23rd International Colloquium Tribology - January 2022 397 On the role of microorganisms for lubricants - Sometimes good, sometimes bad Peter Lohmann Hermann Bantleon GmbH, Ulm, Germany Corresponding author: plohmann@bantleon.de Gerhard Gaule Hermann Bantleon GmbH, Ulm, Germany 1. Introduction Biodegradation is the breakdown of organic material by microorganisms e.g., bacteria and fungi. Lubricants usually are hydrocarbonic substances of organic origin. Accordingly, there are microorganisms having the capability of breaking down lubricants. On the one hand, we welcome microorganisms and their biological degradation processes, namely when it comes to the fact that untraceable or unreachable residues of oily leaks in the environment are to be eliminated. On the other hand, we curse microorganisms that undesirably drive the decomposition and degradation of important components in emulsions of metalworking machines and thus cause quality losses. 2. Microorganisms There is a huge variety of hydrocarbonoclastic microorganisms having the ability to biodegrade hydrocarbons. These ubiquitous microorganisms include bacteria, yeast, and fungi. There are freshwater bacteria, such as e.g., Pseudomonas and Acinetobacter. Others are adapted to a marine environment e.g., Alcanivorax [Fig. 1] and Marinobacter. Aromatic hydrocarbons are mainly degraded by marine Neptunomonas and Cycloclasticus species, while some methane-oxidizing and phototrophic bacteria are partially capable of oxidation of aliphatic hydrocarbons. The bacteria that dominate soils include for example Mycobacterium or Rhodococcus [Fig. 1]. Among the fungi, it is usually geobiontic representatives such as Aspergillus, Fusarium or Penicillium [Fig. 1], which can decompose hydrocarbons. Alkane-oxidizing yeasts can be found, for example especially in the genera Candida [Fig. 1] and Lodderomyces. 3. Mechanisms There are both oil-positive microorganisms that can enter directly into the oil phase and oil-negative microorganisms that remain suspended in the water phase and excrete emulsifying substances for an effective absorption of oil microdrops. Others are equipped with a waxy surface to attach to lipophilic substrates and to facilitate the assimilation into the cell [1]. The subsequent degradation process most frequently starts with a monoor diterminal [2] or, more rarely, subterminal [3] attack on the hydrocarbon by special enzymes. A chain of complex biochemical reactions is ending up in the final degradation via β-oxidation [4, 5, 6]. The corresponding result is that organically bound carbon is converted into carbon dioxide. To put it simple, one can summarize all these reactions as follows: Organic substance + Oxygen → Carbon dioxide + Water + (microbial biomass). Figure 1. Oil-degrading microorganisms. A Candida maltose, B Penicillium spec., C Alcanivorax borkumensis, D Rhodococcus ruber. 4. Conclusion N-alkanes and monoaromatic substances can be degraded relatively easy by microorganisms. In contrast, branched and aliphatic hydrocarbons are hard to degrade, sometimes only by microbial specialists. As a rule, biodegradation of lubricants is a concerted action of various microorganisms having different characteristics. 398 23rd International Colloquium Tribology - January 2022 On the role of microorganisms for lubricants - Sometimes good, sometimes bad References [1] Fuchs, G. 2006 Allgemeine Mikrobiologie. 8. Auflage. Georg Thieme Verlag, Stuttgart, Germany [2] Watkinson, R. J. and P. Morgan. 1991. Physiology of Aliphatic Hydrocarbon-Degrading Microorganisms. In Physiology of Biodegradative Microorganisms, ed. C. Ratledge, 79-92. Springer, Netherlands. [3] Forney, F. W. and A. J. Markovetz. 1970. Subterminal oxidation of aliphatic hydrocarbons. J Bacteriol 102(1): 281-282. [4] Coon, M. J. 2005. Omega oxygenases: nonheme-iron enzymes and P450 cytochromes. Biochem Biophys Res Commun 338(1): 378-385. [5] Krauel, H., R. Kunze, and H. Weide. 1973. Bildung von Dicarbonsäuren durch Candida guilliermondii, Stamm H 17, aus n-Alkanen. Z Allg Mikrobiol 13(1): 55-58. [6] Kester, A. and J. Foster. 1963. Diterminal oxidation of long-chain alkanes by bacteria. J Bacteriol 85(4), 859-869.