Diazotrophic Cyanobacteria are Associated With a Low Nitrate Resupply to Surface Waters in Lake Tanganyika

Frontiers in Environmental Science

Emmanuel A. Sweke¹  Francesco Pomati²  Robert Niederdorfer⁴  Daniel Odermatt⁴  Christian Dinkel⁴  Bernhard Wehrli⁶  Kathrin B.L. Baumann⁶  Benedikt Ehrenfels⁶  Julian Junker⁷  Maciej Bartosiewicz⁸  Ismael A. Kimirei⁹  Tumaini M. Kamulali^1,10  Athanasio Stephano Mbonde^1,10 

[1] 0DSFA, Deep Sea Fishing Authority, Zanzibar, Tanzania;Department Aquatic Ecology, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland;Department Fish Ecology and Evolution, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland;Department Surface Waters – Research and Management, Eawag, Swiss Federal Institute of Aquatic Science and Technology, Kastanienbaum, Switzerland;Department of Environmental Sciences, University of Basel, Basel, Switzerland;Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland;Institute of Ecology and Evolution, University of Bern, Bern, Switzerland;Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland;TAFIRI, Tanzania Fisheries Research Institute, Dar es Salaam, Tanzania;TAFIRI, Tanzania Fisheries Research Institute, Kigoma, Tanzania;

关键词: cyanobacterial blooms;    stratification;    oligotrophic;    dolichospermum;    diazotrophs;    phycocyanin;   

DOI  :  10.3389/fenvs.2021.716765

来源: DOAJ

【 摘 要 】

In Lake Tanganyika, blooms of nitrogen-fixing (diazotrophic) cyanobacteria emerge, when the upper water column re-stratifies after a period of upwelling and convective mixing. During this seasonal transition, diazotrophic cyanobacteria exploit the abundant phosphate and fix nitrogen after other phytoplankton taxa have consumed the available nitrate. However, it remains less clear, which mechanisms favour diazotrophic cyanobacteria under more heavily stratified conditions with lower levels of excess phosphate and persistent nitrate-depletion. Here, we collected profiles of physicochemical parameters, nutrients and photo-pigments, as well as the medium- to large-sized phytoplankton community during two lake-wide cruises to elucidate to what extent the abundance of diazotrophic cyanobacteria in Lake Tanganyika may be controlled by the nitrate resupply through the thermocline into the euphotic zone. At stations where nitrate was depleted, but phosphate remained available near the surface, high densities of diazotrophic cyanobacteria were associated with a low nitrate supply to surface waters. Our data provide first support for two conceptual scenarios, where the relative position of the thermocline and the euphotic depth may create a functional niche for diazotrophic cyanobacteria: when the upward transport of nitrate into the euphotic zone is reduced by a subjacent thermocline, diazotrophic cyanobacteria, comprising Dolichospermum and Anabaenopsis, are key players in the medium-to large-sized phytoplankton community. By contrast, a thermocline located within the euphotic zone allows for a rapid vertical transport of nitrate for a thriving nitrate-assimilating phytoplankton community that evidently outcompetes diazotrophic cyanobacteria. This study highlights that, under nitrogen-depleted conditions, diazotrophic cyanobacteria can also grow in response to a reduced nutrient resupply to the productive surface waters.

【 授权许可】

Unknown