Temperature‐dependent growth and activity in a globally distributed nitrogen‐fixing haptophyte

Dinitrogen (N2)‐fixing microorganisms play a crucial role in supplying nitrogen (N) to the oceans by converting atmospheric N2 into bioavailable N. N2 fixation was thought to be limited to warm oligotrophic ocean waters, but the association between the N2‐fixing UCYN‐A cyanobacterium and specific haptophytes, including Braarudosphaera bigelowii and relatives, has been found in diverse ocean environments, including warm subtropical gyres, temperate coastal systems, and cold polar waters. UCYN‐A2, previously known as the symbiont of B. bigelowii, and now considered an early‐stage organelle that exchanges fixed nitrogen for fixed carbon, has only recently been cultured. This study investigated the growth and activity of B. bigelowii in response to a range of temperatures to better understand its global distribution and ecology. Incubation experiments were conducted with B. bigelowii to determine growth rates, carbon (C) and N2 fixation rates, and cell sizes across a temperature range of 6–26°C. Growth rates were highest between 10°C and 22°C and lowest at 6°C and 26°C. Significant positive correlations were found between cell count‐based growth rates, C‐specific and N‐specific growth rates. Braarudosphaera bigelowii cell size increased at low temperatures. The growth and metabolic activity detected across a wide range of temperatures help to explain the wide geographic distribution of B. bigelowii. This study presents the first growth and activity measurements under a range of temperatures from B. bigelowii, providing vital information needed to understand the unique ecology of this organism and to parameterize its activity in ecosystem models.