Nitrate input simplified community composition
PCoA well substantiated the community dissimilarities between two groups
(Figure 1A), where the nitrate group fell into distinct subgroups per
sampling time but control samples showed no significant difference over
time. For nitrate group, a longer incubation led to a larger
dissimilarity between subgroups. This suggested that the notable
community succession was in response to elevated nitrate but not to the
one-month incubation. We performed one-way ANOSIM to further compare the
pairwise dissimilarities between subgroups with respect to dissimilarity
significance (P ) and distribution (R2). For
convenience, we used “C” to represent control sediments, and “N” to
represent nitrate-amended sediments. For example, “4-C” refered to the
control sediment on Day 4. Most subgroups were statistically different
from each other except 4-C vs. 16-C and 8-C vs. 16-C (Figure 1B). Any
pairwise dissimilarity between C-N and N-N subgroups was significant
(P < 0.001), as PCoA suggested. In contrast, most
control subgroups had comparable within- and between-subgroup
dissimilarities (Figure 1C).
The relative abundance of categorized OTUs at the phylum or class level
suggested a progressive dominance of beta-Proteobacteria in
nitrate-amemded sediments, while classes Anaerolineae,
delta-Proteobacteria and Euryarchaeota dominated the control (Figure
1D). The enrichment of beta-proteobacteria (from 1.79% to 61.81%,
~35 folds) accounted for the dissimilarities among
nitrate subgroups. Only ~1/3 of total 4990 OTUs could be
assigned to a specific genus (Figure 1E). Among those, syntrophic
bacteria (e.g. Smithella , Syntrophphorhabdus ,Syntrophphus and Syntrophobater ) and methanogenic archaea
(e.g. Methanothrix , Methanoregula , Methanolinea ,Methanobacterium and Methanomassiliicoccus ) dominated
control and early nitrate communities. Compositions of control
communities were rather time-steady so we only screened genera
significantly changing over time within the nitrate group by paired
T-test. We found that most genera were gradually outcompeted alongside
the rise of genus Thiobacillus and Luteimonas (Figure 1F),
and the core microbiomes of late nitrate subgroups (i.e., 16N and 32N)
greatly differed in taxonomy and abundance from control and the early
nitrate subgroups.