Water samples were collected near the water surface at a distance of about 8 meters from the shoreline from three points of the Haql coast of the Red Sea, Saudi Arabia (Figure 1). Water quality was assessed using the mean values of three sampling data sets.

The plant samples were comprised of 1) Red algae (Rhodophyta); Gracilaria salicornia (G. salicornia), Digenea simplex (D. simplex) and 2) Green algae (Chlorophyta); Enteromorpha compressa (E. compressa); Sargassum muticum (S. muticum). These plant samples were collected from the same above-mentioned location from where water samples were collected. The collected plant samples were washed to remove surface adhered materials and stored in plastic bottles. Later, the algal samples were washed twice with double distilled water and used to analyze various attributes.

Collected water samples were used for the estimation of following quality parameters: turbidity was measured using nephelometer, and pH using portable pH meter. Total dissolved solids (TDS) were estimated using a conductivity meter. Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) were measured by the volumetric titration method (APHA, 1995) [15]. Total Kjeldahl nitrogen (TKN) was estimated by colorimetric method (EPA, 1978a) [16], ammonia

(NH₃) was estimated by Nessler method as described by Koch and McMeekin (1924) [17], nitrate ( NO 3 − ) was determined according to Rodger et al. (2017) [18] using cadmium reduction method. Fluoride was estimated by SPADNS method adapted from standard methods for the examination of water and wastewater (APHA, 1998) [19]. Total phosphorus (P) and total chlorine residual, and iron (Fe) were analyzed using standard methods as described in method 8048-Hach, 8167-Hach, and 8008-Hach, respectively (APHA, 1995) [15]. Oil and grease contents were quantified using gravimetric method 9070 (Blum and Taras, 1968) [20]. Total organic carbon (TOC) was assessed using EPA method 415.1 (EPA, 1999) [21], while phenols were estimated spectrophotometrically using EPA method 420.1 (EPA, 1978b) [22]. Concentration of following HMs was tested: Arsenic (As), Cadmium (Cd), Copper (Cu), Mercury (Hg), Lead (Pb), Selenium (Se), Barium (Ba), and Zinc (Zn). Concentration of various chemicals in water samples is mentioned in Tables 1-3.

A perusal of the data show that all the studied algal species showed a significant variation in photosynthetic pigments (Table 4). A comparison within the red algae shows that G. salicornia gave higher values for all the studied photosynthetic pigments (Chl a, b, total Chl, carotenoid, phycocyanin and phycoerythrin) except Chl a:b ratio which was higher in D. simplex. The studied green algae performed differently. E. compressa gave higher values for Chl a, b, and total Chl content, while lower values of Chl a:b ratio, carotenoid, phycocyanin and phycoerythrin were recorded as compared to the S. muticum. Overall, highest value of total Chl content was recorded in green alga E. compressa, whereas highest content of carotenoids, phycocyanin and phycoerythrin were found in red alga G. salicornia (Table 4). Photosynthetic pigments are of vital importance in plant life, these pigments are not only responsible for organic food production but also provide protection against high light intensity and also assist in light absorption and energy transfer to the reaction centre. As shown by the results green algae possess a higher concentration of Chl a, Chl b and total Chl, which resulted in enhanced accumulation of carbohydrates (Figure 2). However, the concentration of carotenoids, phycocyanin and phycoerythrin were found higher in red algae (Table 4). Our results corroborate the findings of Pereira et al. (2012) [32] and Nasir et al. (2015) [33] who observed that red strains of seaweeds possess more phycoerythrin than the green strain, they also observed variation in Chl a and phycocyanin. Similar results were also reported by Plastino et al. (2004) [34] and Yokoya et al. (2007) [35]. Seaweeds are continuously exposed to harmful effects of various climatic factors such as high light intensity, temperature, salinity,

heavy metal stress, pollution, turbidity etc. Protection of photosynthetic apparatus against high light exposure is of considerable importance for the survival of seaweeds and increased photosynthetic pigments in response to different light qualities have been reported by Borlongan et al. (2020) [36]. In addition, variation in these photosynthetic pigments in the studied plants was probably due to the quality of the water (Tables 1-3) because presence of heavy metals significantly reduces Chl content in seaweeds (Karimian et al., 2020 [37]; Cheng et al., 2016 [38]). Moreover, variation in the pigment concentration is a response to environmental alterations that allows an organism to adapt under a particular habitat. Higher values of the studied pigments in red algae show their better resistance capability to the changing marine environment.

Proteins and carbohydrates are integral part of any biological system. Carbohydrates provide structure and energy material to the cellular system, while proteins especially act as enzymes in various biochemical reactions. Proteins have been shown to play a significant role in the absorption of UV radiation. Increased anthropogenic activities cause nutrient enrichment of water bodies and induce turbidity which reduces penetration of solar irradiance, thus the nutrients particularly nitrogen and lower solar irradiance induce higher accumulation of proteins (Pereira et al., 2012 [32]). The results show that red algae accumulated more protein and carbohydrate (Figure 2) and thus provided more protection to this group against abiotic stresses. The enhanced increase in protein content was probably due to presence of N in the water (Table 2) which has been shown to induce protein synthesis in microalga Isochrysis galbana (Zarrinmehr et al., 2020 [39]).

Exposure of plants to various natural stresses as well as to uncontrolled anthropogenic activities, induces excessive generation of ROS which causes damage to biological membranes and adversely affects several plant physiological processes (Khan et al., 2020 [40]). In the present study, we analyzed H₂O₂ content and TBARS as stress markers of oxidative stress and lipid peroxidation. Marine plants face various abiotic stresses such as salinity, osmotic stress which induce oxidative stress by excessive generation of ROS. It is evident from Figure 3(A) and Figure 3(B) that red algae exhibited lower levels of H₂O₂ and TBRAS than green algae. However, within the red algal group G. salicornia showed lower values of these stress markers than the red alga D. simplex. While, in the green algal group, E. compressa exhibited lower level of H₂O₂ and TBRAS. The increased level of H₂O₂ and TBARS was probably due to the presence of oil, grease, heavy metals and various other chemical compounds in the water (Tables 1-3). Our results corroborate the findings of Rezayian et al., 2019 [41], and Cheng et al., 2016 [38]. Therefore, it can be postulated that red algae possess a higher capacity of scavenging ROS as shown by decreased concentration of the stress markers. The lower concentration of H₂O₂ and TBRAS in red algae might be due to higher activities of antioxidant enzymes (SOD, POX, and CAT) that scavenged the ROS and resulted in decreased H₂O₂ level. Effect of lower level of H₂O₂ content was also reflected by the lower peroxidation of lipids as shown by decreased value of TBARS (Figure 3(B)).

To cope with detrimental effects of oxidative stress plants are fitted with antioxidant system. The antioxidant enzyme SOD is known as the first line of defense against ROS that dismutase O 2 • − to O₂ and H₂O₂ (Hassan and Scandalios, 1990 [42]) whereas, CAT and POX convert H₂O₂ into O₂ and H₂O. Increased activities of SOD, POX, and CAT were observed in red algae (Figure 4(A) and Figure 4(B)). Therefore, lower level of H₂O₂ in red algae was due to increased activities of these antioxidant enzymes. Whereas in green algae, SOD activity was higher while POX and CAT were lower (Figure 4(A) and Figure 4(B)) that resulted in more accumulation of H₂O₂ than the red algae. Increase in the activities

of antioxidant enzymes in aquatic plants has been reported in response to various environmental stresses (Zou et al., 2014 [43]; Cheng et al., 2016 [38]; Rezayian et al., 2019 [41]). Moreover, the results show that red algae contain more protein that might have contributed to enhanced activities of antioxidant enzymes which ultimately resulted in lower levels of TBARS and H₂O₂ content. It indicates that red algae (G. salicornia, D. simplex) were more tolerant to various pollutants than the green algae (E. compressa, S. muticum).

Plants counter osmotic stress by accumulating osmolytes such as Pro and GB. The results of the study show that red algae synthesized more Pro and GB than the green algae and highest level of proline was found in red alga D. simplex, whereas lowest value was found in green alga E. compressa (Figure 5(A) and Figure 5(B)). The increased accumulation of sugars, and proline, prevents the damaging effects of stresses by maintaining hydration level, protecting enzyme activity and photosynthesis, and scavenging ROS (Ahanger et al., 2018 [44]; Khan et al., 2017 [45], 2020 [40]). Increased proline accumulation under the stress prevents oxidative stress and protects photosynthesis system, influences the functioning of key enzymes, and improves stress tolerance (Khan et al., 2020 [40]). Therefore, higher accumulation of proline in red algae particularly D. simplex, probably caused reduction in H₂O₂ content that resulted in improved photosynthetic pigments and carbohydrate and protein synthesis. It indicates that red algae possess more capacity to counter detrimental effects of excessive concentrations of heavy metals, nutrients, and other environmental stresses than the green algae.