2.1. Antibacterial Activity and MIC Determination
Antibacterial activities expressed as inhibition zone diameters of algal aqueous extracts against the tested bacterial strains are shown in
Table 1. The antibacterial effects of
C. tomentosum aqueous extract were found to be superior to those of
A. fragilis extract in the experiment.
The minimum inhibitory concentration (MIC) is an essential factor that assesses microorganism resistance and sensitivity to specific substances. The MIC of
C. tomentosum and
A. fragilis against the bacterial strains is shown in
Table 2.
Seaweeds are potential renewable resources of bioactive compounds with diverse beneficial effects. Several studies reported that bioactive secondary metabolites isolated from various seaweed exhibit potential to be used as antimicrobial molecules. Nevertheless, species belonging to the genus
Codium have been the least investigated among all members of Chlorophyta for their biological activities and their possible use in food and biomedical applications. Aqueous extracts of
C. tomentosum and
A. fragilis were subjected to antibacterial assay against a wide array of bacterial pathogens. The selected bacteria are among the most common causes of foodborne and infectious diseases. They showed an extended spectrum of inhibitory activity against all the bacterial pathogens [
45].
In addition, there are reported studies that describe the antibacterial capability (derived from secondary and primary metabolites) of seaweeds against medically important pathogenic bacteria. As previously shown in many studies, marine macroalgae have antimicrobial components that inhibit the growth of some bacteria. It has also been reported that the efficacy of macroalgae extracts against microorganisms is mostly influenced by factors such as location and seasonality. Another study of macroalgae showed a high percentage of species with antimicrobial activity, 73% in the case of Chlorophyta (green algae), 69% in Rhodophyta (red algae) and 53% in Phaeophyceae (brown algae) [
46]. Rajauria et al. used aqueous phenolic extract to demonstrate that
Himanthalia elongata have antimicrobial activity against
Listeria monocytogenes ATCC 19115,
Salmonella abony NCTC 6017,
Enterococcus faecalis ATCC 7080 and
Pseudomonas aeruginosa ATCC 27853 [
47].
The findings of the present investigation are analogous to those observed by Elkhateeb et al. [
48], who reported that the crude extract of
C. tomentosum showed strong antimicrobial activity against two Gram-positive bacteria,
Staphylococcus aureus ATCC 6538 and
Bacillus subtilis ATCC 6633, as well as two Gram-negative bacteria,
Escherichia coli ATCC 19404 and
Vibrio alginolyticus MK 170250.
However, in other studies such as that of Koz et al. [
46], antibacterial activity of
Codium fragile extract against
E. aerogenes,
E. coli and
B. subtilis was observed. Antibacterial activity of
C. intricatum extract in opposition to
S. aureus and MRSA is similar to that reported from methanol extracts of
Codium species (
C. tomentosum,
C. tomentosum,
C. dichotomum and
C. fragile), where high inhibitory activity was noted [
45]. Pasdaran et al. [
10] reported the antibacterial activity of
A. fragilis volatile oil against
Pseudomonas aeruginosa,
E. coli and
Staphylococcus aureus. Salem et al. [
49] reported the antibacterial activity of
A. fragilis extracts against
E. coli,
S. aureus,
E. feacalis,
Salmonella sp.,
B. cereus and
P. aeruginosa. Alghazeer et al. [
50] reported that extracts from
C. tomentosum possess in vitro antibacterial activity against eight bacterial strains, namely
S. aureus,
B. subtilis,
Bacillus spp.,
S. epidermidis,
S. typhi,
E. coli,
P. aeruginosa and
klebsiella spp., similar to the findings of this study.
The sensitivity of a specific kind of bacteria to the activity of bioactive substances found in the algal extracts is attributed to the difference in structure and composition of the cell walls. Gram-positive bacteria are marked by dense peptidoglycan in the outer layer of the cell wall, while Gram-negative bacteria have a composite, multilayered cell wall structure that makes the entry of bioactive compounds more difficult [
51].
2.2. Antioxidant Activity
Phenolic compounds are key antioxidant and antibacterial agents with numerous benefits for disease prevention and human health. Flavonoids are natural substances with a polyphenolic structure; as a result, they have antibacterial and antioxidant activity and can help prevent illnesses including Alzheimer’s disease, cancer and atherosclerosis [
52]. Total phenolic and flavonoid contents of
Codium tomentosum and
Actinotrichia fragilis are shown in
Table 3.
Total phenolic contents of
C. tomentosum and
A. fragilis aqueous extract were 32.28 ± 1.63 mg/g and 19.96 ± 1.28 mg/g of extract, respectively, while TFCs were 4.54 ± 1.48 mg/g and 3.86 ± 1.02 mg/g of extract, respectively. TFC’s significance comes mostly from its redox characteristics that might account for its antibacterial and antioxidant activities against a variety of microorganisms and free radicals. Furthermore, the action of TFC as an antioxidant agent is closely related to the hydroxyl groups in its structures, which are responsible for scavenging lipid peroxy-radicals, singlet oxygen, superoxide anion and free radical stabilization [
53,
54,
55].
The DPPH is a significant metric for determining an extract’s antioxidant activity. The IC
50 is the extract concentration needed to scavenge 50% of the DPPH radicals. The IC
50 values of
C. tomentosum and
A. fragilis aqueous extracts are presented in
Table 4. The better the antioxidant properties, the smaller the IC
50 value. According to the results,
C. tomentosum aqueous extract demonstrates the highest antioxidant activity, with an IC
50 value of 75.32 ± 0.07 μg/mL. The IC
50 of L-ascorbic acid as a positive control was 22.71 ± 0.03 μg/mL.
Basically, antioxidants delay oxidation and reduce oxidative damage, which is a significant causative factor in the development of many chronic diseases. Oxidative stress is an important factor in the pathogenesis of various diseases such as atherosclerosis, cancer and aging. Naturally generated reactive oxygen species can attack cell components and then exert several types of biological damage and oxidative stress [
56,
57,
58]. Antioxidants protect against these reactions which occur in vital systems and increase shelf-life when added to lipids and lipid-containing foods [
46,
52]. Although synthetic antioxidants such as butylatedhydroxyanisole, butylatedhydroxytoluene and propyl gallate have been used for many years, they have started to be restricted in recent years because of their carcinogenicity. Thus, there is a gradual increase in the investigations to identify new natural antioxidants [
46,
52].
Seaweeds are considered to be important sources of antioxidants. The results from antioxidant activity screening in the extracts suggested that algae extracts have radical scavenging inhibitor activity. The findings of the present investigation are similar to those observed by Alghazeer et al. [
16], who reported that
C. tomentosum extracts possess antioxidant and antiproliferative activities which might be helpful in preventing or slowing the progress of various oxidative stress-related disorders. The findings of the present investigation are similar to those observed by Baskaran et al. [
59], who reported that methanol crude extracts of
A. fragilis contain different potential antioxidant compounds able to scavenge different types of free radicals.
Acanthophora specifera flavonoid separation reveals a combination of chlorogenic acid (69.64%), caffeic acid (12.86%), vitexin-rahmnose (12.35%), quercetin (1.41%) and catechol (0.59%) [
60]. The antioxidant activity of the flavonoid-enriched extract has been proven to be very high [
61]
Phenolic compounds have exceptional antioxidant properties due to their capacity to function as chelating agents with reactive oxygen species, avoiding oxidative stress and cell damage [
29,
62]. As a result, scavenging of oxidants is critical for disease prevention, and phenolic compounds found in seaweeds are particularly helpful as a natural supply of antioxidant agents [
52]. The study of Agregán et al. [
63] demonstrated that aqueous polyphenolic extracts of
Ascophyllum nodosum,
Bifurcaria bifurcata and
Fucus vesiculosus have antioxidant activity, which stabilize the canola oil oxidation level.
Nonetheless, synthetic ingredient limits in the food business may represent a tipping point for the use of seaweed compounds as safe replacements [
64], since they also exhibit anti-microbial activity against key food spoilage and food pathogenic microorganisms [
65]. Seaweed phenolic antioxidant extracts have been used to improve oxidative stability and to preserve or boost the intrinsic quality and nutritional content of foods [
66,
67]. The antioxidant potential is useful in the food industry not only for nutraceutical compounds on functional food products, where they are indisputably valuable for health improvement (as food supplements), but also to extend the shelf-life period when used in processed food (functional foods) [
68,
69]. Furthermore, the antibacterial activity of seaweed phenolics suggests that they can be valuable in the food industry [
70].
2.5. Sensory Evaluation of Fillet Fish Fortified with Algal Extracts
Regarding the bioactivity and biochemical potential of these two extracts, it is important to understand if they can be used as food additives and applied in the food industry [
52,
71].
Table 8 shows the sensory evaluation scores of fillet fish fortified with algal extracts. According to the findings, fillet fish fortified with algal extracts has been accepted as having high nutritional value for human diet. Interestingly, fillet fish fortified with
C. tomentosum extract demonstrated the greatest overall acceptance score, as well as the highest antibacterial and antioxidant activity.
Water extracts were used because they are safer solvents than other organic solvents. Water extracts contain a variety of constituents, including polysaccharides and polyphenols, but we focused on polyphenols in this study because they are powerful antioxidants and antibacterial agents, as reported previously by several studies. This important observation will be made in subsequent work.
In this preliminary work, we treated the fillet fish with aqueous extracts mixed with spices immediately before frying in oil. Only sensory evaluation was performed on the fillet fish fortified with algal extracts in an attempt to improve its nutritional value in comparison to our standard fillet fish diets.
Our future studies will focus on fortifying fillet fish or yogurt with natural seaweed extracts as natural preservatives instead of chemical ones. pH values, titratable acidities (TAs), total soluble solids, water holding capacity (WHC), thiobarbituric acid reactive substances (TBARS), lipid oxidation, protein oxidation (modified DNPH carbonyl assay) and microbiological properties will be evaluated over 30 days of storage.
The constraints on synthetic components in the food business may be a tipping moment for the use of seaweed compounds as safe replacements [
64], as they also exhibit antimicrobial properties against major food spoilage and pathogenic microbes [
65]. Seaweed phenolic antioxidant extracts have been used to improve oxidative stability and to preserve or boost the intrinsic quality and nutritional content of foods [
66,
67].
The antioxidant potential is beneficial in the food business not only as nutraceutical compounds in functional food items, where they are undeniably valuable for health improvement (as food supplements), but also to extend the shelf-life period when included in processed foods (functional foods) [
68,
69]. Furthermore, the antibacterial properties of seaweed phenolics suggest that they may be valuable in the food business [
70].
Furthermore, bromophenols enriched extract from
Ulva lactuca and
Pterocladiella capillacea were investigated as “marine flavour” agents in farmed fish and other aquatic organisms, because farming final products can differ in flavor from wild catches, and this can be incorporated as a feed ingredient or a seaweed bromophenol-enriched sauce [
72].