# Cyanobacterial Potential for Restoration of Loess Surfaces through Artificially Induced Biocrusts
## 4 Discussion
### Particle Trapping and Stabilization by Cyanobacteria
Strains that achieve higher biomass production under diazotrophic conditions should be preferred, as environmental conditions in arid areas may favour N\({}_{2}\)-fixing species. In this study, the _Nostoc_ sp. L6 strain achieved higher biomass production when cultivated in BG11 N-, than in BG11 N+ medium. However, the biomass yield was still lower than that of the _Tylypotthrix_ sp. L4 strain, although the yields were equal in the first week of cultivation, indicating a higher rate of biomass production by _Tylypotthrix_ sp. L4 strain. In addition, field experiments by [PERSON] et al. [116] showed that _Nostoc_ sp. strain needed much more biomass than _Microcleus vaginatus_, _Phormidium tenure_, or _Scytonema javanicum_ to stabilize unconsolidated sand, which was related to the morphology of the colony. _Microcleus_ strains were very often selected among other strains as they are often the first colonizers of bare soil. However, the cultivation of _Microcleus_ proved to be very slow in terms of biomass production [117], which may affect its application in restoration practice. In view of this all, efficiency in biomass production should be supported by other characteristics when selecting a strain for inoculation. Cyanobacterial EPSs have so far shown an undeniable role in particle/sediment stabilization. For this reason, EPS production is an essential feature of strains used in restoration practice. Cyanobacteria produce two different fractions of EPS in culture: hydrophobic CPS and hydrophilic, water-soluble RPS [66]. Considering sediment stabilization, the role of RPSs is still unproven. [PERSON] et al. [118] and [PERSON] et al. [119] showed that CPSs have a more significant role in particle fixation, while RPSs, hydrophilic in nature, potentially determine surface water repellency [118]. Therefore, RPSs could have an important role in preventing water loss and increasing water availability to biocrust cyanobacteria, which are considered the most important mechanisms for the biocrust' restoration [120]. In this study, the strain _Tylypotthrix_ sp. L4 had the lowest RPS yield under diazotrophic conditions, which could argue in favor of this strain to achieve better particle binding and express greater sediment stabilization. On the other hand, RPSs could play an important role in controlling the amount of water available [118], which is essential for the survival of cyanobacterial species in arid environments. In this study, the strain _Chroococcidis_ sp. L1 had the highest yield of both total polysaccharides and RPSs under diazotrophic conditions, which may be of interest for restoration studies in terms of water availability. The reasons that may have affected EPS production by this strain have been discussed above, but considering the biomass production, this strain would probably not be able to develop biocrusts after application in the field due to the high light and O\({}_{2}\) concentrations, unless combined with another strain that has the ability to fix N\({}_{2}\), e.g., _Tylypotthrix_ sp. L4 strain. Inoculation with the mixture of strains that produce different yields of polysaccharide fractions indicates efficiency in the biocrust formation and sediment stabilization, and should be investigated in the future. Although this study has demonstrated the potential of cyanobacteria for the restoration of loess sediments, their large-scale use requires a comprehensive approach to the selection and preparation of the cyanobacterial inoculum. Besides the importance of the physiological properties of the cyanobacterial strains intended for inoculation to the unconsolidated substrate [121], [PERSON] et al. [118] pointed out that the granulometry of the substrate plays an important role in the determination of the initial inoculum. They found that on the sand of medium grain size, compared to coarse sand, a smaller amount of the initial inoculum is necessary for the development of biocrusts with the same aggregate stability [118].