## 4 Discussion
The cellular structure of _Spirulina_ and _Chlorella_, as well as the presence of fiber and other compounds, may affect the release and solubility of iron in the digestive tract, which is crucial for its absorption. Higher concentrations of algae may create a more complex matrix, potentially hindering iron availability. On the other hand, fermentation processes (as in the case of kefir) have the potential to modify food components and affect mineral bioavailability, but the specific effects of _Spirulina_ and _Chlorella_ in kefir require further study. In their work, [PERSON] et al. (2022) [47] draw attention to the significant variability in iron content in _Spirulina_ supplements, which can reach up to tenfold. They emphasize that the concentration of iron in the microalgae culture medium affects the fluctuations in the amount of this element in the obtained biomass. This fact has been well demonstrated in the work of [PERSON] et al. (2023) [49]; moreover, they showed that increased iron supply by Spirulina may result in a decrease in the content of other nutrients, e.g., lipids and phycocyanin. In turn, the research of [PERSON] et al. (2022) [50] showed that in _Spirulina_ supplements the predominant proportion of iron (82-92%) occurs in the form of Fe\({}^{3+}\) ions, and only a small amount is in the form of more absorbable Fe\({}^{2+}\) ions. This finding may explain the observed low bioavailability of iron from microalgae supplements. Our results suggest that a decrease in iron bioavailability at higher concentrations of algae in kefir may be related to the inhibitory effects of milk components and the potential properties of the algae matrix that impede iron release. Further studies are needed, specifically for kefir with the addition of these microalgae to better understand these interactions and potentially optimize iron's bioavailability. Our results indicate a very low iodine content in the microalgae material, despite the values declared by the seller on the packaging. The amount of iodine in algae strictly depends on the cultivation conditions, which indicates a low content of this element in the cultivation medium. Conclusion: The declarations on the label should not always be trusted; the content of bioactive ingredients should be verified in the context of this product segment, especially such specific ones as iodine. The introduced microalgae were probably grown under strictly controlled conditions, in bioreactors with a medium of a defined composition, and not in semi-open tanks with iodine-rich seawater. The discrepancy between the declared and analyzed iodine content of microalgae supplements may result from multiple factors beyond the composition of the growth medium. It is hypothesized that some technological process (e.g., high-temperature drying) or degradation of iodine over time--especially in the form of volatile iodine compounds or due to light and storage conditions--may also contribute to this variability in the final product. Our findings about vitamin B2 are in line with earlier reports confirming that Chlorella and Spirulina are rich in B-complex vitamins, including riboflavin [5; 51; 52]. [PERSON] et al. [52] report riboflavin contents of 3-6 mg/100 g in dried Chlorella biomass, slightly higher than values observed for Spirulina. However, bioavailability remains a limiting factor. Chlorella's rigid cell wall, composed largely of cellulose, impairs nutrient release during digestion unless cell disruption technologies (e.g., high-pressure homogenization or enzymatic lysis) are applied [3; 53]. Additionally, Spirulina tends to have more digestible cell walls but contains a lower intrinsic amount of B2, which may explain its lower efficacy at higher doses despite better digestibility. Moreover, previous studies have highlighted that while microalgae fortification may increase the nutritional value of fermented products, it also introduces structural complexity that can hinder nutrient release [54; 55].