plants Article Deep-Sea Actinobacteria Mitigate Salinity Stress in Tomato Seedlings and Their Biosafety Testing Pharada Rangseekaew 1,2 , Adoración Barros-Rodríguez 3 , Wasu Pathom-aree 4, * and Maximino Manzanera 3   Citation: Rangseekaew, P.; Barros-Rodríguez, A.; Pathom-aree, W.; Manzanera, M. Deep-Sea Actinobacteria Mitigate Salinity Stress in Tomato Seedlings and Their Biosafety Testing. Plants 2021, 10, 1687. https://doi.org/10.3390/ plants10081687 Academic Editors: Chrystalla Antoniou, Raffaella Maria Balestrini and Vasileios Fotopoulos Received: 1 July 2021 Accepted: 11 August 2021 Published: 17 August 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Doctor of Philosophy Program in Applied Microbiology (International Program) in Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand; Pharada_ra@cmu.ac.th 2 Graduate School, Chiang Mai University, Chiang Mai 50200, Thailand 3 Department of Microbiology, Institute for Water Research, University of Granada, 18071 Granada, Spain; dorysbr@correo.ugr.es (A.B.-R.); manzanera@ugr.es (M.M.) 4 Research Center of Microbial Diversity and Sustainable Utilization, Department of Biology, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand * Correspondence: wasu.p@cmu.ac.th; Tel.: +66-53943346-48 Abstract: Soil salinity is an enormous problem affecting global agricultural productivity. Deep-sea actinobacteria are interesting due to their salt tolerance mechanisms. In the present study, we aim to determine the ability of deep-sea Dermacoccus (D. barathri MT2.1 T and D. profundi MT2.2 T ) to promote tomato seedlings under 150 mM NaCl compared with the terrestrial strain D. nishinomiyaen- sis DSM20448 T . All strains exhibit in vitro plant growth-promoting traits of indole-3-acetic acid production, phosphate solubilization, and siderophore production. Tomato seedlings inoculated with D. barathri MT2.1 T showed higher growth parameters (shoot and root length, dry weight, and chlorophyll content) than non-inoculated tomato and the terrestrial strain under 150 mM NaCl. In addition, hydrogen peroxide (H 2 O 2 ) in leaves of tomatoes inoculated with deep-sea Dermacoccus was lower than the control seedlings. This observation suggested that deep-sea Dermacoccus mitigated salt stress by reducing oxidative stress caused by hydrogen peroxide. D. barathri MT2.1 T showed no harmful effects on Caenorhabditis elegans, Daphnia magna, Eisenia foetida, and Escherichia coli MC4100 in biosafety tests. This evidence suggests that D. barathri MT2.1 T would be safe for use in the environ- ment. Our results highlight the potential of deep-sea Dermacoccus as a plant growth promoter for tomatoes under salinity stress. Keywords: biosafety; Dermacoccus; marine actinobacteria; plant growth promotion; salt stress; sustainable agriculture 1. Introduction Marine ecosystems cover more than 70% of the surface of the Earth, with most parts still under-explored [1]. The deep-sea is still poorly investigated in terms of microbial diver- sity due to the difficulty in obtaining the samples. Nevertheless, actinobacteria are widely distributed in deep-sea environments [2–4]. These marine actinobacteria are considered an excellent source of bioactive compounds. The adaptation of deep-sea microorgan- isms is interesting from an academic perspective and for their potential applications in biotechnology. The deep-sea environment is generally characterized by high pressure, low temperature, lack of light with varying concentrations of oxygen and salinity [2,5]. Deep-sea actinobacteria have to adapt or develop unique abilities to withstand salinity and pressure [6] or piezotolerant properties [7] to survive under these extreme conditions. With the adaptive ability to survive under extreme environments, especially salinity and osmotic stress, these deep-sea actinobacteria are likely to be useful as biostimulants to mitigate salinity stress for plant growth. Salinity stress is considered a major problem causing a massive reduction in agricul- tural production worldwide. The total area of global cultivated land is 1.5 × 10 9 hectares; Plants 2021, 10, 1687. https://doi.org/10.3390/plants10081687 https://www.mdpi.com/journal/plants