PGPB and Microalgae for Restoration of Degraded Soil: Comparison
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Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Lina M. González-González.

Plant-growth-promoting bacteria (PGPB) are bacterial strains isolated from diverse environments with the potential to positively influence the growth and yield of diverse plants, mostly of agricultural importance. Microalgae (including cyanobacteria) and PGPB can be used as promoters of soil recovery.

  • microalgae
  • cyanobacteria
  • plant growth-promoting bacteria
  • degraded soils

1. Introduction

The ability to meet the continuous increase in food demands as a consequence of population growth is one of the biggest challenges of this century. Climate change and freshwater limitations are realities that we must consider when developing sustainable agricultural systems [1]. The current increase in food demands has put severe pressure on soil resources, resulting in significant areas of degraded soil worldwide due to intensive and poor agricultural land management [2,3][2][3]. Similarly, industrial activities, modern agricultural practices, improper waste disposal, and accidental spills of hazardous substances result in soil contamination [4,5][4][5]. Moreover, climate change alters fire regimes, significantly affecting soils and ecosystems [6]. Sadly, more than 33% of global land is degraded, and this percentage will grow if no actions are taken to prevent and reverse the degradation [7].
One of the first steps in tackling the increasing food demands is restoring degraded soils (i.e., degraded farmlands, contaminated soils, and post-fire ecosystems). The use of organic amendments, such as microalgae, especially cyanobacteria, is an increasing field of study that aims to restore soil health and fertility [6,8,9][6][8][9]. These microorganisms have the capability to restore soil structure and aggregate stability by releasing exopolysaccharides and forming soil aggregates, providing O2 to the subsurface, solubilizing and mobilizing macro- and micronutrients, mineralizing simpler organics, and serving as a source of organic matter and nutrients [10,11][10][11]. Additionally, the fertilizer potential of these microorganisms is well documented. Microalgae, including cyanobacteria, contain some plant-growth-promoting substances, such as phytohormones (auxins, cytokinins, abscisic acid, ethylene, and gibberellins), amino acids, vitamins, polyamines, betaines, protein hydrolysates, and polysaccharides, which can be used as biostimulants [12]. Furthermore, microalgae extracts have a recognized potential to improve soil physical and biological properties by acting as organic slow-release fertilizers that can return nutrients (carbon and macro-elements) and ensure the efficient use of resources.

Despite their proven fertilizer and bioremediation potentials, the capacity of these microorganisms to restore degraded soils needs to be better explored. Most studies have focused on the positive effects of microalgae and PGPB on plant growth, and all aspects regarding soil health and fertility still need to be assessed. Moreover, the use of microalgae and bacteria consortia for soil restorations has been poorly studied, even though their synergistic interaction can significantly boost their positive impact on soil fertility. Microalgae and bacteria exhibit positive interactions through substrate exchange, cell-to-cell communication via small signaling molecules, and horizontal gene transfer, conferring adaptive advantages to environmental stressors [23,24][13][14].

2. Use of PGPB and Microalgae for Restoration of Degraded Soil

PGPB are beneficial in harsh and limiting environments because of their role in alleviating stress in plants, making them excellent candidates to assist revegetation of eroded zones. For instance, PGPB can help plants tolerate drought stress by improving their water and nutrient uptake [49][15]. There are several examples of soil restoration with plants inoculated with PGPB; a severely eroded land in the southern Sonoran Desert was restored using native leguminous trees and the giant cardon cactus inoculated with two PGPB (Azospirillum brasilense and Bacillus pumilus), native arbuscular mycorrhizal fungi, and small quantities of compost [16,50][16][17]. Over a decade later, highly eroded land, destroyed for 25 years with almost no topsoil and extremely low mineral quantities to support plant growth, was successfully recovered. Likewise, the outdoor nursery cultivation of mesquite tree transplants was evaluated as a way to restore arid zones [51][18]. The study showed that inoculating the seedlings with PGPB—A. brasilense immobilized in dry alginate microbeads—resulted in the enhancement of all growth parameters of the plants, including biomass, aerial volume, root system, and chlorophyll pigments. Ramachandran and Radhapriya [52][19] explored a similar approach in a highly degraded forest in the Nanmangalam Reserve Forest in the Eastern Ghats of India. The authors planted 12 native tree species inoculated with a consortium of five native types of PGPB, small amounts of compost, and chemical fertilizer. The results of an experiment that lasted almost three years revealed that the PGPB consortium enhanced plant biomass in all the native plants and improved soil quality in the degraded forest. Schoebitz et al. [53][20] evaluated the combined effect of A. brasilense, Pantoea dispersa, and an organic olive residue immobilized in clay in the revegetation of semiarid land. The study revealed that PGPB improved soil properties by increasing phosphorus and potassium content availability by up to 100% and 70%, respectively. The inoculant also increased the total carbon and microbial biomass carbon content and enzyme activities, such as dehydrogenase, urease, and protease.
High soil salinity is another undesirable feature that reduces soil fertility [54][21]. However, salt-tolerant PGPB can significantly enhance salt tolerance in plants through several mechanisms, such as the adjustment of osmosis, protection from free radicals, the excretion of phytohormones that enhance growth parameters, and the release of extracellular polymeric substances (EPSs) that bind with Na+ cations, decreasing its bioavailability for plant uptake [55,56,57][22][23][24]. For instance, the PGPB Bacillus pumilus strain JPVS11, improved the growth performance of rice (Oryza sativa L.), which was negatively impacted by high soil salinity [58][25]. The study also revealed a significant improvement in soil enzyme activities of up to 56%, 46%, 48%, and 56% in alkaline phosphatase, acid phosphatase, urease, and β-glucosidase, respectively. Likewise, Hafez et al. [59][26] evaluated the potential of PGPB—Azospirillum brasilense—to restore saline–sodic soils. Following the inoculation of the strain with eco-friendly organic wastes for 150 days, the authors reported that soil fertility was enhanced with increases in soil organic carbon, dehydrogenase, urease enzymes, micronutrients (Fe, Zn, Mn, Cu, and B), and macronutrients (N, P, and K).
The soil restoration potential of microalgae, especially cyanobacteria, has been studied far more extensively because of the protagonist-like role they play in biological soil crust or biocrust. Biocrust corresponds to a cohesive and thin horizontal ground cover composed of photosynthetic organisms, such as lichens, bryophytes, and microalgae, and their associated bacteria, archaea, and fungi, which are of uttermost importance in stabilizing the soil against erosion [6,60][6][27]. Cyanobacteria, i.e., the first colonizers, stabilize the topmost layers and facilitate the formation of the soil crust with other microalgae groups and bacteria [61][28]. This is particularly important in arid or semiarid lands, desertified soils, and soils affected by fire, where cyanobacteria can be a suitable soil amendment that increases nutrient availability and promotes plant growth [61][28]. Additionally, microalgae act as biostimulants, affecting soil biological activity by enhancing enzymatic activity [62][29].
Wang et al. [63][30] reported the suitability of an artificial consortium composed of the cyanobacterial species Microcoleus vaginatus and Scytonema javanicum to recover the biological soil crust of degraded soil in a desert area in Inner Mongolia. After cyanobacterial inoculation, the authors reported a significant increase in total nitrogen, organic carbon, total salt, calcium carbonate, and electrical conductivity. The inoculation of this consortium with the plant Salix mongolica was later evaluated by Lan et al. [64][31]. Cyanobacteria inoculation quickly formed a biocrust and gradually gave rise to the moss crust, helping vascular plants to regenerate. A similar study reported on the inoculation of a cyanobacterial consortium with the species Anabaena doliolum, Cylindrospermum sphaerica, and Nostoc calcicole in a semiarid clay–loam soil, improved carbon and nitrogen mineralization, increased water-holding capacity, and enhanced hydraulic conductivity. Additionally, in response to the cyanobacterial biofertilizer, pear millet and wheat crops showed an increase in their growth and yield [65][32]. Another artificial consortium co-formed by the filamentous cyanobacteria Microcoleus vaginatus, Phormidium tenue, Scytonema javanicum, Nostoc spp., and the chlorophycea Desmococcus olivaceus efficiently assisted in the stabilization of fine sands, helping to control erosion in aeolian sandy soil in the south-eastern region of the Tenger Desert [66][33]. Similarly, Issa et al. [67][34] evaluated the effect of the cyanobacteria Nostoc spp. on the structural stability of poorly aggregated tropical soil from the Eastern Cape Province of South Africa. Cyanobacterial inoculation increased the resistance of soil aggregates to break down, enhancing soil stability two to four times over the control after six weeks of inoculation. Another study reported the potential of the acid-tolerant microalgae species Desmodesmus spp. and Heterochlorella spp., alone, or in combination, to improve soil health and fertility. The inoculation of strains in two acid soils (Kurosol and Podosol) collected from Queensland, Australia, resulted in the development of algal soil crust. Additionally, the authors reported an increase in the release of exopolysaccharides (more than 200%) which facilitate soil stability, an increase in carbon content (up to a 57%), an increase in dehydrogenase activity (more than 500%), and an increased production of indolacetic acid (between 200 and 500%) [68][35]. Furthermore, the algalization of acid soils with these species enhanced the richness of ecologically important soil bacteria, such as rhizobacteria and diazotrophs [69][36]. Muñoz-Rojas et al. [70][37] also evaluated the potential of a cyanobacteria consortium with Nostoc commune, Tolypothrix distorta, and Scytonema hyalinum to restore mine soil and reported that up to 40% of the soil surface was covered by biocrust after 90 days, as well as a significant increase in soil organic carbon and the promotion of C sequestration.
All of these studies have revealed that the use of PGPB and microalgae, particularly cyanobacteria, is an effective approach in restoring degraded soils, increasing soil fertility, stabilizing the soil against erosion, and promoting plant growth in arid and semiarid regions. Because the inoculation of PGPB and microalgae can be a sustainable and eco-friendly strategy for soil restoration programs, the use of these microorganisms in consortia has also been explored to further enhance the positive effects of these beneficial microorganisms.

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