Requirement of Nickel as an Essential Micronutrient for the Utilization of Urea in the Marine Cyanobacterium Synechococcus sp. PCC 7002

Synechococcus sp. PCC 7002 cells appeared to be nitrogen limited when grown on urea without nickel in the growth medium; the amounts of chlorophyll a and phycobiliproteins decreased and glycogen accumulated to high levels. When nickel was added to the urea medium, the diagnostics of nitrogen limitation was not observed. The activity of urease was dependent on the presence of nickel no matter what nitrogen source was used for cell growth. The ureC gene encoding the α-subunit of urease was constitutively transcribed in cells grown on all nitrogen sources tested. The addition of nickel to the growth medium rapidly led to a 15-fold increase in urease activity in 2 hours. These results indicate that cells of Synechococcus sp. PCC 7002 require a nickel supplement (5 μM NiSO₄ in the growth medium) to utilize urea efficiently, and that Synechococcus sp. PCC 7002 cells are nickel-starved under the laboratory conditions regularly used for this strain. In two fresh water strains of cyanobacteria, Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120, high levels of urease activity were detected without addition of nickel to the growth medium, suggesting that these fresh water strains have a high-affinity uptake system for nickel or that their nickel requirements are lower than Synechococcus sp. PCC 7002.