Bacteria and active biofilms

The diver­si­ty of bac­te­ria in natu­re can hard­ly be sur­pas­sed. Howe­ver, they are so small that we can­not reco­gni­ze them with our eyes. As invi­si­ble patho­gens they are very sus­pi­cious to us humans. Only modern mole­cu­lar bio­lo­gy has shown us that they are necessa­ry on the skin and in the intes­ti­nes to main­tain our health. And bac­te­ria are also indis­pensable in food pro­duc­tion and pro­ces­sing. Cer­tain spe­ci­es have long been used in the pro­duc­tion of foods such as cheese, sourdough bread and sauer­kraut. But what role do bac­te­ria play in fish far­ming, espe­cial­ly in clo­sed aquacul­tu­re systems?

Bac­te­ria in aquaculture

Bac­te­ria can only mul­ti­ply if they are given food. Sin­ce their food is often insi­gni­fi­cant was­te mate­ri­al, this is hard­ly noti­ce­ab­le: Resi­du­al sub­s­tan­ces disap­pe­ar, but a small num­ber of bac­te­ria remains. In aquacul­tu­re, for examp­le, bac­te­ria use lef­tover feed. Howe­ver, they main­ly live in and from the excre­ments of the fish. As in natu­re, they “remi­ne­ra­li­ze” the some­ti­mes-poi­son­ous excre­ments of the ani­mals by decom­po­sing orga­nic food com­pon­ents to such an extent that ulti­mate­ly only CO2, H2O and various inor­ga­nic sub­s­tan­ces (phos­pha­te, ammo­ni­um, nitra­te, atmo­s­phe­ric nitro­gen) remain. In our arti­cle Nitri­fi­ca­ti­on and Deni­tri­fi­ca­ti­on we have exp­lai­ned this degra­dati­on pro­cess in more detail.

Bac­te­ri­cal risks

In addi­ti­on to the indis­pensable con­tri­bu­ti­on of bac­te­ria to water puri­fi­ca­ti­on, the­re are also risks in clo­sed systems:

  1. If the bac­te­ria recei­ve food, they mul­ti­ply. Abo­ve a cer­tain num­ber in a volu­me of water, we noti­ce them becau­se they cloud the water.
  2. Bac­te­ria con­su­me oxy­gen during the con­ver­si­on of sub­s­tan­ces. The fish also need oxy­gen. In bac­te­ria-rich, tur­bid water, this com­pe­ti­ti­on can lead to a dead­ly lack of oxy­gen for the fish. The fish are par­ti­cu­lar­ly end­an­ge­red in sum­mer, becau­se in warm water not only less oxy­gen is dis­sol­ved from the air into the water, but at the same time the oxy­gen con­sump­ti­on and the repro­duc­tion of the bac­te­ria is increased.
  3. Fish keep bac­te­ria away from them­sel­ves by forming sli­me. Muci­la­ge is a form of orga­nic car­bon and the­re­fo­re in princip­le less degrad­able than other orga­nic sub­s­tan­ces, but it is only a ques­ti­on of num­ber when the bac­te­ria mana­ge to break through the lay­er. So, if the­re are a lot of sli­me-degra­ding bac­te­ria in the water, this will affect the self-pro­tec­tion of the fish. Aggres­si­ve bac­te­ria can pene­tra­te con­ta­mi­na­ted water right up to the skin and crea­te ent­ry points for patho­gens the­re. The ani­mals then beco­me more sus­cep­ti­ble to dise­a­ses and their resis­tance to patho­gens and para­si­tes decreases.

It is the­re­fo­re important to pre­vent too many bac­te­ria from being pre­sent in the pro­cess water. In order to pre­vent the risk of bac­te­ria, a quick eli­mi­na­ti­on of the nut­ri­ent sources in the water is as important as a regu­lar remo­val of the bac­te­ria them­sel­ves. Both hap­pens with the help of mecha­ni­cal fil­ter units, the drum fil­ter and the skim­mer, which remo­ve both lar­ge and small par­ti­cles from the plant. This not only redu­ces the nut­ri­ent sup­ply for the bac­te­ria, but also their num­ber. Con­se­quent­ly, the germ load of the fish and the com­pe­ti­ti­on for oxy­gen bet­ween bac­te­ria and fish remains tech­ni­cal­ly controllable.

Bio­films in recir­cu­la­ti­on systems

In natu­re, bac­te­ria often orga­ni­ze them­sel­ves in bio­films: Che­mi­cal pro­ces­ses such as nitri­fi­ca­ti­on and deni­tri­fi­ca­ti­on are the joint efforts of dif­fe­rent types of bac­te­ria, which they per­form in a very small space, the bio­film. For other bac­te­ria, a lack of nut­ri­ents cau­ses them to form an extracel­lu­lar, sti­cky matrix with which they can attach them­sel­ves to sur­faces and extract small par­ti­cles as well as dis­sol­ved nut­ri­ents from the pas­sing water. Due to the abo­ve descri­bed risks cau­sed by bac­te­ria, care is taken in the bea­ring tank and thus in the immedia­te vicini­ty of the ani­mals to ensu­re that the sur­faces (e.g. tank walls, nets, pro­bes etc.) are as small as pos­si­ble for bio­films and easi­ly acces­si­ble for the plant ope­ra­tor. This should allow the plant ope­ra­tor to regu­lar­ly wipe off bio­films. In the indi­vi­du­al bio­fil­ters (nitri­fi­ca­ti­on and deni­tri­fi­ca­ti­on), on the other hand, struc­tu­red car­ri­er mate­ri­als („bio­fil­ter pel­lets“ or „bio-car­ri­ers“) pro­vi­de the bac­te­ria with a par­ti­cu­lar­ly lar­ge sur­face area for colo­niz­a­ti­on. In addi­ti­on, the living con­di­ti­ons for bac­te­ria are opti­mi­zed in terms of pro­cess tech­no­lo­gy. For examp­le, a fil­ter is par­ti­cu­lar­ly well ven­ti­la­ted so that the bac­te­ria find a lot of oxy­gen and pre­fer to stay in this fil­ter ins­tead of in the tank. During their stay in the fil­ter, the bac­te­ria then meta­bo­li­ze the excre­ments of the fish so that the con­cen­tra­ti­on of was­te pro­ducts in the tank always remains low and does not end­an­ger the fish.

In sum­ma­ry, the secret of a healt­hy clear-water aquacul­tu­re con­sists of an effec­ti­ve mecha­ni­cal remo­val of solids and bac­te­ria and a small sur­face-to-volu­me ratio in the tank. In addi­ti­on, it is important to design the various bio­fil­ters suf­fi­ci­ent­ly in size and sur­face area to ensu­re that the bac­te­ria can effec­tively degra­de the resi­du­al sub­s­tan­ces by means of the opti­mi­zed con­di­ti­ons pre­vai­ling there.

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— Weus­ter-Botz, D.; Chmiel, H.; Takors, R.: Bio­pro­zess­tech­nik. 4. Auf­la­ge, Sprin­ger-Ver­lag, 2018.
— Ernst, A.: MOSA1 – Mikro­biel­le Opti­mie­rung von Stan­dard­kom­po­nen­ten in der Aqua­kul­tur. 1: Becken­ab­tren­nun­gen: Net­ze oder Rechen?. Vor­trag zum Abschluss­be­richt, 17.09.2019.
— School­tink, H.: Mikro­biel­le Bio­fil­me – Gemein­sam zur Atta­cke. Phar­ma­zeu­ti­sche Zei­tung, Aus­ga­be März 2015.

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