Temporal and spatial dynamics of Bacteria, Archaea and protists in equatorial coastal waters

Abstract

Singapore, an equatorial island in South East Asia, is influenced by a bi-annual reversal of wind directions which defines two monsoon seasons. We characterized the dynamics of the microbial communities of Singapore coastal waters by collecting monthly samples between February 2017 and July 2018 at four sites located across two straits with different trophic status, and sequencing the V6-V8 region of the small sub-unit ribosomal RNA gene (rRNA gene) of Bacteria, Archaea, and Eukaryota. Johor Strait, which is subjected to wider environmental fluctuations from anthropogenic activities, presented a higher abundance of copiotrophic microbes, including Cellvibrionales and Rhodobacterales. The mesotrophic Singapore Strait, where the seasonal variability is caused by changes in the oceanographic conditions, harboured a higher proportion of typically marine microbe groups such as Synechococcales, Nitrosupumilales, SAR11, SAR86, Marine Group II Archaea and Radiolaria. In addition, we observed seasonal variability of the microbial communities in the Singapore Strait, which was possibly influenced by the alternating monsoon regime, while no seasonal pattern was detected in the Johor Strait.

Figure 1

Salinity, chlorophyll, phosphates and DIN (dissolved inorganic nitrogen) during the 18-month time series in Singapore coastal waters. Highlights in grey and blue represent NE and SW monsoon, respectively.

Figure 2

Shannon Index (H′, left) and Richness (Number of ASVs, right) of microbial communities at 4 stations in Singapore coastal waters for Archaea (top, A,B), Bacteria (middle, C,D) and Eukaryota (bottom, E,F).

Figure 3

Tree map with the major taxonomic groups in Singapore and Johor Straits.

Figure 4

Non-metric multidimensional scaling (nMDS) analysis of Bray-Curtis similarity index. Each sample is labelled by location and monsoon period. Arrows represent environmental parameters with p < 0.001 when performing an envfit analysis The ellipses represent 95% confidence intervals for samples collected at the same station. Top. All stations. Bottom. Only Singapore Strait samples (STJ and EC stations).