Exploring changes in bacterioplankton community structure in response to tannic acid, a major component of mangrove litterfall of Sundarbans mangrove ecosystem: a laboratory mesocosm approach

Tannic acid is a secondary compound produced by vascular plants and is a major component of mangrove litterfall. Tannic acid is water soluble, leaches out from mangrove litterfall and contributes to DOC and DON pools in adjacent estuaries. About 50% of the litterfall may be degraded and channelized into the marine microbial loop. The influence of tannic acid on bacterioplankton community structure was tested by setting up laboratory-based barrel experiments. Estuarine water from Stn3 of Sundarbans Biological Observatory Time Series (SBOTS) was enriched with tannic acid, and the change in concentration of dissolved nutrients was determined on a daily basis over a span of 15 days. Concentrations of tannic acid, gallic acid and other dissolved nutrients such as nitrate and ortho-phosphate were determined using a UV-Vis spectrophotometer. Tannic acid significantly affected the concentrations of gallic acid and dissolved nitrate in the barrels. Degradation of tannic acid was tracked by a decrease in concentration of tannic acid and generation of gallic acid. The influence of tannic acid on bacterioplankton community structure was analysed on the start (day 0), intermediate (day 3, day 5, day 7 and day 9) and end (day 15) of the experiment. Bacterioplankton community structure was elucidated by sequencing the V3–V4 region of 16S ribosomal RNA on an Illumina MiSeq platform. Proteobacteria was found to be the most dominant bacterial phylum in control and tannic acid–enriched barrels (barrels 1 and 2) on day 0. With the progression of experiment, the abundance of Proteobacteria altered significantly in the control barrel indicating the possible role of this phylum in the breakdown of tannic acid within estuarine mangroves. The abundance of Proteobacteria in the tannic acid–enriched barrels remained high, indicating that members of Proteobacteria may be capable of using tannic acid as a source of carbon and nitrogen. Tannic acid appeared to inhibit most of the other bacterioplankton phyla including Actinobacteria, Acidobacteria and Verrucomicrobia that existed in large abundance in the control barrel on day 15 but were almost absent in the tannic acid–enriched barrels. At class level, Bacteroides was found to be present in highest abundance in the tannic acid–enriched barrels. Tannic acid appeared to strongly influence the abundant bacterioplankton phyla and families as indicated by Pearson’s correlation coefficient and non-metric multidimensional scaling ordination plots. Gallic acid is one of the final products of tannic acid degradation. Breakdown of tannic acid could influence the marine nitrogen and carbon cycling by releasing DON and DOC, respectively, into the adjacent estuaries. Information of breakdown and remineralization of components of litterfall such as tannic acid would also be important for calculation of carbon and nitrogen budgets of coastal ecosystems including in mangroves.