Abstract
Coastal zones are important transitional areas between the land and sea, where both terrestrial and phytoplankton supplied dissolved organic matter (DOM) are respired or transformed. As climate change is expected to increase river discharge and water temperatures, DOM from both allochthonous and autochthonous sources is projected to increase. As these transformations are largely regulated by bacteria, we analyzed microbial community structure data in relation to a 6-month long time-series dataset of DOM characteristics from Roskilde Fjord and adjacent streams, Denmark. The results showed that the microbial community composition in the outer estuary (closer to the sea) was largely associated with salinity and nutrients, while the inner estuary formed two clusters linked to either nutrients plus allochthonous DOM or autochthonous DOM characteristics. In contrast, the microbial community composition in the streams was found to be mainly associated with allochthonous DOM characteristics. A general pattern across the land-to-sea interface was that Betaproteobacteria were strongly associated with humic-like DOM [operational taxonomic units (OTUs) belonging to family Comamonadaceae], while distinct populations were instead associated with nutrients or abiotic variables such as temperature (Cyanobacteria genus Synechococcus) and salinity (Actinobacteria family Microbacteriaceae). Furthermore, there was a stark shift in the relative abundance of OTUs between stream and marine stations. This indicates that as DOM travels through the land-to-sea interface, different bacterial guilds continuously degrade it.
Highlights
Dissolved organic matter (DOM) is the largest pool of organic carbon in the global oceans (∼660 pg C) and it is up to 200-fold greater than that in organic particles or marine life (Hansell et al, 2009; Jiao et al, 2010)
Non-metric multidimensional scaling ordination based on Bray– Curtis dissimilarity showed that the inner estuary had a microbial community composition that was significantly different from the outer estuary (PERMANOVA, p < 0.01; Figure 1)
As the streams discharged into the higher saline estuary the prokaryotic community composition abruptly changed (Figure 2B). This stark shift in the relative abundance of operational taxonomic units (OTUs) at the stream and marine stations indicated that as DOM travels through the land-to-sea interface different bacterial guilds, in addition to Betaproteobacteria, continuously degrade it
Summary
Dissolved organic matter (DOM) is the largest pool of organic carbon in the global oceans (∼660 pg C) and it is up to 200-fold greater than that in organic particles or marine life (Hansell et al, 2009; Jiao et al, 2010). Compared to allochthonous DOM, the biological DOM produced in situ is typically considered to have a lower molecular weight and constitutes a labile carbon source for heterotrophic microbes (Jiao et al, 2010). This labile autochthonous organic matter is rapidly metabolized by heterotrophic bacteria (Hansell, 2013; Asmala et al, 2018a), and the organic matter pool in the marine environment is eventually turned into refractory DOM that can last for millennia (Jiao et al, 2010). Considering that the surface temperature in the global oceans is increasing (Rhein et al, 2013) and this will enhance algal blooms (Beaulieu et al, 2013), climate change is likely to increase the production of autochthonous DOM in coastal systems and microbial populations associated with degradation of this carbon source
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