The Application of Potassium Diformate in Aquaculture: Comparison
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Potassium diformate (KDF) is an organic acid salt. It is a dimer formed through hydrogen bonding between one molecule of formic acid and one molecule of potassium formate. The chemical formula of KDF is HCOOH·HCOOK, with a molecular weight of 130.14. It is a white or slightly yellow crystalline powder with no discernible pungent odor. KDF dissolves in water and exhibits a pronounced hygroscopic nature. Its aqueous solution is acidic and remains stable under acidic conditions, while it decomposes into formate and formic acid under neutral or slightly alkaline conditions. Compared with formic acid, KDF overcomes the irritability, corrosiveness, and instability of formic acid. Therefore, KDF is a more suitable additive in feed, providing a safer and more stable solution in maintaining the balance of microbial communities in aquatic animals.

  • potassium diformate (KDF)
  • aquaculture
  • growth performance
  • feed efficiency
  • disease resistance
  • pH

1. Introduction

Antibiotics are the primary measures employed in the prevention and treatment of diseases in aquaculture. However, the frequent utilization of antibiotics can significantly impact the growth and reproduction of aquatic organisms, resulting in water pollution. The European Union (EU) has prohibited antibiotic additives in animal feed. Potassium diformate (KDF) represents the first non-antibiotic feed additive approved by the EU as a viable alternative to antibiotics. Its application in animal nutrition has been validated, demonstrating beneficial health effects.
KDF can enhance the growth performance, improve the feed efficiency, heighten immunomodulation, increase the disease resistance, and regulate the intestinal microbiota balance of aquatic organisms [21][1] (Figure 1).
Fishes 09 00085 g002
Figure 1. Addition of potassium diformate to aquaculture organisms. In chemical structures, black represents carbon atoms, red represents oxygen atoms, grey signifies hydrogen atoms, and blue symbolizes potassium atoms.

2. Enhancing Growth Performance and Feed Efficiency

Since KDF is applied as an additive in aquatic feed, researchers continuously explore its health-promoting effects on aquaculture organisms. Relative to other feed additives, KDF has a wider range of applications. Besides improving the growth performance of aquatic animals, it also benefits the preservation of feed. In a study on Osphronemus goramy, it was noted that the addition of KDF significantly increased their feeding rate. This phenomenon is attributed to the elevated levels of KDF in the feed, subsequently influencing the osmoregulatory system of the fish. Because KDF is an organic acid salt, the energy for growth is only available in small amounts, and the remainder is utilized to balance the osmoregulatory system of fish. Additionally, KDF has the capability to eliminate pathogenic bacterial cells in the digestive tract, increase the population of symbiotic bacteria, and enhance the absorption of nutritional substances in the feed. This makes the nutrients in the feed more easily absorbed, thereby improving the efficiency and value of feeding. KDF induces acidity in an animal’s digestive tract. This contributes to the enhancement of assimilative enzyme activity, thereby increasing the feed intake to promote weight gain. Following the addition of 0.3% KDF, Osphronemus goramy achieved an average daily growth rate of 1.31%. In aquaculture experiments, a 1% daily growth rate is considered favorable for Osphronemus goramy, as it is a species known for its relatively slow growth [31][2]. The incorporation of KDF in the feed enhanced mineral absorption in the intestine stimulated the secretion of certain enzymes, such as proteases, thereby promoting growth [36][3]. In the experiment involving juvenile Beluga (H. huso), Sayah et al. [32][4] combined the use of KDF and calcium diformate (CaDF) to assess their impact on the growth of juvenile Beluga. KDF and calcium diformate (CaDF) share similar functions, allowing for synergistic research. These experiments indicate that feeding KDF can improve the feed conversion ratio and specific growth rate in juvenile Beluga. This is attributed to the stimulation of digestive enzyme secretion upon incorporating KDF into the feed, leading to heightened feeding interest in juvenile Beluga, and consequently promoting their growth performance [37][5]. Overall, the aforementioned studies underscore the significant role of KDF supplementation in the growth performance and digestion of aquaculture species [38][6].

3. Improving Disease Resistance

In numerous aquatic organisms, the addition of KDF to feed has demonstrated beneficial effects in enhancing their immune system and disease resistance. An increased or decreased white blood cell count can serve as an indicator of an organism’s disease resistance [39][7]. White blood cells play a crucial role in the defense system of fish against pathogenic infections [40][8]. They possess non-specific immune defense functions, enabling them to eliminate pathogens through phagocytosis [41][9]. The fish’s defense system releases neutrophils triggered by external chemical stimuli or chemotaxis in the event of infection [42][10]. The typical white blood cell count range for freshwater fish is 2.0–15.0 × 10−4 cells/mm3 [43][11]. In a cultivation experiment with Osteochilus hasselti, KDF addition to the feed enhanced the immune response of O. hasselti [43][11]. The lowest average white blood cell count was observed in the group with no KDF, while the highest increase was observed in the KDF group. KDF can disrupt the osmoregulatory system within an organism [44][12]. According to Lantu, an excess of salt within a fish’s body can lead to the fish expending more energy to stabilize its osmoregulatory system [34][13]. Red blood cells contain oxygen-binding hemoglobin, and their quantity is influenced by a fish’s activity, water temperature, species, age, gender, and nutritional status. The control group showed the lowest average increase in red blood cells, measuring 2.05 × 106 cells/mm3, while the KDF-added group (0.3%) exhibited the highest average increase, reaching 2.49 × 106 cells/mm3. The normal range of red blood cells in bony fish is generally between 1.00 and 3.00 × 106 cells/mm3. An increase in red blood cells leads to a higher concentration of hemoglobin, allowing for the binding of more oxygen. The results indicate that among all the treatments, Nilem fish exhibited a significant decrease in the red blood cell count. However, 0.1%, 0.3%, and 0.5% KDF added to the feed caused relatively higher red blood cell counts than those in the control group (0%). This may be attributed to the enhanced resistance of the organisms due to the addition of KDF, leading to an increase in white blood cell count in Nilem fish. Consequently, the groups given KDF showed lower infection levels, allowing organs such as the liver, spleen, and spinal cord to continue forming red blood cells.
KDF possesses the potential to alleviate the symptoms of infection in aquatic animals. Aeromonas hydrophila, a commonly found bacteria in aquatic environments, is the primary bacterial species responsible for fish mortality [45][14]. Infection with A. hydrophila results in noticeable symptoms, such as a loss of appetite, surface lesions, gill hemorrhage, edema, and ulcers, which can lead to the swelling and damage of the liver, kidneys, and spleen, resulting in death [46][15]. KDF can alleviate the aforementioned symptoms [47,48][16][17]. One researcher noted that fish infected with A. hydrophila exhibited pathological changes, such as darkening of their body color, weakness, unresponsiveness to feed, and local hemorrhaging [49][18]. The exotoxins released by A. hydrophila circulate throughout the body via the bloodstream, leading to hemolysis and ruptured blood vessels. This vascular and tissue rupture leads to visible hemorrhaging on the body surface [50][19]. The condition of the body’s surface deteriorated between Day 3 and Day 9. Nilem (O. hasselti) fingerlings infected with A. hydrophila exhibited symptoms such as hemorrhaging, exophthalmos, ulceration, and dropsy [50][19]. Hemorrhaging is the initial response to A. hydrophila infection, resulting in tissue damage [51][20]. Dropsy is attributed to the secretion of the aerolysin cytotoxic enterotoxin (ACT) gene, which contributes to tissue damage. According to Noor El Deen et al., fish infected with A. hydrophila exhibited superficial skin lesions, leading to imbalanced swimming, a darkened body color, exophthalmia, and hemorrhaging [52][21]. However, administering KDF can treat the complex infection symptoms caused by A. hydrophila in aquatic animals within a relatively short period. Elala et al. supplemented the diet of A. hydrophila-infected Oreochromis niloticus with 0.3% KDF. Beyond the 14th day, the injuries tended to heal in response to the KDF treatment. This recovery is attributed to the potent antibacterial and antiviral activities of KDF, thereby enhancing the overall health status of the host [18][22]. The fish fed an organic acid blend and potassium diformate as feed additives showed a significant reduction in the total bacterial count per gram of feces. Da Silva et al. identified organic acids (butyrate, propionate, and acetate) as having the most pronounced inhibitory effects against Vibrio sp. in marine shrimp, Litopenaeus vannamei [53][23]. These acids can permeate the membranes of Gram-negative bacteria, releasing protons into the cytoplasm [54][24]. The bacteria undergo substantial ATP expenditure in their efforts to extrude protons to preserve the equilibrium of the intracellular pH [55][25], leading to the depletion of cellular energy resources and culminating in cellular demise. The serum total proteins play a key role in immune responses and serve as a fundamental indicator of the health status of fish [56][26]. 

4. Modulation of the Gut Microbiota

KDF exhibits inhibitory effects on most harmful intestinal bacteria. Other studies indicate that KDF demonstrates stronger antibacterial activity against Enterobacteriaceae and Salmonella compared to Lactobacillus, enhancing the overall health of the intestinal microbiota in cultured animals. Research has shown that the addition of varying levels of KDF, specifically 1, 1.5, and 2 g per kg, to the diet of H. huso led to an increase in intestinal lactic acid bacteria (LAB) in H. huso after 60 days [32][4]. Organic acids have a regulatory effect on the intestinal health of H. huso, such as enhancing their intestinal balance and digestion, removing toxins, and improving their immune status [32][4]. Saliva et al. indicated that KDF as well as propionates, butyrates, and acetates had inhibitory effects on Vibrio species [58][27]. Formic acid inhibits V. cholerae, V. harveyi, V. parahaemolyticus, V. vulnificus, and V. anguillarum [59][28]. Abu-Elala and Ragaa [23][29] conducted a cultivation experiment with O. niloticus and added KDF (0, 1, 2, and 3 g/kg) to the feed. The addition of KDF to the feed improved the feed efficiency of O. niloticus. The feed containing KDF significantly reduced the total bacterial count in the feces, which is attributed to lipophilic organic acids being able to diffuse into the cell membranes of Gram-negative bacteria and acidifying them, leading to bacterial death. The addition of KDF to the feed enhanced the relative abundance of intestinal bacteria in O. niloticus. Zhou et al. [27][30] similarly found that the addition of KDF to the daily diet can impact the intestinal microbiota of external bacteria by selectively increasing the relative bacterial abundance (RBA, %) of the genus Bifidobacterium, such as α-Proteobacterium IMCC1702-like, Streptococcus P14-like, and three uncultured bacterial species.
L. vannamei is one of the most extensively cultured shrimp species globally, representing a significant portion of shrimp aquaculture. Investigating feed additives to enhance the production of the South American white shrimp is of paramount importance. Other researchers explored the effects of KDF, sodium formate (SF), and a combination of KDF+SF. The addition of KDF to the daily diet significantly reduced the total culturable bacterial count (TCBC) and presumptive Vibrio spp. count (PVC) in all the treatment groups [35][31]. Similarly, the total bacterial count in the feces of red hybrid tilapia (Oreochromis sp.) significantly decreased when using different doses of a mixture of organic acids and 0.3% KDF [60][32]. The addition of KDF (2%) to L. vannamei feed for 15 days resulted in a reduced TCBC and PVC. When varying amounts of butyrate salts were added to the feed of L. vannamei, the count of Vibrio species decreased at 27 and 47 days [58][27]. These findings carry important implications for animal farming, particularly shrimp farming.

5. Reducing Intestinal pH Levels

Under typical conditions, the gastrointestinal pH of aquatic organisms is generally maintained between 5 and 8. Proteins undergo degradation due to amino acids in this slightly acidic-to-mildly alkaline environment. The dissociation state of these amino acids can significantly influence their absorption rate [17,18,19,31,60][2][22][32][33][34]. By supplementing KDF in the feed of aquaculture organisms, a microbial balance is achieved in the digestive tract, maintaining an appropriate pH, eliminating pathogenic microorganisms, and preserving the health of the organisms. Adding KDF to the feed is a preventive alternative for maintaining the health of farmed fish. This can lower the intestinal pH [62][35], suppress the growth of pathogenic bacteria, mainly Gram-negative bacteria, aid in nutrient digestion and absorption, and have beneficial effects on animals’ production performance [19][34]. KDF enhances the feed performance. Within the gastrointestinal tract, KDF can lower the pH in the small intestine by transporting H+ ions. KDF inhibits the growth of Gram-negative bacteria by dissociating the acids in bacterial cells after entering the intestine [63][36]. At pH < 5, the number of several Gram-negative bacterial species decreases. A low pH forms a natural barrier against bacteria from the ileum and large intestine. Low-molecular-weight KDF is lipophilic and can penetrate the cell membranes of Gram-negative bacteria [64][37]. The supplementation of KDF can reduce the population of pathogenic bacteria in the gastrointestinal tract, while increasing the population of acid-resistant beneficial bacteria, such as lactobacilli.
The intestinal pH of O. goramy decreased after adding KDF to the feed. In the feed with 0.3% KDF, the initial pH was 7.01, and the final pH was 6.61. In the feed with 0.5% KDF, the initial pH was 7.01, and the final pH was 6.49. In the feed with 0.8% KDF, the initial pH was 7.11, and the final pH was 6.12. Fish receiving KDF had a lower stomach pH compared to those without KDF. The reduction in stomach pH allows for faster digestion of the feed, and a lower pH facilitates better feed breakdown, promoting fish growth [31][2].

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