## 1 Introduction  
Endophytes are microorganisms, mostly fungi and bacteria that inhabit the inner plant tissue; they can be found in all plants in nature [1]. They form a mutualistic relationship with the plant host which provides the endophyte with a living niche and food, while the endophyte provides tolerance to, and protection against biotic and abiotic stressors. Endophytes have been found to secrete specialized metabolites that contribute to their survival in hostile environments [2; 3], termed secondary metabolites. Secondary metabolites can be categorized into various chemical groups: alkaloids, flavonoids, phenolic acids, fatty acids, quinones, steroids, alcohols, tannins, terpenoids, ketones, xanthones, and many others [4]. Endophytes use their secondary metabolites as antifungals, antibacterial, antivirals and antioxidants [5; 6; 7], as part of their defense against competitors. As a side effect, those metabolites benefit the host plant by protecting them against pests and other stressors. Some of those secondary metabolites are also known for their pharmaceutical properties (i.e., as anticarcinogens, anti-inflammatory, antiarthritics, antidiabetics, and immunosuppressants [4; 8]). Endophytes comprise many phyla. For example, Actinobacteria--gram-positive bacteria, many of which are producers of medicinal antibiotics--can be found in terrestrial and aquatic environments [5; 9; 10; 11]. One such actinomycete is _K. flava_, first described by [PERSON] et al. (2008) as yellow, aerobic, non-motile and coccoid cells. It can be found in many environments, such as air [12], soil, plants [13; 14], roots [15], marine sediment [16], sponges [17] and algae [18; 19]. _K. flava_ is known for its versatile abilities, showing methanol catalytic activity, thermotolerance, antifouling activity, chromate tolerance, plant growth induction and more [15; 19; 20]. We previously isolated _K. flava_, isolate ABp5, as an endophyte from the marine macroalgal _Bryopsis plumosa_[18]. _B. plumosa_ is a seaweed of the phylum Chlorophyta. _Bryopsis_ sp. is known for their ability to attract, select, and maintain specific endophytes within their cells; they use these endophytes for wound repair and propagation in the face of biotic stresses [21]. In our previous study, we isolated seven different endophytes from _B. plumosa_, all of them were found to be biologically active against aquaculture pathogens. _K. flava_ isolate ABp5 was active against _S. parasitic_ in in vitro assays [18]. _S. parasitic_ is an oomycete (water mold) causing saprolegniasis, a disease affecting freshwater fish eggs and juvenile fish in hatcheries worldwide. In mature fish, it is characterized by visible white or gray patches of filamentous mycelium on the body or fins. _Saprolegnia_ infections were previously controlled with Malachite green, but since this compound was banned internationally in 2002, this oomycete has become a serious problem in the aquaculture industry, mainly of catfish (_Ictalurus_ spp. ), salmon (_Salmo_ spp. ), and trout species [22; 23]. Aquaculture is the fastest growing food industry. The increasing demand for healthy animal-sourced protein and a rapidly growing economy have resulted in an annual growth rate of 5.8% for the aquaculture industry since 2001 [24]. The salmon farming industry has grown exponentially in the last three decades, with Norway and Chile as the major suppliers. This industry holds a 4.5% of the world's aquaculture industry with fish yield of 2.5 million tons in 2018. It has become an integral part of the global aquaculture market due to its large economic share. It is estimated that 10% of all hatched salmon die due to _Saprolegnia_ infections, causing major financial losses for this industry [23; 25; 26]. In this study, we isolated and identified an active compound from the endophytic bacterium _K. flava_. The compound was tested for bioactivity against the aquaculture pathogen _S.