Sustainable Biostimulation in Chili Cultivation: Effects of PGPMs and Marine Algal Extracts on the Physiological Performance of Serrano Pepper Crop

(1) Background: The increasing soil and water pollution in agriculture is mainly due to the uncontrolled use of synthetic fertilizers. As the responsibility to adopt sustainable agricultural practices grows, biofertilizers may offer a solution to reduce the use of chemical inputs and improve crop productivity. This study focused on evaluating the physiological effects of Trichoderma asperellum , Bacillus sp., and seaweed extracts ( Ulva lactuca and Solieria spp.) on the cultivation of serrano pepper plants. (2) Methods: Five treatments were carried out: control (T1), T. asperellum (T2), Bacillus sp. (T3), seaweed extract (T4), and their combination (T5). The microbial inoculants were applied to the root zone, while the seaweed extracts were applied to the foliage. Leaf samples were collected at the end of the vegetative phase to evaluate physiological and agronomic traits. (3) Results: The application of T3 significantly increased leaf area (12.34%), biomass (11.91%), and yield (10.7%) while decreasing the SPAD, chlorophyll, and carotenoid contents. T4 resulted in the highest nitrate reductase activity, while T5 resulted in the peak total chlorophyll content. No significant differences were observed in nitrate reductase activity between T4 and the control or in the carotenoid content between T1, T2, T4, and T5. (4) Conclusions: Bacillus sp. demonstrated agronomic benefits despite a decrease in pigments, supporting its application in the sustainable production of peppers.