Abstract
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.
Highlights
The surface water of the southeastern Mediterranean Sea (SEMS) is a warm, sunlit marine environment with ultra-oligotrophic conditions, resulting in low bacterial and phytoplankton biomass and low productivity[11,12,13]
We suggest that the new N derived from T. erythraeum N2 fixation, along with the additional nutrients that were introduced by the sewage, may induce primary and bacterial production rates, resulting in prompted microphytoplankton growth (Fig. 2, Table 2)
Our findings suggest that apart from the calm sea, sunlit conditions and the somewhat low N:P ratio (12:1), two mechanisms may have been in play and triggered the development of the T. erythraeum bloom: (i) The high concentrations of N and P that were introduced to the coastal environment with the sewage supplied the initial cellular metabolic needs for the growth and proliferation of T. erythraeum cells. (ii) The high dissolved organic carbon (DOC) concentration that was supplied with the sewage may have provided assimilable carbon source to the coastal environment
Summary
The surface water of the southeastern Mediterranean Sea (SEMS) is a warm, sunlit marine environment with ultra-oligotrophic conditions, resulting in low bacterial and phytoplankton biomass and low productivity[11,12,13]. Phytoplankton productivity is considered either N-limited[14] or N and P co-limited[15, 16], whereas bacteria are either P-limited[17] or C-limited[14] These conditions are supposedly ideal for diazotrophy to occur by autotrophic diazotrophs that can fix C and N using the sunlight energy[18]. We focused on the possible links between sewage outburst and the development of Trichodesmium bloom and corresponding microbial community structure. To this end, we followed a Trichodesmium erythraeum bloom in the coastal water of the SEMS which was triggered by an outburst of municipal sewage.
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