## 4 Discussion
The chemical nature of the products from hydrocarbon degradation was not analyzed in this study. The composition of crude oil includes several thousands of different hydrocarbons that could potentially represent a carbon source for _Alternaria_ sp. The products resulting from microbial degradation by _Alternaria_ are also a complex mixture of polar compounds hard to resolve by conventional analytical chemistry tools. However, hydrocarbon degradation products' chemical natures have been extensively studied [65; 66]. Different enzymestransform saturated and aromatic hydrocarbons, increasing the relative abundance of polar fractions. Various pathways, such as terminal oxidation, subterminal oxidation, \(\omega\)-oxidation, and \(\beta\)-oxidation, are involved in the degradation of aliphatic hydrocarbons. The initial alkane oxidation results in the formation of an alcohol, which can then be metabolized by the \(\beta\)-oxidation pathway of fatty acids. On the other hand, fungi's initial oxidation of aromatic hydrocarbons is mediated by monooxygenases forming a trans-diol. The benzene ring is then cleaved in different ways by appropriate enzymes leading to the formation of central intermediates, which are further converted to tricarboxylic acid (TCA) cycle intermediates. Finally, cyclic alkanes are converted to cyclic alcohols and dehydrogenated to ketones by an oxidase system. Then, a monooxygenase system forms lactonates, and a lactone hydrolase finally opens the ring. Our study proved that _Alternaria_ sp. has a great affinity for the aromatic fraction of EHCO, showing an extensive transformation of 91.3% of this fraction. The aromatic fraction that was transformed by _Alternaria_ sp. to more polar compounds migrated to the resin-asphaltene fraction in the degraded hydrocarbons, as suggested by the percentage increase of this fraction. Despite this increase, it has been previously well-established that the polar compounds appearing in the resin-asphaltene fraction as a result of the biodegradation of saturates and, especially, of the aromatic fraction, are more biodegradable than the original hydrocarbons, and they can be more accessible for degradation by other microorganisms [67; 68]. The station N2 (Peraldo region), from where _Alternaria_ sp. was isolated, did not display any remarkable difference in salinity, temperature, or dissolved oxygen concentration compared with the rest of the stations (Supplementary Table S2). The Gulf of Mexico has a two-layer system; the upper layer (above 800-1000 m) is highly regulated by the Loop Current and its associated eddies (LCEs) entering the Gulf of Mexico through the Yucatan Channel [69]. The river discharges, the temperature variation, and the freshwater input do not impact the salinity or temperature beyond the upper 100 m of the water column [70]. Consequently, the main driver in the hydrographic conditions in the GoM where the sediments samples were taken was the local presence of LCEs [70]. Regardless of the wide distribution of the sites sampled in the Gulf of Mexico, it is not possible to recognize a difference between the Perdido and Coatzacoolcos Stations. Temperature and dissolved oxygen variation respond mainly to the depth of the sediments, and there is an insignificant variation in the salinity in all the stations. Recent estimations report that unconventional oil reserves, which include HCO, EHCO, and bitumen, could exceed six trillion barrels [71]. The ineludible extraction of these reservoirs will inevitably produce oil spills. It is essential to explore native fungal microorganisms capable of degrading HCO and, especially, EHCO to mitigate the environmental impact of accidental petroleum spills. Offshore oil spills are of tremendous concern due to their potential impact on the economy and especially on ecological systems [72]. The successful bioremediation of marine oil spills has been reported [73].