Microalgae are photosynthetic microorganisms found mainly in freshwater and marine environments (Colling Klein et al. 2018; Fulbright et al. 2018). They are known for their easy adaption to extreme conditions, high photosynthetic efficiency, and biomass nutritional properties (Huang et al. 2014; Ma et al. 2016). Neochloris oleoabundans is a very interesting microalga for sustainable oil production, biopolymers, and wastewater treatment due to its high growth rate and culture conditions (Gouveia et al. 2009; Santos et al. 2012; Peng et al. 2015; Jaeger et al. 2018).
For large-scale production of N. oleoabundans, open systems are the most likely strategy due to the possibility of producing larger quantities of biomass that can be used in different types of industries with lower production costs. However, open systems are susceptible to microbial contamination which causes. There is little information on the influence of biological pollutants on algae cultures; therefore, it is vital to understand their behavior to control them. Competition, parasitism, and predation are some of the interactions that can affect the development of cultures. Biomass production can be reduced up to 10 times because of competition of nutrients, while direct predation can reduce cultures by up to 90% in 2–5 days (Wang et al. 2013; Ramanan et al. 2016; Molina et al. 2019). Though, there are different types of bacteria in the natural microalgae phycosphere (the environment surrounding microalgae); bacterial cell concentration is often determined as “total bacteria,” without distinction of species and genera and can reach cell counts between 1x106 to 1x109 cells.mL− 1 (Fulbright et al. 2018; Di Caprio 2020).
Currently, several physical and chemical technologies have been developed to control the presence of different predatory organisms. Nevertheless, the efficiency of these methods depends on several factors, including detection stage, type of contaminant, and culture systems (Moreno-Garrido and Cañavate 2000; Wang et al. 2018; Deore et al. 2020). Chemical treatments can inhibit or kill biological pollutants in microalgae cultures. Pesticides were firstly used to reduce zooplankton. Antimicrobial agents, such as antibiotics, sodium hypochlorite, sulfonamides, potassium tellurite, and different types of detergents (e.g., tween, sodium lauryl sulfate) have also been used (Wang et al. 2013; Vaz et al. 2014; Lam et al. 2018; Vu et al. 2018). Even though the addition of chemical compounds can pose as a possible solution, the use of these substances can be problematic due to the production of toxic by-products during the process and affect the growth of the target microalga (Scholz 2014; Van Haute et al. 2015; Day et al. 2017; Molina-Aulestia et al. 2021).
For this reason, the use of active compounds derived from natural sources is important and necessary. Essential oils such as clove or peppermint oils have known antimicrobial activity. These oils are rich in hydrophobic terpenes and terpenoids that affect the permeability of cells, showing activity against a wide variety of microorganisms such as fungi and bacteria. Terpenoids such as eugenol, thymol, and carvacrol can cause coagulation of cell contents and, consequently, cell damage (Bakkali et al. 2008; Cox-Georgian et al. 2019; Hanif et al. 2019).
In previous studies, the toxicity of six terpenes (eugenol, linalool, limonene, alpha, and beta-pinene, and trans- cinnamaldehyde) on the microalga N. oleoabundans UTEX 1185 were evaluated (Molina-Aulestia et al. 2021). Currently, techniques such as MIC (minimal inhibitory concentration) make it possible to determine the minimum concentration at which a substance inhibits the growth of microorganisms (Moreira et al. 2005; Parvekar et al. 2020). This paper assesses the microbial diversity and identification of contaminants from cultures of the microalga N. oleoabundans UTEX 1185, and the antimicrobial activities of six essential oils.