Abstract
Picocyanobacteria are among the smallest autotrophs on the planet, forming the base of the microbial foodweb in many aquatic ecosystems, from the poles to the tropics. Freshwater picocyanobacteria play a vital role in carbon fixation, nutrient cycling, and oxygen production in freshwater environments. Enhancing knowledge on the potential picocyanobacterial response to forecast changes such as climate shifts and increasing eutrophication is essential. Such predictions require an understanding of the diversity, distribution, drivers, and function of lacustrine picocyanobacteria at the genotype level. This information is largely lacking from the current scientific literature. To address these knowledge gaps, I used a combination of molecular (environmental DNA metabarcoding and genomics) and traditional (epifluorescence microscopy and culturing) approaches to characterise the diversity, distribution, potential drivers, and function of lacustrine picocyanobacteria across a range of lake types in New Zealand.
The horizontal-spatial abundance and community structure of picocyanobacteria was assessed in two contrasting (oligotrophic and hypertrophic) lakes, revealing that abundance and community composition differed significantly both between and within lakes. In both lakes, community structuring appeared to be driven by localised environmental conditions, suggesting that picocyanobacterial genotypes may respond differently to environmental change. To further explore these potential genotype responses, temporal shifts in abundance and diversity were assessed in relation to potential environmental drivers in five contrasting lakes over one year. Here, cell abundances were inconsistently related to different environmental variables across the lakes, while the addition of metabarcoding data revealed temporally dynamic and diverse picocyanobacterial communities strongly associated with specific environmental drivers in each lake.
With this new identification of strain-specific responses and community adaptation, the potential drivers of picocyanobacterial distributions and community assembly were then assessed in a nation-wide study covering 128 lakes across broad environmental gradients. The influence of deterministic and stochastic processes in shaping picocyanobacterial distributions and community assembly were explored along with the occupancy distribution of genotypes across lakes. Interestingly, no genotypes were found to inhabit all studied lakes whereas many genotypes were restricted to single lakes, resulting in a strongly unimodal lake occupancy distribution. Picocyanobacteria were not dispersal-limited across New Zealand, while richness and community structure differed in relation to specific environmental drivers.
Finally, to begin characterising the functional potential of strains, in particular to determine whether alternative nutrient metabolism is utilised in lakes of contrasting trophic state, 25 monoclonal picocyanobacterial strains were isolated from six lakes. Fifteen strains were isolated from oligotrophic lakes and ten strains from eutrophic and hypertrophic lakes. Of these, seven were selected for genome sequencing in which preliminary results suggest differential nutrient metabolism between strains, although not necessarily related solely to trophic state. Further genomic analysis and experimental approaches will continue to reveal the complex life strategies of lacustrine picocyanobacterial strains which will enable a deeper understanding of their function and potential responses to changes in our freshwater environments.