Abstract
(1) Background: Paleolimnological studies use sediment cores to explore long-term changes in lake ecology, including occurrences of harmful cyanobacterial blooms. Most studies are based on single cores, assuming this is representative of the whole lake, but data on small-scale spatial variability of microbial communities in lake sediment are scarce. (2) Methods: Surface sediments (top 0.5 cm) from 12 sites (n = 36) and two sediment cores were collected in Lake Rotorua (New Zealand). Bacterial community (16S rRNA metabarcoding), Microcystis specific 16S rRNA, microcystin synthetase gene E (mcyE) and microcystins (MCs) were assessed. Radionuclide measurements (210Pb, 137Cs) were used to date sediments. (3) Results: Bacterial community, based on relative abundances, differed significantly between surface sediment sites (p < 0.001) but the majority of bacterial amplicon sequence variants (88.8%) were shared. Despite intense MC producing Microcystis blooms in the past, no Microcystis specific 16S rRNA, mcyE and MCs were found in surface sediments but occurred deeper in sediment cores (approximately 1950′s). 210Pb measurements showed a disturbed profile, similar to patterns previously observed, as a result of earthquakes. (4) Conclusions: A single sediment core can capture dominant microbial communities. Toxin producing Microcystis blooms are a recent phenomenon in Lake Rotorua. We posit that the absence of Microcystis from the surface sediments is a consequence of the Kaikoura earthquake two years prior to our sampling.
Highlights
Lake ecosystems worldwide are experiencing dramatic changes in water quality, nutrient concentrations and phytoplankton community composition due to global warming, eutrophication and introductions of non-native species [1,2]
We posit that the absence of Microcystis from the surface sediments is a consequence of the Kaikoura earthquake two years prior to our sampling
In deeper sediment core abundance with increasing sediment depth (Figure and Figure layers, cyanobacterial amplicon sequence variants (ASVs) abundance was below 0.1% of total bacterial community (Figure 6a and Relative abundance of; (a) Microcystis and other cyanobacteria in the total bacterial community, and (b) Cyanobacteria genera in the total cyanobacteria community based on 16S rRNA sequences
Summary
Lake ecosystems worldwide are experiencing dramatic changes in water quality, nutrient concentrations and phytoplankton community composition due to global warming, eutrophication and introductions of non-native species [1,2]. Many bloom-forming species produce cyanotoxins, which have neurotoxic, cytotoxic, and hepatotoxic effects on organisms [5]. Toxins 2020, 12, 580 contaminated with cyanotoxins can lead to poisoning, or death of humans, pets, stock, and wildlife [5]. The cyclic heptapeptide microcystins (MCs) are the most frequently found [6,7,8,9,10]. Cyanobacteria genera known to produce MCs include Planktothrix, Oscillatoria, Dolichospermum and most prominently Microcystis [5]. The mcy gene cluster (mcyA-J) encoding these biosynthetic enzymes has been employed as the target region for the molecular detection of potentially toxin producing cyanobacteria strains in water and sediment [6,14,15]
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