## 5 Animal Nutrition
### Chemical Methods
Currently, fermented plant protein has been used as an excellent substitute for fish meal in aquafeed to increase and sustain the production of sustainable aquaculture [137]. Fermentation (the process by which complex substances are broken down into simpler substances by microorganisms) is an ancient, simple, and inexpensive method to process plant raw materials (among others), reduce antinutritional components, and increase digestibility. In the literature, duckweed has been reported to contain antimicrobial active substances that inhibit the growth of microorganisms [138]. However, [PERSON] and colleagues (2021) found that both _Lenna minor_ and _Spirodela polyrhiza_ duckweeds are suitable substrates for the propagation of _B. subtilis_ KATMIRA1933 and _B. amyloliquefaciens_ B-1895. Both bacilli grew on duckweed and reached the 9.48-11.31 Log CFU/g cell concentration after 48 h of growth. However, the variety and origin of the duckweed are also important, as it was observed that the best growth was found with _Lenna minor_, followed by _Spirodela polyrhiza_, while the commercial duckweed product was a less favourable substrate [139]. The fermentation process can also cause significant changes in certain cases and components. When fermentation was carried out with five different lactic acid bacteria, significantly lower moisture contents and lower crude fibre contents were measured in the fermented duckweeds compared to the non-fermented one, but with no significant differences in protein content. There were also no significant differences in free amino acid contents. However, a much lower (61%) plytic acid content was detectedin fermented than in unfermented duckweed [140]. Meanwhile, [PERSON] and co-workers (2002) [124] reported that the fibre content of leaf meal was reduced from 11.0% to 7.5%, and the antinutritional factors tannin and phytic acid were reduced from 1.0% to 0.02% and from 1.23% to 0.09%, respectively, after fermentation, when _Lemma polyrhiza_ was fermented by a cellulose-degrading fish-gut _Bacillus_ sp. bacterium. Similarly to the findings of [PERSON] and co-workers (2014), fermented duckweed showed a decrease in crude fibre, but in this case, a decrease in crude protein content and a significant increase in free amino acid contents were also observed [140]. [PERSON] and colleagues (2011) fermented various aquatic macrophytes with a mixed culture containing _Lactobacillus plantarum_ strains DSM 8862 and DSM 8866 and found that, while fibre and ash were slightly reduced in the fermented product, the protein content increased slightly [141]. However, there was a significant decrease in antinutritional components (trypsin inhibitors, phytates, tannins) compared to the unfermented original sample. Not only bacteria but also fungi can grow on duckweed and cause significant changes in its composition. Fermentation using _Trichoderma harzianum_ and _Saccharomyces cerevisiae_ has significant effects (\(p<0.05\)) on the dry matter, ash, and crude fat in duckweed. The fermentation process increased the contents of minerals, and the content of crude fat in fermented _Lemma_ meal increased by up to 5.76% after fermentation. However, the crude fibre content in the fermented duckweed reduced by 15.27% [141]. In case of a mixed fermentation, _Lemma minor_ fermentation was performed by the probiotic microorganisms _Lactobacillus casei_ and _Saccharomyces cerevisiae_. It was found that the proportion of protein increased significantly after fermentation, while crude fibre and ash decreased slightly. The results showed that the fermentation process could improve the protein quality of the duckweed meal; however, the bacteria could be an important source of high-quality protein [124]. It is considered that duckweed is a highly nutritious plant due to its high protein content compared to other aquatic weeds; however, the presence of antinutritional factors limits the direct use of the weed as a dietary ingredient.