Free-living heterocystous cyanobacteria in the tropical marginal seas of the western North Pacific

Abstract

Calothrix rhizosoleniae and Richelia intracellularis are widespread heterocystous nitrogen-fixing cyanobacteria in nitrate-limited tropical oceans. Most of them are symbiotic with diatom hosts (SYM), but some are free-living (FL). We investigated FL abundance in the South China Sea and the Kuroshio on 36 cruises between 2001 and 2014. The field results showed overall FL abundances of 16-239 × 103 filaments m-2 and mean %FL (FL/(FL+SYM)) of 8.5 ±1.0%. The morphological examination of 1382 FL filaments showed that 87.3% were Calothrix and 12.7% with features of Richelia were probably associated with Rhizosolenia. FL abundance increased with a deepening nitracline, increasing surface seawater temperature, and abundances of Chaetoceros-Calothrix and Rhizosolenia-Richelia symbioses. %FL decreased with a deepening nitracline, indicating symbiosis prevailed when nitrate was depleted. As the nitracline deepened further, however, %FL increased with a deepening phosphacline, implying that phosphate shortage would enhance dissociation even under nitrate depletion. An enrichment study showed that %FL increased when nitrate was added; conversely, the symbiosis was enhanced by added phosphate. The results indicate that nutrient fluctuation regulates the inter-related dynamics of FL and SYM and this nutrient dependence might be an ecological strategy important for the survival of the heterocystous cyanobacteria.