Benthic proliferations of Microcoleus autumnalis (basionym Phormidium autumnale) and closely related taxa are being reported with increasing frequency in streams worldwide. This cyanobacteria species commonly produces the potent neurotoxin anatoxin, and exposure to this has resulted in animal fatalities and human health concerns. Bacterial communities within cyanobacterial assemblages can facilitate processes such as nutrient cycling and are posited to influence cyanobacterial growth and function. However, there is limited knowledge on spatial variability of bacterial communities associated with benthic cyanobacterial and anatoxin content and quotas.

The overarching goal of this study was to investigate the bacterial community assemblages and anatoxin variability in M. autumnalis-dominated mats collected across different spatial scales. Five samples were collected from six sites of two streams in New Zealand and a suite of physiochemical parameters were sampled or measured at each site. Bacterial communities were characterised using 16S rRNA metabarcoding and anatoxin content was determined using liquid chromatography-mass spectrometry (LC-MS). Droplet digital PCR was used to determine anaC copies number which allowed anatoxin quotas (amount of anatoxin per cell) to be determined. We explored three questions; (1) how does the structure of bacterial communities in M. autumnalis-dominated mats vary within sites, within streams and between streams, (2) are anatoxin quotas as spatially variable as total anatoxin content in M. autumnalis-dominated mats across varying spatial scales, and (3) are there relationships between the structure of associated bacterial communities, anatoxin content and quota and stream water column physicochemical factors.