Occurrence of Trichodesmium in south-southeastern Brazilian shelf slope: distribution, nutrient acquisition and coexistence with other microplanktonic organisms

The marine cyanobacterium Trichodesmium occur in tropical and subtropical oceans and contribute significantly to the nitrogen and carbon cycle. Although their occurrence is broadly reported, just a few are located in southern Atlantic Ocean. This work aimed to collect and identify the species near south-southeastern Brazilian shelf slope during two consecutive autumns (June/2013 and May/2014)(Figure 01). We observed their vertical distribution, unique strategies for nutrient acquisition and identified other microplanktonic groups co-occurring.

Overall, Trichodesmium trichomes were more abundant at surface (> 50 m), under lower wind speed (< 8 knots) and shallow mixed layer depth (< 40 m). Besides the previous reports of T. erythraeum in the coastal and shelf regions, other four species (T. clevei, T. hildebrandtii, T. radians and T. thiebautii) were identified. The highest concentrations of trichomes (> 157,000 trichomes L⁻¹) were located at the northernmost stations. The study area had low concentrations of phosphate (< 1 μM) and total dissolved nitrogen (< 1.5 μM), which characterize the region as oligotrophic. Groups of microplankton typical from poor nutrient regions were observed, some of them in association with nitrogen-fixers as Ornithocercus spp. (in association with picocyanobacteria), and Rhizosolenia spp. (in association with Richelia intracellularis)(Figure 01).

In contrast, by the use of scanning electron microscopy coupled with EDS X-ray, we could observe particles that were concentrated inside the colonies of Trichodesmium. The analysis of the content revealed a potential source of elements for the organism, since we found high concentrations of iron (10-20% of the total composition), phosphorus and zinc. Some colonies, moreover, seemed to be influenced by the La Plata River sediments due to the presence of titanium. Also, the trichomes were in association with heterotroph bacteria that could be involved in dissolution processes, increasing the availability of essential nutrients.

Figure 01: Map of the study area showing the location of sampling stations. The main contributions of microplanktonic species are represented as sectors of pie charts, except when Trichodesmium spp. contributed to > 90% to total amount of microplankton assemblages.

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