The specialization-disturbance hypothesis (SDH) states that disturbances in an ecosystem have a disproportionately negative impact on specialist members, while favoring generalist members. In the context of prokaryotic communities, however, the expectation is often assumed to be reversed due to the large and diverse species pools: at least one specialist member of the community should be better adapted to the nature of the disturbance, outcompeting other (less specialized) populations. The BP Macondo oil spill in the Gulf of Mexico (April 2010) provided an unintended experimental ground for testing this hypothesis. Sediment samples from Pensacola beach (FL, USA) were taken a month before the oil reached the shoreline, as well as 2, 4, and 12 months after. A total of 16 metagenomic and 35 16S ribosomal RNA amplicon datasets were obtained from these samples to characterize the responses of the microbial communities to the oil disturbance and their succession pattern. In the absence of a reliable estimation of niche breadth for every population in the communities, we evaluated community features under SDH, including taxonomic/functional diversity, minimum doubling time, and average genome size.
Results/Conclusions
The effect of oiling was evident in the functional and taxonomic profiles, with significant separation between oiled and non-oiled samples. The taxonomic diversity in the communities decreased after oiling, but increased to levels above pre-disturbance after a year of recovery. Hence, the expectations that diversity in the community would decrease –due to oil toxicity and an overabundance of typically rare hydrocarbon-degrading species– and recover after community succession, were met. However, this pattern was not observed in the functional profiles, in which diversity increased upon oiling, and upon recovery decreased to levels similar to or below pre-oiling. While these results seem contradictory under the assumption of invariant functional diversity per species (i.e., constant niche breadth), they follow SDH expectations. Generalists would be expected to contribute a larger functional pool to the community than specialists. Therefore, a relative increase in the contribution of generalist populations upon disturbance (SDH prediction) would result in an overall increase in functional diversity, and a decrease in taxonomic diversity, as observed in this study. Moreover, wider niche breadths are often associated with faster growth rates and larger genomes. Consistent with the latter interpretations, we identified an increase in growth rates upon disturbance from ribosomal gene codon biases.
Recommended reading: Rodriguez-R et al., ISME J 2014.