Abstract
Glyphosate is a widely used herbicide that can potentially be a phosphorus (P) source for phytoplankton and microbes when discharged into the coastal ocean. In contrast to bacteria, few eukaryotic phytoplankton species appear capable of directly utilizing glyphosate. In this study, we observed, after a long delay (>60 days), Prorocentrum donghaiense, a dinoflagellate known to cause major harmful algal blooms in the East China Sea, could grow in a medium with glyphosate as the sole P source; suggesting that P. donghaiense growth was through bacterial mediation. To understand how the bacteria community might respond to glyphosate, we analyzed the 16S rRNA genes of the microbial community present in P. donghaiense cultures when grown under lower (36 μM) and higher (360 μM) glyphosate concentrations. Based on both Sanger and Illumina high throughput sequencing, we obtained more than 55,323 good-quality sequences, which were classified into six phyla. As the concentration of glyphosate rose, our results showed a significant increase in the phyla Proteobacteria and Firmicutes and a decrease in the phylum Bacteroidetes. Further qPCR (Quantitative PCR) analysis showed higher abundances of two specific phylotypes in the higher-glyphosate P. donghaiense cultures when compared to the lower-glyphosate and no-glyphosate cultures. Correspondingly, qPCR displayed the same trend for the abundance of a gammaproteobacterial type of phnJ, a gene encoding Alpha-D-ribose 1-methylphosphonate 5-phosphate C-P lyase, which is responsible for phosphonate degradation. In addition, Tax4Fun analysis based on our 16S rRNA gene sequences results in higher predicted abundances of phosphonate metabolizing genes in glyphosate-treated cultures. This study demonstrates that glyphosate could selectively promote the growth of particular groups of bacteria within an algal culture and in glyphosate enriched coastal waters, this interaction may potentially further facilitate the growth of alga.
Highlights
Glyphosate [N-(phosphonomethyl) glycine] is a broad spectrum, post-emergent, herbicide widely used in agriculture and silviculture to inhibit the growth of grasses
We found that P. donghaiense was unable to directly utilize glyphosate as a sole P-source; after a long delay, P. donghaiense was able to grow on glyphosate
DIP concentrations in the f/2 cultures decreased steadily whilst those in the f/2−P and f/2−P+Gly treatments were low to begin with and further dropped to below the detection limit from day 4 (Figure 1B). These results indicated that P. donghaiense could not hydrolyze glyphosate to obtain phosphate
Summary
Glyphosate [N-(phosphonomethyl) glycine] is a broad spectrum, post-emergent, herbicide widely used in agriculture and silviculture to inhibit the growth of grasses. It works through the inhibition of 5-enolpyruvyl shikimic acid-3-phosphate (EPSP) synthase and disrupts aromatic amino acid biosynthesis (Duke, 2003). Glyphosate infiltrated into the soil is subjected to degradation by the microbial community; pulses of coastal water contamination can be expected when rainfall occurs directly after application and when flood events increase river sediment load (Giesy et al, 2000). Considerably higher concentrations have been detected under direct aquatic application and in isolated wetland environments (Giesy et al, 2000; Battaglin et al, 2009)
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