Abstract
During the summer of 2017, recurrent extensive blooms of the diazotrophic cyanobacterium Trichodesmium invaded the beaches and coastal waters of the Canary Islands, causing great social alarm. Some local media and public sectors ascribed, without any strong scientific evidence, the origin and reactivation of these blooms to untreated sewage outfalls distributed along the coasts. In order to test whether sewage outfalls could have any influence on the metabolic activity of Trichodesmium, we performed 13C and 15N2 uptake experiments with colonies experiencing three different bloom development stages, incubated both with clear seawater and sewage water from an outfall south of Gran Canaria island. Our results showed that sewage outfalls did not promote any increase in dinitrogen (N2) fixation in Trichodesmium, supporting the hypothesis that decaying blooms were generated offshore and transported shoreward by local currents and winds, accumulating mostly leeward of the islands. The combination of unusually warm seawater temperatures, enhanced and sustained stratification of the upper water column and recurrent dust deposition events would have favored the development of the Trichodesmium blooms, which lasted for at least four months.
Highlights
Trichodesmium is a colonial filamentous cyanobacterium capable of atmospheric dinitrogen ( N2) fixation
When all limiting resources are depleted, Trichodesmium cells induce apoptosis, referred to as programmed cell death (PCD)[19], a process that can make Trichodesmium blooms almost disappear in a matter of hours to days[20]
Our experiments showed that N 2 fixation rates in clear water incubations were not significantly different from those of sewage incubations (Fig. 3, t-test p = 0.98), which indicates that sewage did not promote N 2 fixation in Trichodesmium
Summary
Trichodesmium is a colonial filamentous cyanobacterium capable of atmospheric dinitrogen ( N2) fixation. We collected Trichodesmium colonies at different stages of development of the bloom (Fig. 2), and performed incubations with clear seawater and wastewater from an urban sewage outfall to the south of Gran Canaria Island and examined its impact on N 2 and carbon fixation rates. We incubated Trichodesmium from three different bloom development stages (sparse, slick and PCD-like, Fig. 2) with clear and sewage waters.
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