Results from this study revealed that gasoline emissions lacking oxygen, low in hydrocarbons but high in carbon monoxide and carbon dioxide, negatively influenced growth of R. stolonifer and F. oxysporum although there was an initial stimulation of spore germination in R. stolonifer. Generally however, spore germination and germ tube lengths of R. stolonifer decreased with exposure. The greatest inhibitory effect of emissions observed at the highest exposure periods for the two fungi show the harmful effect of the emissions with prolonged exposure. This might have been due to the high concentration of carbon monoxide and carbon dioxide coupled with the complete absence of oxygen. R. stolonifer was reported to grow significantly at 0% oxygen but high concentrations of CO2 suppressed mycelium growth of the fungus (Wells and Uota 1969). On the contrary in another report, the spores of R. stolonifer did not germinate when molecular oxygen was rigorously excluded (Bussel et al. 1969). The initial stimulation observed in this study, may be due to the low levels of inhibitory oxides of carbon which the mold fungus was exposed to at the initial phase. The observed increase in levels of oxides of carbon with time of running the power generator supports this explanation. In addition, R. stolonifer being a faster-growing fungus, may have initiated germination in the first few minutes of exposure before the first test at 5 min such that no negative effect was observed at that period of exposure. Germ tube emergence occurred in F. oxysporum from 4 h when grown on potato dextrose agar (Kumari et al. 1975), while for R. stolonifer , modeling experiments showed that evidences of germination were expected from 30 s at 25 °C in water (Gabler et al. 2004). Longer exposures however had effect as the emitted gas constituents may have altered the chemical composition of the medium after dissolving in water to various extents.
Anaerobic environment, created by the absence of oxygen and elevated CO2 and CO levels may have led to anoxia within the fungal cells causing the yield of less metabolic energy (Atwell et al. 2015) and energy is needed for growth to take place. This likely inhibited fungal growth measured as spore germination and germ tube extension. Therefore, it was not unexpected that increase in CO and CO2 concentrations in the atmosphere inhibited growth of the two filamentous fungi tested. The oxides of sulfur and nitrogen which are also reported to be present in gasoline emissions (Tavares et al. 2010) though not detected by the equipment used in this study might have also formed acidic solutions in water causing direct and indirect inhibition of growth. Pure cultures of Cladosporium cladosporioides (Fres.) De Vries and Coniothyrium olivaceum Bonord were inhibited by sulfur dioxide (SO2) concentrations of < 0.053 μl/l (Wookey et al. 1991). Concentration dependence of fungal pathogen response to SO2 revealed that high concentrations were suppressive, while low concentrations caused greater severity of fungal plant diseases (Khan and Khan 2011). There are inconsistent reports on the influence of nitrogen oxides on fungal growth (Depayras et al. 2018; Orr and Nelson 2018; Strohm et al. 2019).
The survival of the two fungi at such long exposure to gasoline emissions is indicative of their tolerance to the unfavorable environment. Despite exposure to emissions, the organisms may therefore still be active in growth for long periods. Their pathogenic abilities in vivo therefore need to be investigated. The oxides of carbon dissolve in water forming weak carbonic acid. The carbonic acid together with other acids formed by emitted oxides of nitrogen and sulfur in solution probably also contributed to inhibition of fungal growth measured as hyphal extension. This is because the acids formed must have lowered the pH of the medium. The varied effects of pH on growth of F. oxysporum have been reported (Gordon et al. 2019).
The hydrocarbons in gasoline detected in the emissions most likely also played a role in negatively influencing growth of fungi. The observed decrease in hydrocarbons level with length of running the power generator coinciding with the much higher level of growth in Fusarium than Rhizopus at the higher exposure periods signify that Fusarium thrived better at reduced hydrocarbons concentration even when the concentrations of carbon oxides increased. Aromatic hydrocarbons from marine gas oil (MGO) inhibited growth of soil fungi by stopping hyphal extension (Hughes et al. 2007). In this study, R. stolonifer however seemed more affected by CO2, CO, and hydrocarbon levels of emissions. The consistent reduction in germination of R. stolonifer spores compared with fluctuations in that of F. oxysporum are evidences of the relative resistances of the fungi, with R. stolonifer appearing less susceptible to the negative effects of the emissions than F. oxysporum. The fact that the most effective inhibition of germ tube elongation of R. stolonifer occurred from 25 min exposure when hydrocarbon content had decreased confirms that all gasses in the emissions had more serious effect on the fungus, though the hydrocarbons might have exerted more effect. Fungi from gasoline polluted soils in liquid cultures showed favorable growth on volatile aromatic hydrocarbons in a combination of low pH and low water activity using the hydrocarbons as their sole carbon and energy sources (Prenafeta-Boldu et al. 2001). The source of carbon in the present study was solely from the emissions which also lowered the pH of the liquid in which the fungi were suspended. It is probable that R. stolonifer and F. oxysporum employed the emissions for their carbon supply. More fungi isolated from air filters exposed to hydrocarbon-polluted gas streams assimilated volatile aromatic hydrocarbons for their carbon and energy requirements (Prenafeta-Boldu et al. 2006).
For F. oxysporum spores, the fluctuating germination percentage with length of exposure after an initial stimulation whereby two peaks of decreased height were observed however, demonstrate the ability of the fungus to adjust to unfavorable environment. Progressive decrease in microbial growth of Fusarium sp. occurred on PDA with increase in concentration of crude oil (Nwadinigwe and Obinwa 2006). Gasoline, itself from crude oil, probably played a role in negatively influencing growth of fungi in the present study. The initial reduction in germination and germ tube length of F. oxysporum suggest that the organism being slower growing than R. stolonifer was from the onset more affected by the unfavorable environment created by gasoline emissions. Thereafter, F. oxysporum physiologically adjusted and adapted, even though growth was inhibited in both fungi. F. oxysporum adjusted better to the unfavorable environment in its vegetative growth, so the fungus may survive longer than R. stolonifer especially as hyphal fragments, when gasoline emissions pollute the environment. This was evidenced by the longer germ tubes of F. oxysporum even at high exposure periods.