Microalgae represent a promising alternative as biofertilizers and biostimulants, providing essential nutrients and bioactive compounds that support plant growth. In this study, a screening of seven microalgal species—including Arthrospira platensis, Nannochloris sp., Chlorella sp., Chlorella vulgaris, Acutodesmus obliquus, Parachlorella kessleri, Coelastrella vacuolata—and one isolated mixed culture was conducted to evaluate their potential as biostimulants and biofertilizers under autotrophic cultivation conditions. Whole cultures and corresponding supernatants were directly applied, without any pretreatment, reducing potential processing costs. Their biostimulant activity was evaluated through multiple bioassays, including germination index and auxin- and cytokinin-like responses, while nitrogen, phosphorus, and potassium content was analyzed to assess biofertilizer potential. The results revealed that biostimulant effects were strongly influenced by species, concentration, and sample fraction. Chlorella species consistently showed high performance across assays, combining strong germination and rooting responses with high nitrogen content (8.2–8.8% w/w), while A. platensis and Nannochloris sp. showed inhibitory effects in many cases. Overall, under the cultivation and application conditions tested, C. vulgaris, mixed culture, and A. obliquus are identified as promising candidates for combined biostimulant and biofertilizer applications. This study is a primary step in identifying the most promising species as an alternative to synthetic fertilizers, enabling further optimization towards more sustainable agricultural practices.