Articles | Volume 15, issue 19
Biogeosciences, 15, 5761–5778, 2018

Special issue: Interactions between planktonic organisms and biogeochemical...

Biogeosciences, 15, 5761–5778, 2018

Research article 02 Oct 2018

Research article | 02 Oct 2018

Trichodesmium physiological ecology and phosphate reduction in the western tropical South Pacific

Kyle R. Frischkorn1,2, Andreas Krupke3,a, Cécile Guieu4,5, Justine Louis4,b, Mónica Rouco2,c, Andrés E. Salazar Estrada1, Benjamin A. S. Van Mooy3, and Sonya T. Dyhrman1,2 Kyle R. Frischkorn et al.
  • 1Department of Earth and Environmental Sciences, Columbia University, New York, NY, USA
  • 2Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
  • 3Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA
  • 4Sorbonne Université, CNRS, Laboratoire d'Océanographie de Villefranche, LOV, 06230 Villefranche-sur-mer, France
  • 5The Center for Prototype Climate Modeling, New York University in Abu Dhabi, Abu Dhabi, UAE
  • apresent address: Thermo Fisher Scientific, Life Science Solutions, 180 Oyster Point Blvd., San Francisco, CA, USA
  • bpresent address: University Rennes 1, Saint-Brieuc, France
  • cpresent address: K=1 Project, Center for Nuclear Studies, Columbia University, New York, NY, USA

Abstract. N2 fixation by the genus Trichodesmium is predicted to support a large proportion of the primary productivity across the oligotrophic oceans, regions that are considered among the largest biomes on Earth. Many of these environments remain poorly sampled, limiting our understanding of Trichodesmium physiological ecology in these critical oligotrophic regions. Trichodesmium colonies, communities that consist of the Trichodesmium host and their associated microbiome, were collected across the oligotrophic western tropical South Pacific (WTSP). These samples were used to assess host clade distribution, host and microbiome metabolic potential, and functional gene expression, with a focus on identifying Trichodesmium physiological ecology in this region. Genes sets related to phosphorus, iron, and phosphorus–iron co-limitation were dynamically expressed across the WTSP transect, suggestive of the importance of these resources in driving Trichodesmium physiological ecology in this region. A gene cassette for phosphonate biosynthesis was detected in Trichodesmium, the expression of which co-varied with the abundance of Trichodesmium Clade III, which was unusually abundant relative to Clade I in this environment. Coincident with the expression of the gene cassette, phosphate reduction to phosphite and low-molecular-weight phosphonate compounds was measured in Trichodesmium colonies. The expression of genes that enable use of such reduced-phosphorus compounds were also measured in both Trichodesmium and the microbiome. Overall, these results highlight physiological strategies employed by consortia in an undersampled region of the oligotrophic WTSP and reveal the molecular mechanisms underlying previously observed high rates of phosphorus reduction in Trichodesmium colonies.

Short summary
Trichodesmium is a keystone genus of marine cyanobacteria that is estimated to supply nearly half of the ocean’s fixed nitrogen, fuelling primary productivity and the cycling of carbon and nitrogen in the ocean. In our study we characterize Trichodesmium ecology across the western tropical South Pacific using gene and genome sequencing and geochemistry. We detected genes for phosphorus reduction, providing a mechanism for the noted importance of this organism in the ocean's phosphorus cycle.
Final-revised paper