Abstract
BackgroundOne of the central objectives of microbial ecology is to study the distribution of microbial communities and their association with their environments. Biogeographical studies have partitioned the oceans into provinces and regions, but the identification of their boundaries remains challenging, hindering our ability to study transition zones (i.e. ecotones) and microbial ecosystem heterogeneity. Fuzzy clustering is a promising method to do so, as it creates overlapping sets of clusters. The outputs of these analyses thus appear both structured (into clusters) and gradual (due to the overlaps), which aligns with the inherent continuity of the pelagic environment, and solves the issue of defining ecosystem boundaries.ResultsWe show the suitability of applying fuzzy clustering to address the patchiness of microbial ecosystems, integrating environmental (Sea Surface Temperature, Salinity) and bacterioplankton data (Operational Taxonomic Units (OTUs) based on 16S rRNA gene) collected during six cruises over 1.5 years from the subtropical frontal zone off New Zealand. The technique was able to precisely identify ecological heterogeneity, distinguishing both the patches and the transitions between them. In particular we show that the subtropical front is a distinct, albeit transient, microbial ecosystem. Each water mass harboured a specific microbial community, and the characteristics of their ecotones matched the characteristics of the environmental transitions, highlighting that environmental mixing lead to community mixing. Further explorations into the OTU community compositions revealed that, although only a small proportion of the OTUs explained community variance, their associations with given water mass were consistent through time.ConclusionWe demonstrate recurrent associations between microbial communities and dynamic oceanic features. Fuzzy clusters can be applied to any ecosystem (terrestrial, human, marine, etc) to solve uncertainties regarding the position of microbial ecological boundaries and to refine the relation between the distribution of microorganisms and their environment.
Highlights
One of the central objectives of microbial ecology is to study the distribution of microbial communities and their association with their environments
Neritic waters (NW), characterised by high temperatures and low salinities (13.15 ± 1.25 °C, 34.32 ± 0.28 PSU), were the first to be crossed by the transect and finished between 9 and 19 km offshore depending on the sampling month
The subtropical front itself was described as a separate cluster by the FCM (11.30 ± 0.88 °C, 34.42 ± 0.13 PSU), with a width between 11 km and 22 km, and occurred between the end of the subtropical waters (STW) and the beginning of the sub-Antarctic waters (SAW), which began between 41 and 53 km offshore
Summary
One of the central objectives of microbial ecology is to study the distribution of microbial communities and their association with their environments. Initial qualitative assessment of boundaries within or between ecosystems can be done to direct sampling effort [7], but this approach is restricted to environments where delineations are clear (e.g. forest patches, [8], seagrass meadows, [9], agricultural land, [10]) Many ecosystems challenge such àpriori designs by presenting no obvious patchiness [11, 12], gradualness [13, 14], by being dynamic [15, 16], because the kind of organisms under consideration are too small for visual assessment [17, 18], or a combination of all or some of the above. These uncertainties reduce the potential comparability between studies [23] and opportunities for broader investigations across ecosystems
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