Bioremediation efficiency in the removal of dissolved inorganic nutrients by the red seaweed, Porphyra yezoensis, cultivated in the open sea

Abstract

The bioremediation capability and efficiency of large-scale Porphyra cultivation in the removal of inorganic nitrogen and phosphorus from open sea area were studied. The study took place in 2002–2004, in a 300 ha nori farm along the Lusi coast, Qidong County, Jiangsu Province, China, where the valuable rhodophyte seaweed Porphyra yezoensis has been extensively cultivated. Nutrient concentrations were significantly reduced by the seaweed cultivation. During the non-cultivation period of P. yezoensis, the concentrations of NH₄-N, NO₂-N, NO₃-N and PO₄-P were 43–61, 1–3, 33–44 and 1–3 μmol L⁻¹, respectively. Within the Porphyra cultivation area, the average nutrient concentrations during the Porphyra cultivation season were 20.5, 1.1, 27.9 and 0.96 μmol L⁻¹ for NH₄-N, NO₂-N, NO₃-N and PO₄-P, respectively, significantly lower than in the non-cultivation season (p<0.05). Compared with the control area, Porphyra farming resulted in the reduction of NH₄-N, NO₂-N, NO₃-N and PO₄-P by 50–94%, 42–91%, 21–38% and 42–67%, respectively. Nitrogen and phosphorus contents in dry Porphyra thalli harvested from the Lusi coast averaged 6.3% and 1.0%, respectively. There were significant monthly variations in tissue nitrogen content (p<0.05) but not in tissue phosphorus content (p>0.05). The highest tissue nitrogen content, 7.65% in dry wt, was found in December and the lowest value, 4.85%, in dry wt, in April. The annual biomass production of P. yezoensis was about 800 kg dry wt ha⁻¹ at the Lusi Coast in 2003–2004. An average of 14708.5 kg of tissue nitrogen and 2373.5 kg of tissue phosphorus in P. yezoensis biomass were harvested annually from 300 ha of cultivation from Lusi coastal water. These results indicated that Porphyra efficiently removed excess nutrient from nearshore eutrophic coastal areas. Therefore, large-scale cultivation of P. yezoensis could alleviate eutrophication in coastal waters economically.

Introduction

Eutrophication is generally considered as the principal cause of the globally observed red tides and the deterioration of marine coastal environments (Schramm et al., 1996). Moreover, it has been accelerated by human activities in recent decades (Schramm, 1999; Capriulo et al., 2002). In China, coastal areas are the centers of population and industrialization. In recent years, fast growth in population and human activities, such as various agricultural practices, discharge of industrial wastewater, urban runoff, burning of fossil fuels and large-scale finfish and shrimp aquaculture, have resulted in the increase of nutrient inputs that are many times more than that generated by the natural processes (Victor et al., 2002). Today, coastal waters of China are the primary recipients of nutrients from land and finfish and/or shrimp aquaculture, and many areas exhibit typical symptoms of eutrophication (Xu and He, 2006). Rapid growth of intensive mariculture adds a continuous or pulsed release of nutrients, which contribute to coastal eutrophication (Troell et al., 2003; Neori et al., 2004). Increased nutrient inputs would generate an array of complex problems, including the excessive growth of harmful microalgae, which, in turn, may lead to more frequent occurrence of red tides (Fei, 2004).

Among different measures to control eutrophication, seaweed cultivation is receiving more and more attention, because of the low cost of cultivation and possible pollutant removal by the seaweed (Xu and He, 2006). The red seaweed Porphyra (Nori) is the most valuable maricultured seaweed in the world, worth approximately US$18,000/t dry biomass, with an annual market value of over US$ 1.3 billion (FAO, 2006). Porphyra grows fast and requires less than 40 days from seeding to the first harvest in a net culture. And it can be harvested repeatedly every 15 days (Sahoo and Yarish, 2005). Cultivated Porphyra grows well with higher nitrogen and other nutrients (Carmona et al., 2006). In Porphyra yezoensis cultivation, high yield and good quality were obtained in those regions where nitrogen concentrations (NO₃-N+NH₄-N) are over 100 mg m⁻³ (>7.2 mM) (S.E.P.E. and S.A.T.M., 1978; Fei, 2004). The high productivity, along with its higher capability (63–170% higher) to rapidly assimilate nitrogen and phosphorus nutrients than other seaweeds, makes Porphyra an excellent choice for eutrophication abatement via large-scale cultivation (Chopin and Yarish, 1998, Chopin and Yarish, 1999; Chopin et al., 1999; Neori et al., 2004). Moreover, Porphyra cultivation also provides a valuable product upon harvest (Cuomo et al., 1993).

Until now, however, most of the bioremediation studies have focused on the green seaweeds Ulva, the red seaweed Gracilaria (Anderson et al., 1999; Mariachiara and Pierluigi, 2002; Suzuki et al., 2005; Hernández et al., 2006; Yang et al., 2006) and the kelp, Laminaria (Chopin et al., 2003; Yang et al., 2004) for nutrient bioremediation. The cultivations of Ulva and Gracilaria are well established, but their market value is lower than that of Porphyra. It is highly desirable to develop the cultivation of the high-valued seaweed species, such as Porphyra, to reduce eutrophication and improve the quality of coastal waters.

Previous investigations have been carried out using cultured gametophytes of Porphyra in the laboratory as potential biofilters. They include P. amplissima, P. leucosticta, P. purpurea and P. umbilicalis from the east coast of North America; P. haitanensis, P. katadai, P. yezoensis and P. dentata in Asia; and P. dioca in Europe (Chung et al., 2002; Kraemer et al., 2004; Carmona et al., 2006; Pereira et al., 2006). The positive role of Porphyra in removing nitrogen and phosphorus at the sites of experimental nori/salmon integrated aquaculture has also been reported (Chopin and Yarish, 1999; Chopin et al., 1999; McVey et al., 2002). However, to our knowledge, there are few published studies in which cultivated Porphyra is used to remediate coastal water eutrophication in an open-water system, where changes in the water environment and nutrients are difficult to monitor and control.

The aim of the present study was to investigate the fluctuation of the levels of dissolved inorganic nutrients in seawater along the Lusi coast of China and the bioremediation potential of large-scale cultivation of Porphyra yezoensis to alleviate coastal eutrophication. Simultaneously, the biomass production and seasonal variations of the levels of tissue nitrogen and phosphorus of P. yezoensis are reported during this 20-month assessment.