Microalgae play a crucial role in the Earth’s ecosystem in addition to being utilized by various industries as a raw material for production of valuable goods. This prompts the question of how their growth conditions can be optimized. This work focuses on one of the crucial nutrients they need in order to grow and live - bioavailable iron. However, in natural open water most of the iron remains in a less soluble form, such as nanoparticulate ferrihydrite. The effects this specific iron oxide has on growth of Chlorella vulgaris, a freshwater unicellular green alga, have been compared to those of other forms of iron. The topic of bioavailability has further been tested using different organic agents, which have been shown to facilitate and improve uptake, the focal point being leonardite humic acid (LHA). Growth of C. vulgaris has been analyzed using spectrophotometry and cell count, however, the differences between various forms of iron and organic agents have mostly been determined statistically insignificant. Physical and chemical properties of ferrihydrite and LHA (such as iron content, surface charge and particle size) have been examined using XPS, DLS and FTIR. Zeta potential measurement by DLS has showed that ferrihydrite particles are positively charged at biological pH range, as opposed to the negative charge on the surface of microalgae. ICP-OES analysis has shown that the binding between ferrihydrite and microalgae or LHA occurred within several hours. These analytical methods have allowed a better understanding of the interactions between microalgae, ferrihydrite and LHA in a solution. These interactions require further investigation, especially in cold-adapted species.