A cascade process was developed for the production of a micro-algae based plant biostimulant. This process is intended to assure a high recovery of the main components of plant biostimulant (phytohormones, betaines, oligosaccharides elicitors, micro-algal protein hydrolysates), without affecting yield of micro-algae lipids extraction. The main steps of the process are: micro-algae cell walls lysis by pressure homogenization and hydrolysis with mixture a lytic enzymes; separation of phytohormones, osmoprotectants, free amino acids and solubles carbohydrates by tangential ultrafiltration; extraction of lipids from ultrafiltration retentate; enzymatic hydrolysis of protein from defatted retentate; mixing protein hydrolisate with ultrafiltrate retentate, resulting the micro-algae based plant biostimulant. The composition of the resulted plant biostimulant dry matter is: 88.26% hydrolyzed algal protein, from which 5.63% is proline, 9.41% carbohydrates, 2.23 others compounds, including 0.023% total betaines and 0.012% cytokinins, kinetin activity equivalents. Micro-algae based biostimulant was tested on water stressed and non-water stressed tomato plants. In non-water stressed plants the biostimulant obtained from micro-algae determined a better development of root length (108.08% control), and an increased leaves number (120.31% control) and of leaves area (105.16% control), comparing to a commercial seaweed extract–105.98% control, 106.25% control, and, respectively, 104.45%. On leaves number and leaves area the differences are statistically significant, micro-algae based bio-stimulant being more active. These stimulatory effects of both algae based biostimulants compensate the negative influence of water stress. On water stressed tomato plant height and root length decrease by almost 20%(from an average of 58.45 cm on control, well watered and nottreated with biostimulants, to 48.87 cm on water stressed tomato plant and not treated with plant biostimulants) and, respectively, by more than 25%(from 58.82 cm average on control plants, to 39.85 cm on water stressed tomatoes). Application of algae based biostimulant alleviated the effects of water stress on plants root development and reduced by almost 50% the negative influences on plants height. Average production of treated tomato plants (expressed as production per 30 days cycle of flowering and fructification) is similar for both tested macro-and micro-algae based plant biostimulants. Mechanisms of action of the main components of algae plant biostimulants are discussed and further solutions for optimization of micro-algae based biostimulant are envisaged.