Arsenic accumulation by the aquatic fern Azolla: Comparison of arsenate uptake, speciation and efflux by A. caroliniana and A. filiculoides

Abstract

Arsenic accumulation and efflux differ between strains of the aquatic fern Azolla. This study investigates As accumulation and tolerance of the aquatic fern Azolla. Fifty strains of Azolla showed a large variation in As accumulation. The highest-and lowest-accumulating ferns among the 50 strains were chosen for further investigations. Azolla caroliniana accumulated two times more As than Azolla filiculoides owing to a higher influx velocity for arsenate. A. filiculoides was more resistant to external arsenate due to a lower uptake. Both strains showed a similar degree of tolerance to internal As. Arsenate and arsenite were the dominant As species in both Azolla strains, with methlyated As species accounting for <5% of the total As. A. filiculoides had a higher proportion of arsenite than A. caroliniana. Both strains effluxed more arsenate than arsenite, and the amount of As efflux was proportional to the amount of As accumulation. The potential of growing Azolla in paddy fields to reduce As transfer from soil and water to rice should be further evaluated. a r t i c l e i n f o

Arsenic accumulation and speciation in the submerged macrophyte Ceratophyllum demersum L.

“…Compared with other aquatic plants, such as Azolla caroliniana, Azolla filiculoides, and W. globosa, C. demersum has a much higher arsenate influx rate (about three-to tenfold) and a lower K m (about half to one tenth), indicating the higher affinity of transporters for arsenate. For the arsenite influx, the V max and K m values were comparable to those for W. globosa (Zhang et al 2008(Zhang et al , 2009). However, a maximum accumulation of As (963 μg g −1 DW) in plants exposed to 10 μM arsenite after 4 days was higher than that (862 μg g −1 DW) in plants exposed to 10 μM arsenate (P<0.05) (Fig.…”

“…Whereas, in this study, plants were preserved in liquid N 2 after sampling and extracted by PBS solution with EDTA in it to prevent arsenite oxidation (Bednar et al 2002), followed by immediate analysis of As speciation using HPLC-ICP-MS. Also, the extraction rate was much higher (about 60 %) in our study. The same phenomenon in which arsenite was the dominant species of As in C. demersum when supplied with inorganic As was also observed in other higher plants, such as rice (Oryza sativa), tomato (Lycopersicon esculentum), A. caroliniana, A. filiculoides, and W. globosa, which were also preserved in liquid N 2 before analysis of As speciation with immediate testing using HPLC-ICP-MS after extraction (Xu et al 2007;Zhang et al 2008Zhang et al , 2009). Arsenate added to the solution was rapidly converted to arsenite (Fig.…”

“…It accumulated As in 4 days, twice more than other aquatic plants in ≥7 days, such as Azolla, H. verticillata, and M. propinquum, all of which accumulated <400 μg As g −1 DW after exposure to >20-μM arsenate solutions (Robinson et al 2006;Srivastava et al 2007;Zhang et al 2008). Its accumulation of As was similar to that of W. globosa, which has recently been reported as a strong accumulator of As (1,057 and 1,070 μg As g −1 DW accumulated after 7 days of exposure to 15 μM arsenite and 30 μM arsenate, respectively).…”

“…In addition, the reduction of arsenate and complexation of arsenite by thiol peptides are considered to be important mechanisms of As detoxification Zhao et al 2009). Recent studies by Xu et al (2007) and Zhang et al (2008Zhang et al ( , 2009) also showed that rice and tomato roots as well as some species of duckweed rapidly reduce arsenate to arsenite and extrude arsenite to the external medium. However, little information is available on As accumulation and metabolism in submerged macrophytes.…”

“…However, little information is available on As accumulation and metabolism in submerged macrophytes. Studies on Azolla, Wolffia globosa, and the roots of rice and tomato suggest that As (V) in plant cells is rapidly reduced to As (III), followed by rapid efflux from the plant into external solutions (Xu et al 2007;Zhang et al 2008Zhang et al , 2009. If this is one of the detoxification mechanisms for As (V) in submerged macrophytes, the macrophytes themselves would become new sources of metal contamination to surrounding areas.…”