## 4 Discussion
The use of flue gas or fume as a CO\({}_{2}\) or a nutrient source may thus lead to contamination of the resulting biomass. Hence, careful consideration of production processes and analysis of the resulting biomass are necessary before it is used as a feed ingredient. In general, commercial laboratories analyze feed samples for four major heavy metals, namely lead (Pb), cadmium (Cd), arsenic (As) and mercury (Hg). Both inorganic and organic forms of As are present in foods, and the major source for dietary As exposure is marine food products. Luckily, in marine food products As is primarily present as organic compounds [171], which are less toxic than the inorganic toxic ones, such as arsenobetaine (AsB), arsenosugars and arsenolipids [172]. Further data on As content in marine species are warranted to improve dietary exposure and risk assessment. Selenium (Se) has been suggested to provide protective effects against Hg toxicity by decreasing the bioavailability of Hg / mercury. Studies have showed that the Se: Hg molar ratio \(\geq\)1 may offer protection against Hg toxicity [173]. The allowed content or levels, based on the European Union [174] food safety regulations on undesirable substances in products intended for animal feed, are as follows on a dry weight basis (relative to feed with a moisture content of 12%): Total As: <10 mg kg\({}^{-1}\) in calcareous marine algae and <2 mg kg\({}^{-1}\) in general, Pb: <15 mg kg\({}^{-1}\) in calcareous marine algae and <10 mg kg\({}^{-1}\) in general, Cd: <1 mg kg\({}^{-1}\) and Hg: <0.1 mg kg\({}^{-1}\). The analysis of our diatom fermented with and without fume demonstrated that the content of these heavy metals was within the safety limits and comparable within the natural variation of marine food products  
Finally, persistent organic pollutants (POPs) such as dioxins and PCBs have high chemical stability and are known to accumulate in the environment, animals and food chains. As mentioned, microalgae may sequester certain POPs from their growth environment and even remove, break down or metabolize such pollutants. However, due to the non-polar hydrophobic nature of PCBs and non-dioxin-like PCBs, the concentrations of these POPs are extremely low in seawater without lipid-containing organic matter. Our results, although this is just a single screening, indicate that the levels of polychlorinated dibenzodioxins, dibenzofurans and biphenyls were below the limits required by the European Union food safety regulations on undesirable substances in products intended for animal feed. The allowed content of dioxins (sum of PCDDs and PCDFs) is 1.25 ng WHO-PCDD/F-TEQ kg\({}^{-1}\) and the sum of dioxins and dioxin-like PCBs (sum of PCDDs, PCDFs and PCBs) is 4.5 ng WHO-PCDD/F-PCB-TEQ/kg, based on the previously referred to EFSA [174] directives for undesirable substances in animal feed. It is well established that marine diatoms can produce a wide array of secondary metabolites and many of these compounds might have useful applications that can increase the value of the biomass produced in the large-scale bioreactors [33]. In our bioactivity screening of extracts of the biomass, we have focused on antibacterial, cytotoxic, anti biofilm, anti-inflammatory and anti-oxidative activities. Some of the activities that we have characterized could be of value in certain applications of the diatom biomass itself: when using compressed algal cells for fish feed applications, the presence of anti-oxidative compounds will help preserve the quality of the feed during storage whereas the presence of antibacterial compounds will reduce the risk of microbial growth. Such aspect is also useful during the cultivation itself. The presence of compounds that inhibits not only bacterial growth but also biofilm formation and subsequently biofouling in the photobioreactors is contributing to keeping the mass cultures healthy and the photobioreactors clean over prolonged periods of time. This is essential for the success of continuous cultivation of diatoms to produce larger amounts of biomass.