AME 78:107-119 (2017)  -  DOI: https://doi.org/10.3354/ame01810

ABSTRACT: Members of the genus Trichodesmium are commonly found in nutrient-limited tropical and subtropical ocean basins due to their capacity to fix atmospheric nitrogen (N₂). However, N₂ fixation requires supplementary iron and phosphorus, which can potentially become limiting nutrients for the growth of Trichodesmium. In order to understand the ecology and distribution of Trichodesmium in a poorly studied region of the South Atlantic Ocean, we collected physical, chemical, and biological data from dense populations. Despite relatively low concentrations of phosphate (<0.35 µM), we found Trichodesmium throughout the region, with the northernmost stations having the highest abundances in the surface layer (>157000 trichomes l^-1). Using light microscopy, mixtures of biogenic and lithogenic particles were found attached to the colonies; composition analysis revealed high concentrations of iron (12-29% of the average particle composition), phosphorus (1-3%), and zinc (1-5%). Particles from the southernmost stations were unique in their titanium composition (5-6%), which was considered a tracer for the regional input of sediments by the La Plata River plume. Bacteria were associated with some Trichodesmium colonies, suggesting particle dissolution and remineralization as a mechanism for increasing the availability of nutrients, thus facilitating N₂ fixation. This study provides new information about the distribution of Trichodesmium in the South Atlantic Ocean and its association with particles and bacteria as an adaptation for extra nutrient acquisition.

KEY WORDS: Iron uptake · Shelf slope · Scanning Electron Microscopy · Bacterial association · Phytoplankton ecology