumulation will kill most life, while its dissolved charged (ionic) counterpart ammonium, is less so. The reciprocal ratios of these toxicants are critical considerations as more ammonia is present at higher temperatures and pH [4].
Be that as it may, fish are very sensitive to ammonia and ammonium. The physical removal of debris is not the primary function of aquarium filtration. Over a period of around a month to a month and a half, the biofilter develops films of bacteria that grow on sponges or other integral items that transform ammonia to less injurious nitrite, while others transform nitrite to far less toxic nitrate. New filters lack this kind of biology and thus fish must be purchased gradually. Bottled cultures of bacteria are not a reliable substitute.
A new aquarium must be filled and run for around a week before the first solitary fish is purchased. Before release, a fish must be acclimated to the temperature in the aquarium by floating the bag for around 20 to 30 minutes. The first fish is exposed to significant ammonia so it must be hardy, whilst its nutritional intake must be kept to a minimum because fish will eat far beyond their needs, whereas more food contains more protein which is destined to become waterborne ammonia and ammonium. Raised and extremely toxic concentrations of ammonia, cloud water. If the water becomes dull during the first month and a half, it is important to stop feeding and perform a 25 percent water change, whereafter, feed sparingly. See below for important guidance. Our stomachs are around the size of a fist, where a fish’s proportions are not too dissimilar.
Regular removal of sponges and other filtration media to clean them under mains water is the first mistake made by newcomers. Tap water contains sterilizing levels of chlorine and chloramines which kill the beneficial water-purifying bacteria and thus the biofilter never matures. Hence all the fish purchased by newcomers tend not to survive. Just as it is not ordinary to trip over cadavers on the sidewalk, it is not normal for fish to die. Repeated squeezing in aged tank water recently removed from the display, is the only safe method of cleaning the sponges of aquarium filters and their other media. If you ever need to replace the sponges because they are degrading, cut them into four and replace one quarter every fortnight. For the first two months of running an aquarium, clean only the glass with an algae magnet and buy one small fish every two weeks from the second week. Acrylic aquaria need special magnets and algae pads or else the aquarium will be scratched.
From two months, perform a 25 percent weekly water change with water that is of the same temperature as that of the aquarium which has had its chlorine and chloramines neutralized with a generous glug of “tap safe”. Chlorine is very deadly to fish and chloramines are persistent which gradually release chlorine and ammonia. “Tap safe” formulated from sodium thiosulphate is somewhat harmless, so as these chloramines breakdown over several days, surplus thiosulphate neutralizes their chlorine, whereas the biofilter detoxifies their ammonia. The tips of human fingers can detect fractions of a degree in temperature [1], so new water to replace that in the aquarium can be accurately matched using the hot tap and your fingers: first in the aquarium and then in the new water. Water can be simply syphoned out; however, it is also beneficial to use a gravel syphon or vacuum to clean some of the aquarium substrate. Remember to manipulate the sponges and other biofilter contents in the 25 percent aged aquarium water you removed before it is thrown out.
Fig 1. Ultra-high magnification transmission electron micrographs of sections through the cellular architecture of fish gills. [D] clusters of intracellular symbiotic ammonium-detoxifying bacteria close to blood vessels [V] filled with nucleated red blood cells with a 5-micron (5 x 10-6 meters) scale bar. [E & F] illustrating the intracellular nature of these clusters of symbionts with respective scale bars of 5 microns and 500 nanometers (5 x 10-7 meters). Courtesy of van Kessel et al. 2016 and The Creative Commons Attribution-NonCommercial-NoDerivs License.
If fish begin to die after several months to a year or more, test the water for nitrate, because 40 milligrams per liter (mg l-1; parts per million; ppm) is toxic to most freshwater fish. Weekly water changes should stabilize nitrate to around 20 to 25 ppm. Perform a 40 percent water change if nitrate is too high and then moderate food so that it remains around 20 to a maximum of 27 ppm (mg l-1). Place a fresh bag of prerinsed granular activated carbon (GAC) in your filter for merely 48 hours every one and a half to two months.
Amazingly, the cells of the gills of captive freshwater zebra danios (Danio rerio) and common carp (Cyprinus carpio) nurture clusters of internal bacteria that transform bloodborne ammonium into nitrite which is then converted by others to dissolved dinitrogen gas which leaves the aquarium at the surface (Fig 1.). A proportion of the nitrogenous waste produced by these fish is thus neutralized and eradicated before it can enter the water [7]. The reduction of nitrite to dinitrogen commonly referred to as denitrification, usually occurs in microenvironments where dissolved oxygen (DO) is negligeable or absent, whereas these remarkable symbiotic bacteria perform these biochemical reactions surrounded by DO. However, these kinds of competencies are common amongst rapid-doubling aerobic heterotrophic bacteria that utilize organic carbon for growth [6], whereas typical nitrifying communities that convert ammonia to nitrate are slow-multiplying chemolithoautotrophs that respire oxygen and use inorganic sources of carbon [2]. Whether these intracellular microbes have evolved since the introduction of these fish into recirculating environments such as aquaria or they are there to help fish survive in droughts when bodies of water shrink, can perhaps form the foundation of a novel study.
References
- Aslett, C., G. (2024) The Complete Reef Aquarist, for hobbyists, the trade and academics – A Conservation Manual. Aslett, C., G. (ed.). Reef Ranch Publishing Ltd, Leeds, West Yorkshire, UK. https://ebay.us/m/ALPZd6
- Hovanec, T. (2019) Dr. How to harness bacteria to cycle your saltwater tank quickly! | MACNA 2019. BrsTV. https://www.youtube.com/watch?v=zDI7sxqC-ss
- Moyle, P., B. & Cech, J., J., Jr. (2000) An Introduction to Ichthyology. Ryu, T., Tarabokjia, L., Schiaparelli, K. & Dellas, K. (eds.). Prentice Hall Inc., Upper Saddle River, NJ, U.S. p 108.
- Noga, E., J. (2010) Fish Disease: Diagnosis and Treatment, Second Edition. Noga, E., J. (ed.). John Wiley & Sons Inc. p 93.
- Stryer, L. (1995a) Biochemistry 4th Edition. Stryer, L. (ed.). W. H. Freeman and Company, New York. p 634.
- Sun, Y., L., Li, A., Zhang, X., N. & Ma, F. (2015) Regulation of dissolved oxygen from accumulated nitrite during the heterotrophic nitrification and aerobic denitrification of Pseudomonas stutzeri T13. Appl Microbiol Biotechnol. 99, 3243-3248.
- van Kessel, M., Mesman, R., Arshad, A., Metz, J., Spanings, F., van Dalen, S., van Niftrik, L., Flik, G., Wendelaar Bonga, S., Jetten, M., Klaren, P. & Op den Camp, H. (2016) Branchial nitrogen cycle symbionts can remove ammonia in fish gills. Environmental microbiology reports. 8(5), 590-594.
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