Patterns in Rhizosolenia- and Guinardia-associated Richelia abundances in the tropical marginal seas of the western North Pacific

Richelia is a widespread, heterocystous, N2-fixing cyanobacterium that occurs in oligotrophic seas. It can be symbiotic within rhizosolenoid hosts, including Rhizosolenia (R-R) and Guinardia (G-R), and occasionally occurs as free-living (FL-R) filaments. We investigated the Richelia abundances in the South China (SCS) and Philippine Seas (PS) during the northeast (NEM) and southwest monsoon (SWM) seasons between 2001 and 2016. Field results showed that G-R was most abundant in the SCS during the SWM. Both R-R and FL-R were more abundant in the PS than the SCS and during the SWM versus the NEM. In general, R-R were more abundant than G-R, except at the SCS-SWM stations where abundances were comparable. Multivariate statistical models indicate that as R-R and G-R were abundant in warmer seawaters, nitrate plus nitrite was a key factor negatively controlling R-R abundance while silicate was more positively related to G-R. Positive correlation between G-R and FL-R abundances and microscopic observations imply that like Rhizosolenia-associated Richelia, filaments released from Guinardia hosts can remain viable outside their diatom hosts for time periods on the order of days. Overall, results reveal different distribution patterns between the two rhizosolenoid-Richelia symbioses and both G-R and R-R likely contribute to FL-R in the field.