Microorganisms form exocellular structures, trophosomes, to facilitate biodegradation of oil in aqueous media

Abstract

Cytochemical staining and microscopy were used to study the trophic structures and cellular morphotypes that are produced during the colonization of oil-water interfaces by oil-degrading yeasts and bacteria. Among the microorganisms studied here, the yeasts (Schwanniomyces occidentalis, Torulopsis candida, Candida tropicalis, Candida lipolytica, Candida maltosa, Candida paralipolytica) and two representative bacteria (Rhodococcus sp. and Pseudomonas putida) produced exocellular structures composed of biopolymers during growth on petroleum hydrocarbons. Four of the yeasts including S. occidentalis, T. candida, C. tropicalis and C. maltosa excreted polymers through modified sites in their cell wall ('canals'), whereas C. lipolytica and C. paralipolytica and the two bacterial species secreted polymers over the entire cell surface. These polymers took the form of fibrils and films that clogged pores and cavities on the surfaces of the oil droplets. A three-dimensional reconstruction of the cavities using serial thin sections showed that the exopolymer films isolated the ambient aqueous medium together with microbial cells and oil to form both closed and open granules that contained pools of oxidative enzymes utilized for the degradation of the oil hydrocarbons. The formation of such granules, or 'trophosomes,' appears to be a fundamental process that facilitates the efficient degradation of oil in aqueous media.

Modifications of the cell wall of yeasts grown on hexadecane and under starvation conditions

“…Thus, it appears that the local modification of the cell wall during adaptation to growth on hexadecane results in the replacement of polysaccharides within the 'canals' by substances of a different nature. These are likely proteins, as shown in our earlier studies on the localization of oxidative enzymes (Dmitriev et al , ). A protein nature of 'canals' has also been confirmed here, using pronase treatment of hexadecane‐grown cells, in which both ultrathin sections and surface replicas showed that the substance of the 'canals' had been destroyed by pronase (Figure A, B). Under starvation conditions, the dynamics of quantitative changes in polysaccharides during the course of 'canal' formation was different.…”

“…This assumption was supported by our electron‐microscopic data on the localization of the product of a cytochemical reaction to oxidative enzymes in the 'canals'. This allowed us to assert that 'canals' are the sites of primary oxidation of hydrocarbons (Dmitriev et al , ).…”

“…In our earlier studies (Dmitriev et al , , , ), we reported local modifications in the cell walls of yeasts that are grown on hydrocarbons (hexadecane), and that were conventionally referred to as 'canals'. These 'canals' were demonstrated to be lipophilic, and thus to provide a channel for diffusive transport of hydrophobic substances.…”

“…These 'canals' were demonstrated to be lipophilic, and thus to provide a channel for diffusive transport of hydrophobic substances. Using electron‐microscopic cytochemistry and immunocytochemistry, we have since shown the presence of oxidative enzymes and cytochrome P 450 in the 'canals' and exocellular fibrillar components of yeasts that facilitate participation of 'canals' in the primary oxidation of hydrocarbons (Dmitriev et al , ). As yet the literature lacks any other vivid evidences of structural changes in the yeast cell wall in response to changes in the environment.…”

“…Here we report new observations in which we show that the 'canals' are also formed in the yeast cell wall under conditions of carbon starvation. If the participation of the 'canals' in the utilization of hydrocarbons by yeasts is not in doubt (Dmitriev et al , ), it is unclear why yeasts form 'canals' in the absence of a carbon source. To better understand this phenomenon, we have now carried out comparative cytobiochemical studies to localize glucan and mannan, major cell wall polysaccharides, in the cell walls of yeasts grown on glucose or hexadecane or subjected to carbon starvation, using a time course to follow the dynamic changes in the content of these polysaccharides over time.…”