Sewage outburst triggers Trichodesmium bloom and enhance N₂ fixation rates

The southeastern Mediterranean Sea (SEMS) is a warm and sunlit marine environment with low ambient N concentration, thus considered ideal for diazotrophy by autotrophic diazotrophs such as Trichodesmium. Despite the favorable conditions, N₂ fixation rates are often low and Trichodesmium has hardly been spotted in the SEMS. This study reports on the occurrence of a Trichodesmium bloom in the SEMS which was ascribed to T. erythraeum according to DNA fingerprinting of the nifH gene. We found that this bloom (1407 ± 983 cells L^-1) was triggered by an intense outburst of raw sewage that supplied high concentrations of N, P and dissolved organic carbon (DOC), which resulted in low N:P (∼12:1) and exceptionally high C:P (∼1340:1) ratios. We surmise that these conditions provided favorable conditions for Trichodesmium bloom to form via mixotrophic metabolism. As a result, a fourfold increase in N₂ fixation was recorded, which contributed ∼70% to new primary production and spur a sharp increase in phytoplankton activity and biomass. The conclusions of this study point on a new paradigm for bloom-forming T. erythraeum which is tightly linked to anthropogenic sources and prompt microbial productivity in oligotrophic marine environments such as the SEMS.