Seaweed Loads Cause Stronger Bacterial Community Shifts in Coastal Lagoon Sediments Than Nutrient Loads.

Tânia Aires,Gerard Muyzer,Ester A Serrão,Aschwin H Engelen

doi:10.3389/fmicb.2018.03283

Tânia Aires, Gerard Muyzer + Show 2 more

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https://doi.org/10.3389/fmicb.2018.03283

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Abstract

The input of nutrients from anthropogenic sources is the leading cause of coastal eutrophication and is usually coupled with algal/seaweed blooms. Effects may be magnified in semi-enclosed systems, such as highly productive coastal lagoon ecosystems. Eutrophication and seaweed blooms can lead to ecosystem disruption. Previous studies have considered only one of these factors, disregarding possible interactive effects and the effect of the blooming species’ identity on sediment bacterial communities. We tested the effect of experimental nutrient loading and two common blooming seaweeds (Ulva rigida and Gracilaria vermiculophylla) in coastal lagoon sediments, on the structure of bacterial communities (using 16S rRNA amplicon sequencing) and corresponding putative functional potential (using PiCRUSt). At the Operational Taxonomic Unit (OTU) level, the addition of nutrients reduced bacterial community α-diversity and decreased the abundance of sulfate reducers (Desulfobacterales) compared to sulfur oxidizers/denitrifiers (Chromatiales and Campylobacterales), whereas this was not the case at the order level. Seaweed addition did not change bacterial α-diversity and the effect on community structure depended on the taxonomic level considered. The addition of Gracilaria increased the abundance of orders and OTUs involved in sulfate reduction and organic matter decomposition (Desulfobacterales, Bacteroidales, and Clostridiales, respectively), an effect which was also detected when only Ulva was added. Nutrients and the seaweeds combined only interacted for Ulva and nutrients, which increased known sulfide oxidizers and denitrifiers (order Campylobacterales). Seaweed enrichment affected putative functional profiles; a stronger increase of sulfur cycling KEGG pathways was assigned to nutrient-disturbed sediments, particularly with the seaweeds and especially Ulva. In contrast, nitrogen and sulfur cycle pathways showed a higher abundance of genes related to dissimilatory nitrate reduction to ammonium (DNRA) in Ulva+nutrients treatments. However, the other seaweed treatments increased the nitrogen fixation genes. Thiosulfate reduction, performed by sulfate-reducing bacteria, increased in seaweed treatments except when Ulva was combined with nutrients. In conclusion, the in situ addition of nutrients and the seaweeds to intertidal sediments affected the bacterial communities differently and independently. The predicted functional profile suggests a shift in relative abundances of putative pathways for nitrogen and sulfur cycles, in line with the taxonomic changes of the bacterial communities.

Highlights

Coastal systems are affected by multiple anthropogenic stressors of both marine and terrestrial origin (Tallis et al, 2008), such as eutrophication
In order to predict metagenomes, closed-reference Operational Taxonomic Units (OTUs) picking was performed using the subsampled reads against the GreenGenes reference database using the pick_closed_reference_otus.py script from QIIME (Caporaso et al, 2010). This was done separately from the bacterial community composition analysis because PiCRUSt requires an older version of GreenGenes, as well as the closed reference OTU picking method, which would limit a more comprehensive and broad diversity analysis
PiCRUSt (Langille et al, 2013) was used online in the Galaxy environment following the PiCRUSt workflow from Huttenhower’s Lab3. This included 16S copy number normalization, metagenomes/function prediction, and function categorizing into KEGG pathways

Summary

Introduction

Coastal systems are affected by multiple anthropogenic stressors of both marine and terrestrial origin (Tallis et al, 2008), such as eutrophication. One of its main causes is the high input of phosphorus (P) and, more importantly, nitrogen (N) from agricultural fertilizer runoff and wastewater discharge (Nixon, 1995) This will, lead to elevated atmospheric N concentrations and increased atmospheric deposition of N (Holland et al, 2005), resulting in a dramatic rise of the amount of bioavailable N in natural habitats (Galloway and Cowling, 2002). N is often a limiting nutrient (Taylor et al, 1995), but excessive nutrient inputs drive microbial communities to create biogeochemical feedbacks that promote the availability of N and P (Howarth et al, 2011) This positive feedback stimulates eutrophication and has negative effects, such as harmful or opportunistic blooms of seaweed (hereafter referred as alga/algae) and the outgrowth of phytoplankton and epiphytic algae (Hauxwell et al, 2001). These blooms are the primary cause for oxygen depletion, which leads to hypoxia/anoxia with consequences for biodiversity (Isbell et al, 2013; Leff et al, 2015) and society (Leliaert et al, 2009)

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