Plant growth promoting potential of Arthrobacter spp., an early colonizer on the volcanic deposits of Mt. Merapi, Indonesia, as a restorative agent for degraded land

Exploration of novel and effective plant growth-promoting bacteria (PGPB) from unexplored environments is an intriguing option for restoring degraded land in Indonesia caused by mining activity. Due to their ability to develop in the absence of carbon, PGPB derived from volcanic environments may be a sustainable alternative. Previous research on the volcanic deposits of Mt. Merapi, Indonesia and Miyake-Jima, Japan, revealed that Arthrobacter spp. dominated the pioneer bacterial communities. Consequently, this research aimed to investigate the biochemical and physiological characteristics of volcanic Arthrobacter groups and assess their potential as PGPB. Using the conventional API system (20NE, 50CH, Zym), biochemical characteristics were evaluated. In vitro evaluations of plant growth-promoting activities such as siderophore production, phosphate solubilization, growth in media devoid of nitrogen, heavy metal resistance and weathering activity were conducted. The results showed broad substrate utilization and enzyme activity profiles of the Mt. Merapi isolates (BRU 37, BRU I.13, and BRU I.45). During 15 days of incubation on a minimal medium containing potassium feldspar, glucose, and NH4Cl, all of the isolates grew and produced acids. In nitrogen-free media, all of the isolates grew. These isolates showed positive reaction for the phosphate solubilization assay using Pikovskaya plates. After one day of incubation, all isolates produced yellow zones on the blue-green pigmented media (CAS-overlay media) and were therefore classified as potent siderophore producers. The heavy metal resistance assay showed that BRU37 grew in the heavy metal media. These results indicate the potential of selected indigenous Mt. Merapi Arthrobacter to enhance plant growth. Further characterization of plant growth-promoting traits is now in progress.