Abstract <p>Nickel ion (Ni²⁺) contamination has increased as a result of human activity, particularly in aquatic habitats. High concentrations of Ni²⁺ can have detrimental effects on the growth, development, and reproduction of aquatic organisms like algae. Since algae are the primary producers in aquatic environments, harmful effects on them may have a negative influence on people and other consumers in the food chain. In this study, two unicellular green algae, <i>Chlamydomonas</i> sp. GUEco1016 and <i>Chlorococcum</i> sp. GUEco1018, were isolated from their native habitat and their physiological processes were examined in relation to Ni²⁺. Four doses of Ni²⁺ (5, 10, 15, and 20 ppm) for <i>Chlamydomonas</i> sp. GUEco1016 and (15, 20, 25, and 30 ppm) for <i>Chlorococcum</i> sp. GUEco1018 were selected for both algae after the lethal concentration (LC₅₀) was established. Augmented Ni²⁺ exposure significantly showed reduced growth, pigment synthesis, protein, and carbohydrate levels in both algae. Furthermore, enhanced malondialdehyde (MDA) concentrations in <i>Chlamydomonas</i> sp. GUEco1016 and <i>Chlorococcum</i> sp. GUEco1018 was observed at 20 and 30 ppm, respectively, it indicates that Ni²⁺ induces oxidative stress. Similarly, both the algae displayed significant increase in the total phenol content (TPC) indicating toxicity of Ni²⁺. Both algae synthesised different quantities of ascorbic acid (AsA) depicting differential sensitivity to Ni²⁺ exposure. Likewise, morphological studies using light microscopy and SEM microphotographs also demonstrated significant effect of Ni²⁺ toxicity on algal cells. The results in the study suggests that <i>Chlorococcum</i> sp. GUEco1018 is more resilient to Ni²⁺ stress than <i>Chlamydomonas</i> sp. GUEco1016 and it is suggested as a better choice for Ni²⁺ mitigation in contaminated water.</p>