Diversity trumps acidification: Lack of evidence for carbon dioxide enhancement of <i>Trichodesmium</i> community nitrogen or carbon fixation at Station ALOHA

Abstract

We conducted 11 independent short‐term carbon dioxide (CO 2 ) manipulation experiments using colonies of the filamentous cyanobacteria Trichodesmium isolated on three cruises in the North Pacific Subtropical Gyre (NPSG). Dinitrogen (N 2 ) and carbon (C) fixation rates of these colonies were compared over CO 2 conditions ranging from ∼ 18 Pa (equivalent to last glacial maximum atmospheric ) to ∼ 160 Pa (predicted for ∼ year 2200). Our results indicate that elevated has no consistent significant effect on rates of N 2 or C fixation by Trichodesmium colonies in the NPSG under present environmental conditions. Differences between treatments were not modulated by phosphorus amendments, iron amendments, or light level. Sequencing the hetR, nifH, 16S, and internal transcribed spacer genes of Trichodesmium colonies revealed a highly diverse community of Trichodesmium and other N 2 ‐fixing colony‐associated organisms. The species composition of Trichodesmium demonstrated spatiotemporal variability, but over half of total sequences were phylogenetically closely related (&gt; 99% hetR sequence similarity) to isolate H9‐4 of T. erythraeum, which showed no response to elevated in previous laboratory experiments. Our handpicked Trichodesmium colonies included a substantial number of organisms other than Trichodesmium with the metabolic capacity for N 2 and C fixation. We suggest that the diverse assemblage of Trichodesmium species and coexisting microorganisms within the colonies can explain the lack of an observed CO 2 enhancement of N 2 or C fixation rates, because different species are known to have different specific affinities for CO 2 .