1. Introduction

Marine fungi (MF) have been recently recognized as a diverse group and an excellent source of natural products. Obligate MF grow and sporulate exclusively in sea water and their spores are capable of germinating in sea water. On the other hand, facultative MF have undergone physiological adaptations to live either in fresh water or terrestrial environment that allow them to grow and possibly also sporulate in the marine environment ^[1]. MF grow in stressful habitats, such as under cold, lightless and high-pressure conditions or in association with other organisms, sometimes even in oil-spill polluted site. In order to survive in different environmental conditions, they produce many interesting secondary metabolites, with relevant bioactivities, making them attractive in different application fields ^[2]. 

The presence of crude oil in the environment for millions of years have led to the evolution of many microorganisms able to activate their metabolism to use it as a major or sole source of carbon and energy^[3]. One way is to increase the bioavailability of hydrocarbons through the production of biosurfactants (BS) ^[4]. Biosurfactants are generally referred as low molecular weight microbial products, composed of sugars, amino acids, fatty acids and functional groups such as carboxylic acid. They are known to lower the surface and interfacial tension between different phases, possessing a low critical micelle concentration (CMC) and allowing the formation of stable emulsions. While proteins, lipoproteins and hetero/lipopolyssacharides are best classified as Bioemulsifiers (BE), since they can efficiently emulsify immiscible liquids even at low concentrations, thus possessing a good emulsifying activity, but are less effective at surface tension reduction, hence they show a low surface activity ^[5].

Two groups of small, secreted cysteine-rich proteins can act indeed as good biosurfactants, hydrophobins (HFB) and Cerato-platanins (CP).

Hydrophobins (HFBs), self-assembling proteins typical of filamentous fungi, are described as the most powerful surface-active proteins. HFBs are small (about 100 amino acids) amphiphilic proteins that play multiple biological roles in fungal biology, lowering the surface tension of the liquid medium in their soluble form and coating aerial structures such as hyphae, fruiting bodies and spores for their easy growth and dispersal in the air, for fungal adhesion to surfaces and host-pathogen interactions ^[6] [7].

2. Cerato-platanins (CP)

Cerato-platanins (CP) are another class of fungal proteins known as surface-active biomolecules ^[8]. They are small (about 120 amino acids) cysteine-containing proteins (two disulfide bonds), released into the culture filtrate, but also found in the cell wall of fungal hyphae and spores. CP proteins constitute a well-conserved family, with a 70% similarity at some conserved motifs ^[9]. Similar to HFBs, their solutions lead to strong foam formation and they self-assemble at hydrophobic:hydrophilic interfaces into ordered, amphipathic layers ^[10] [11]. However, they are not HFB-like proteins, and some studies indicated that their behavior is opposite to that of HFBs, i.e., it has been reported that the CP EPL1 from Trichoderma atroviride increased the polarity of solutions and surfaces ^[12]. Moreover, structural analysis of CP from Ceratocystis platani revealed that there are significant structural differences between CPs and HFBs, i.e., the surface of the molecule shows no large hydrophobic patch  ^[10]. On the other hand, the CP structure (a double ψβ-barrel fold with six β-strands and two α-helices) is homologous to the N-terminal domain of expansins, which are proteins associated with carbohydrate binding and loosening of the cellulose scaffolds in plant cell walls ^[13]. Expansins are mainly found in plants, where they have various roles in growth and developmental processes, beyond the cell wall-loosening activity. Their action was shown for the first time as a weakening activity on filter paper ^[14]. Similar to expansins, CPs are able to weaken cellulose substrate, disrupting its non-covalent bonds without any hydrolytic activity. This action could be useful to efficiently hydrolyze cellulosic substrates; in fact, cellulases must have complete access to the cellulose chains that are tightly packed^[15]. However, gene knockout experiments have not yet permitted the identification of a clear biological function of CPs, and to date, it is not easy to answer the question of why fungi produce these proteins. CPs can act both as virulence factor and as elicitors. In some plant pathogenic fungi, CPs have been reported to act as phytotoxins inducing cell necrosis. On the other hand, beneficial fungi of the genus Trichoderma were shown to induce plant defense responses, activity known as eliciting activity ^[16]. CPs, secreted by marine fungi grown on oil as sole carbon source, show very good performance in reducing surface tension and in stabilizing emulsions, thus possessing both surfactant and emulsifying activities. The presence and abundant expression of CPs in fungi with all types of lifestyle suggests that their biological functions are related to general aspects of fungal growth.