Diversity of Spirogyra in New Zealand, and its distribution with respect to environmental drivers

Spirogyra is an extremely common, high-biomass, and species-rich green alga of freshwater ecosystems. Conjugating material with zygospores is traditionally required to achieve identification beyond genus. Although filament width classes have been related to habitat status elsewhere, the phylogenetic content of these classes is unknown. We report the results of a survey of 90 sites in New Zealand, designed to determine species distributions in contrasting stream habitats, using rbcL sequencing of microscopically separated filaments to resolve diversity. We found 22 phylospecies of Spirogyra inhabiting 51 sites, with traditional identification using fertile material where possible. Subgeneric resolution was obtained in essentially all samples (vs Spirogyra, with implications for species-level diversity, and we found substantial variation in filament diameter and chloroplasts per cell within phylospecies. Environmental drivers of species distribution, chloroplasts per cell (inferred ploidy), and zygospore formation all differed. Mean summer temperature and pH were the strongest drivers of site occupancy by phylospecies. However, chloroplasts per cell, thought to reflect ploidal state, was mainly driven by habitat enrichment, resulting in larger forms being biased towards lowland waters. A long-standing hypothesis proposes that genome size is limited by nutrient availability, since nucleic acids demand high nitrogen and phosphorus; Spirogyra may offer an excellent experimental system to address this question. The width of zygospores, which are preserved in sediments and found during palaeoecological studies, is correlated with filament diameter and chloroplasts per cell, suggesting that these relationships with trophic status offer potentially useful palaeoecological information. Specimens of narrow (presumably haploid) phylotypes were encountered fertile far more often than were larger phylotypes; whether this reflects polyploidy-related incompatibilities or differences in population turnover rates are unknown.