Comparative assessment of bioactive compounds, antioxidant capacity and nutritional quality of red seaweeds and water spinach

Abstract

Each food form is considered to have greater affinity if enriched with nutritional and bioactive compounds. This study was undertaken to evaluate the bioactive compounds, antioxidant properties, and proximate composition of two seaweeds i.e., Hypnea musciformis and Gelidium pusillum and a freshwater weed i.e., Ipomoea aquatica, in search of potential plant food sources with greater medicinal values. Bioactive compounds, e.g., total phenolic content (TPC), carotenoid, total flavonoids, ascorbic acid, tannin, and antioxidant activity by means of total antioxidant content (TAC), 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging activity, and Ferric reducing antioxidant power (FRAP) were measured. The proximate compositions (e.g., protein, carbohydrate, etc.) of these study samples were also determined. Ipomoea exhibited 36.19% less antioxidant activity than seaweed samples (ranging from 19 to 22 mg AAE/g). The samples TPC (mg GAE/g) followed the order Gelidium (48.37 ± 0.96) > Hypnea (11.37 ± 0.92) > Ipomoea (9.4 ± 0.36). The DPPH radical scavenging activity of I. aquatica was 4 times less than the seaweed. Nevertheless, freshwater weed’s total carotenoids content was one and a half times higher than seaweed samples. The amount of ascorbic acid, tannin, and FRAP were also found higher in seaweeds. However, flavonoid content was higher (8.09 ± 1.4 mg QE/g) in freshwater weed. However, proximate compositions were about 1.5 times higher in seaweed. The non-parametric statistical analysis showed a significant difference between Hypnea versus Ipomoea and Gelidium versus Ipomoea ($\alpha =0.05$; p<0.05) in each of the antioxidant and bioactive compound parameters. The study results suggested that red seaweeds are verily more compatible in contrast with freshwater weed for most of the parameters and could be used as a prospective source of human diets.

Introduction

In recent years, researchers are looking for cheaper sources of natural antioxidants and bioactive compounds. Several previous studies were accompanied to study the phytochemicals and antioxidant activity of natural sources like fruits, vegetables, plants, etc. Holdt and Kraan (2011) and Zzaman et al. (2021). However, constituents of phytochemicals in plants, especially in green leafy vegetables, raised interest among researchers, consumers, producers, and all relevant stakeholders. Among various Bangladeshi freshwater green leafy vegetables, water spinach is one of the most abundant but it is underused- people may not understand its value in terms of nutritional quality and antioxidant properties, which originates from China for the first time (Gupta et al., 2005). On the other hand, seaweeds have been used in Japan and China since the fourth and sixth centuries, respectively, as an essential diet. Besides, Asian people eat seaweed, and it is widespread in Korea (Gates, 2010). In contrast, Bangladeshi people have used seaweed slightly due to insufficient knowledge about its qualities. The nutritional value, taste consistency, colour and texture support seaweed as a healthy diet. Some edible seaweeds contain essential proteins, lipids, minerals, vitamins and possess rich antioxidant properties (Norziah and Ching, 2000). Seaweed nutrient content varies by species, geographic location, season, moisture, and temperature (Kaehler and Kennish, 1996). Seaweed has potential antibacterial, fungal, antioxidant and anti-viral activity (Venugopal, 2008). Compared with other marine resources, seaweed can play a critical role in many ways. Better attention should be given to marine algae since marine algae may contribute to future global food security for the ever-growing (Moomaw, 2017). Red seaweeds (under the Rhodophyta group) possess high protein levels, which was selected in this study (Cian et al., 2015). Consequently, this work focuses on assessing the nutritional properties of two seaweed species, i.e., Gelidium and Hypnea, accessible in almost every country on the coast. Bangladesh’s east coast, in particular, is filled with seaweed.

Water spinach is a major part of any healthy diet in green freshwater vegetables and is a good source of nutrients, particularly for more impoverished families due to low prices and abundance (Fu et al., 2011). Ipomoea is a freshwater weed typically found in wet monsoon (summer) to dry season (winter). It can also grow without any intensive care enriched with bioactive components (likely carotenoids, phenolic content, etc.) and act as a degenerative disease barrier (Gates, 2010). I. aquatica is a sizeable aquatic crop in Southern Asia, India, and southern China. It is an enduring watery herb in the Convolvulaceae group (Umar et al., 2007a). Its drifting character has a trailing vine throughout the aquatic environment that can have a maximum length of 3 cm and no need to be attached with soil. Also, leaves and stems can grow to 15 cm tall (Zhang et al., 2014). In this analysis, the proximate composition and antioxidant properties of Ipomoea were selected as the freshwater weed to be contrasted with the seaweeds. Green vegetables are the key source of nutrition in most developing countries, with food scarcity and famine. Despite the nutritional and medicinal value of these plants, very limited studies were conducted.

Free radicals play a major role in the human body, causing disease by increasing oxidation. Nevertheless, the use of oxygen in the human body is varied by the individual; it may cause the condition to become imbalanced and produces reactive oxygen species (ROS), which lead to carcinogenicity (Gunathilake and Ranaweera, 2016). Naturally generated ROS could be naturalized by the virgin antioxidant that came into our body through the ingestion of various green vegetables, seaweeds, fruits, etc. Bhattacharjee and Islam (2014). Synthetic antioxidants may inject the human body to treat many diseases, whereas natural antioxidants are often preferred (Franco et al., 2019, Preedy and Patel, 2020). Although all bioactive compounds can play a vital role against ROS, carotenoid has significant impacts; it may function to reduce cancer, cataracts, and other degenerative diseases (Perera and Yen, 2007).

Carotenoids are essentially plant pigments important for colouring in various fruits and vegetables, but only responsible for yellow, orange, and red. Besides this, carotenoids can also serve as a source of vitamin A found in fruits and vegetables. Moreover, $\beta$ carotene is also a form of carotenoid that can function against ROS; generally, several degenerative diseases (Perera and Yen, 2007). Carotenoid, however, has contributed to the antiphotoxidant process, namely this antioxidant called 1,1-diphenyl-2-picrylhydrazyl (DPPH). DPPH stops RBC (red blood cell) from being killed and fat oxidized in the body (World Health et al, 2011). Thus the free radical scavenging activity was assessed in samples to determine whether the sample has sufficient antioxidant potential or not (Ajila et al., 2007). Total antioxidants, however, can delay and inhibit rancidity, slow the growth of oxidative products, and increase shelf-life. Total antioxidant capacity (TAC) is developed in the field of chemistry but has been applied in the area of biology and medicine. So, here we introduced to calculate the amount of TAC in both samples, i.e., water spinach (Ipomoea) and seaweed (Hypnea, Gelidium), and clearly showed the difference between two vegetables of water bodies. Ferric acid Reducing Antioxidant Power assay (FRAP) is an essential tool to measure the antioxidant activity of several polyphenols, vitamin C, Vit-E, uric acid, etc. Several studies were conducted to assess the effects of flavonoids. They found astonishing benefits, especially in reducing inflammatory reaction on heart valves, thereby declining the risks of infective endocarditis and rheumatic fever, among the major CVS (cardiovascular system) threats in our society (Rees et al., 2018). Polyphenols, which are consumed mostly in the form of flavonoids, have certainly reduced the risk of developing prostate cancer, arguably the second most prevalent neoplasm present in the elderly male population (El-Din and El-Ahwany, 2016). Tannic acid, well-documented antimicrobial biomedicine, is consumed by a larger share of our community by drinking tea (Chung et al., 1998). However, freshwater weed and seaweed could be potential sources of this essential agent (Tannin) prescribed by this study. An exogenously synthesized water-soluble vitamin is also present in seaweed (Nielsen et al., 2021). Given that the human body cannot generate ascorbic acid (vitamin C) (Marcus, 2013), this plant source can greatly value our health. In Bangladesh, the consumption of seaweed in meals supplement is rare compared to those of freshwater species due to the assumption that marine species contain less nutrition and unsafe to consume. Owing to that, we aim to provide data on the nutritional values of seaweed and compared them to the commonly available freshwater weed. Two red seaweed species that are abundant, i.e., Hypnea musciformis and Gelidium pusillum, have the most significant value (nutritional value, medicinal value, industrial value) compared to other species chosen in this study. This research thus aims to analyse the bioactive compounds, antioxidant activity, and nutritional composition of seaweeds and freshwater weed; and figure out the best one among these three species.