The Effect of Salmon Food-Derived DOM and Glacial Melting on Activity and Diversity of Free-Living Bacterioplankton in Chilean Patagonian Fjords.

Paulina Montero,Bárbara Jacob,Marcelo H Gutiérrez,Giovanni Daneri

doi:10.3389/fmicb.2021.772900

Abstract

Fjord ecosystems cycle and export significant amounts of carbon and appear to be extremely sensitive to climate change and anthropogenic perturbations. To identify patterns of microbial responses to ongoing natural and human-derived changes in the fjords of Chilean Patagonia, we examined the effect of organic enrichment associated with salmon aquaculture and freshening produced by glacial melting on bacterial production (BP), extracellular enzymatic activity (EEA), and community diversity of free-living bacterioplankton. We assayed the effects of salmon food-derived dissolved organic matter (SF-DOM) and meltwaters through microcosm experiments containing waters from Puyuhuapi Fjord and the proglacial fjords of the Southern Patagonia Icefield, respectively. Rates of BP and EEA were 2 times higher in the presence of SF-DOM than in controls, whereas the addition of autochthonous organic matter derived from diatoms (D-DOM) resulted in rates of BP and EEA similar to those measured in the controls. The addition of SF-DOM also reduced species richness and abundance of a significant fraction of the representative taxa of bacterioplankton of Puyuhuapi Fjord. In the proglacial fjords, bacterioplankton diversity was reduced in areas more heavily influenced by meltwaters and was accompanied by moderate positive changes in BP and EEA. Our findings strongly suggest that SF-DOM is highly reactive, promoting enhanced rates of microbial activity while could be influencing the diversity of bacterioplankton communities in Patagonian fjords with a strong salmon farming activity. These findings challenge the traditional view of phytoplankton production as the primary source of labile DOM that fuels heterotrophic activity in coastal ecosystems impacted by anthropogenic organic enrichment. Given the intensive local production of salmon, we analyze the significance of this emerging source of rich “allochthonous” organic substrates for autotrophic/heterotrophic balance, carbon exportation, and hypoxia in Patagonian fjords. The effect of human DOM enrichment can be enhanced in proglacial fjords, where progressive glacial melting exerts additional selective pressure on bacterioplankton diversity.

Highlights

Chilean Patagonia (41◦–56◦S) is one of the most extensive fjord regions in the world (Iriarte et al, 2014a), characterized by variable hydrobiological regimes associated with strong seasonal and latitudinal patterns of precipitation, river and meltwater discharges, and light availability (Aracena et al, 2011; Pantoja et al, 2011a and references therein)
Average bacterial production (BP) and extracellular enzymatic activity (EEA) in salmon food-derived dissolved organic matter (SF-Dissolved organic matter (DOM)) enrichment incubations increased by a factor of 2 compared to the rates measured in controls (Figure 2)
Integrated BP rates for surface waters were significantly higher than in subsurface waters under control conditions (Mann-Whitney, p-value < 0.05), and rates of EEA for both derived from diatoms (D-DOM) treatment and controls were significantly higher in surface than subsurface water incubations (Mann-Whitney, p-value < 0.05) when the data set was analyzed as a whole (Figure 3)

Summary

Introduction

Chilean Patagonia (41◦–56◦S) is one of the most extensive fjord regions in the world (Iriarte et al, 2014a), characterized by variable hydrobiological regimes associated with strong seasonal and latitudinal patterns of precipitation, river and meltwater discharges, and light availability (Aracena et al, 2011; Pantoja et al, 2011a and references therein). Patagonian fjords are considered to act both as a net sink of atmospheric CO2 (Torres et al, 2011) and a site of significant burial of sedimentary organic carbon (Sepúlveda et al, 2011; Smith et al, 2015) Due to their significance to global carbon fluxes and their vulnerability to anthropogenic and climatic pressure, fjord regions have recently been classified as Aquatic Critical Zones (Bianchi et al, 2020). The organic fraction of these allochthonous inputs combined with high production of autochthonous organic matter by phytoplankton (Montero et al, 2011, 2017a,b), provides a heterogeneous cocktail of organic substrates available to be exploited by heterotrophic microbes

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Conclusion