# The Potential of _Phaeodactylum_ as a Natural Source of Antioxidants for Fish Oil Stabilization
The question, therefore, arises whether photoautotrophic n-3 LC-PUFA rich microalgae could also serve as a source of natural antioxidants on top of being considered as an alternative/additional source of n-3 LC-PUFA. To the best of our knowledge, no former research has been performed on the potential of n-3 LC-PUFA rich microalgae for fish oil stabilization. Research on microalgae as a source of antioxidants in bulk oil has up to now been focused on vegetable oils. Moreover, apart from [PERSON] et al. [19] showing inhibition of soybean oil oxidation by the addition of _Nunnochloropsis_ biomass, all studies in this regard focused on non-n-3 LC-PUFA rich' species to which either microalgal biomass or a microalgal extract is added. Studies with microalgal biomass addition encompassed the addition of either _Chlorella_ or _Arthospira_ to olive oil, and all reported enhanced oxidative stability attributable to the gradual release of antioxidants [20; 21; 22; 23; 24]. Studies with microalgal extract addition were more diverse in terms of species (_Haematococcus_[25; 26], _Scenedesmus_[27], _Chlorella_[28], and _Anabaena_[6]), extraction method (solvent extraction with methanol, hexane, or acetone or supercritical fluid extraction with CO2) and vegetable oil (soybean, sunflower, olive, or others) but also reported an increase in oxidative stability explainable by the presence of carotenoids. Only [PERSON] et al. [28], investigating the addition of a _Chlorella_ supercritical CO2 extract to soybean oil, did not observe an antioxidative effect. Based on these studies, it is clear that microalgae show potential for the inhibition of bulk oil oxidation. The aim of this research was to investigate whether this also holds for fish oil as an n-3 LC-PUFA rich and therefore oxidatively more susceptible oil, using n-3 LC-PUFA rich microalgae known to be stable towards n-3 LC-PUFA oxidation. For this, _Phaodactylum_ sp. was selected. On a dry weight basis, this microalga contains around 20% of lipids [29], of which eicosapentaenoic acid (EPA) (an n-3 LC-PUFA), C16:0, C16:1, and C14:0 form the most important fatty acids [30]. It is also rich in carotenoids, with fucoxanthin being the major one next to diatoxanthin and \(\beta\)-carotene [18]. [PERSON] et al. [31] have already suggested the antioxidative capacities of _Phaodactylum_, although only based on fast antioxidant assays. The experimental set-up was twofold. In the first part, the antioxidative effect of supplementing fish oil with _Phaodactylum_ sp. biomass was investigated by analyzing both primary (PV by FOX) and secondary (volatiles by HS-SPME Arrow GC-MS) oxidation products throughout an accelerated storage experiment (3 weeks at 37 \({}^{\circ}\)C). In the second part, an elucidating approach was followed to obtain more insight into the required amount of microalgal antioxidants needed to inhibit n-3 LC-PUFA oxidation and to explain the results of the first experimental set-up. For this, a total lipid extract (chloroform-methanol 1:1) of the _Phaodactylum_ biomass was made and mixed with fish oil in different proportions to obtain six different systems (containing 100, 79, 49, 6, 1, and 0 weight% of the _Phaodactylum_ total lipid extract). Again, the oxidative stability was evaluated by analyzing primary and secondary oxidation products throughout an accelerated storage experiment (10 weeks at 37 \({}^{\circ}\)C). Other than the first experimental set-up, the second was not aimed at a practical application. Rather than investigating the potential of the addition of _Phaodactylum_ total lipid extract as an antioxidant, the objective was to clarify the n-3 LC-PUFA oxidation inhibition capacity of the antioxidative compounds present in this extract from a scientific point of view.