Abstract
Marine picocyanobacteria Synechococcus exhibit highly diverse pigment types (PTs) and hence possess great advantage to utilize different spectrum of light effectively and to occupy a wide range of light niches. In this study, we explored the diversity of Synechococcus PTs in the eastern Indian Ocean (EIO), surface water of Strait of Malacca (SSM), and coastal waters of Sri Lanka (SSL). All the detected PTs were phycourobilin (PUB) containing PT 3 and showed distinct distribution patterns. Low PUB PT 3a and partial chromatic acclimater PT 3eA dominated in coastal and shallow waters (SSM and SSL). In contrast, high PUB and chromatic acclimaters PT 3dA and PT 3c/3dB were mainly distributed in open ocean (EIO). PT 3dA and PT 3c/3dB occurred at similar depths of the lower euphotic layers but showed distinct distribution pattern that are partially exclusive, indicating that they compete with each other for the same light niche. Interestingly, the newly described PT 3f was detected with high relative abundances at all stations and particularly dominated in the upper euphotic layer in EIO, which was confirmed with PT-specific quantitative polymerase chain reaction (qPCR). The relative abundance of PT 3f was negatively correlated with nutrient level, implying that PT 3f is adapted to oligotrophic waters. Pronounced niche partition of different PTs was observed in the upper and lower layers of euphotic zone in EIO and SSM/SSL. Light, nutrients, and strong stratification may play important roles in the niche partition of different PTs. Further analysis about ecologically significant taxonomic units revealed high diversity within each PT at different locations, which provided insights for understanding specific PT with wide range of niches.
Highlights
The ubiquitously distributed marine Synechococcus are one of the most abundant autotrophic microorganisms in the global ocean (Liu et al, 1997; Veldhuis et al, 1997; Partensky et al, 1999; Zwirglmaier et al, 2008)
dissolved inorganic nitrogen (DIN) and phosphate were depleted in the surface water of eastern Indian Ocean (EIO), ranging from 0.191 to 0.494 and 0.013 to 0.077 μmol/L, respectively (Supplementary Figure 1)
Both DIN and phosphate concentration were remarkably higher in the lower euphotic layers than that in the upper euphotic layers, and maximum values were observed in 75 m, generally (Supplementary Figure 2)
Summary
The ubiquitously distributed marine Synechococcus are one of the most abundant autotrophic microorganisms in the global ocean (Liu et al, 1997; Veldhuis et al, 1997; Partensky et al, 1999; Zwirglmaier et al, 2008). Synechococcus exhibit diverse physiological characteristics and ecological niches, resulting in wide geographical distributions from subpolar to equatorial regions, including estuarine, coastal, and oligotrophic oceanic waters (Ahlgren and Rocap, 2012; Mazard et al, 2012; Flombaum et al, 2013; Xia et al, 2015, 2017a; Farrant et al, 2016). On the basis of distribution pattern, various Synechococcus clades could be further divided into different genetically related subgroups, which were named ecologically significant taxonomic units (ESTUs) (Farrant et al, 2016). Strains within each ESTU that share the same distribution pattern can be linked to specific environmental conditions (Farrant et al, 2016)
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