Diversity trumps acidification: Lack of evidence for carbon dioxide enhancement of Trichodesmium 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 P CO2 ) to , 160 Pa (predicted for , year 2200). Our results indicate that elevated P CO2 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 P CO2 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 (. 99% hetR sequence similarity) to isolate H9-4 of T. erythraeum, which showed no response to elevated P CO2 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 .

A Three-Dimensional Model of the Marine Nitrogen Cycle during the Last Glacial Maximum Constrained by Sedimentary Isotopes

“…On the other hand, Ren et al (2009Ren et al ( , 2012 interpret δ 15 N records from the western tropical North Atlantic and Pacific as reduced N 2 fixation, in response to lower N-loss in the glacial ocean. Some specific N 2 -fixing Trichodesmium species may also be limited by CO 2 concentrations of surface waters (Barcelos e Ramos et al, 2007;Hutchins et al, 2007), which could have reduced N 2 fixation during the LGM, although other studies investigating Trichodesmium community assemblages (Gradoville et al, 2014) and unicellular diazotrophs (Law et al, 2012) found no consistent effect of CO 2 on N 2 fixation.…”

Five decades of N2 fixation research in the North Atlantic Ocean

“…However, experiments including several different diazotrophic strains suggest that the response to increased pCO 2 is strain-specific and depends on the CO 2 uptake kinetics of each diazotroph type (Hutchins et al, 2013). Similarly, field studies have found divergent responses to increasing pCO 2 levels, indicating that different phylotypes are unequally affected by acidification, which results in a diluted effect on bulk N 2 fixation field measurements (e.g., Gradoville et al, 2014). Heterotrophic N 2 fixation in hypoxic zones (Hamersley et al, 2011), suggests that the predicted expansion of OMZs (Stramma et al, 2008b) will likely enhance global N 2 fixation rates too, either by in situ heterotrophic N 2 fixation, or by the enhancement of diazotrophy in other oceanic areas in order to balance increased denitrification occurring in the OMZs (Deutsch et al, 2007).…”

An updated synthesis of the observed and projected impacts of climate change on the chemical, physical and biological processes in the oceans

“…There is low to medium confidence on the effects of ocean acidification on nitrogen (N 2 ) fixing cyanobacteria. A wide range of N 2 fixation responses under RCP8.5 conditions have been observed in laboratory experiments Eichner et al, 2014;Gradoville et al, 2014), possibly due to speciesspecific differences in the mechanisms of N 2 fixation (Eichner et al, 2014).…”