Effects of experimental warming on small phytoplankton, bacteria and viruses in autumn in the Mediterranean coastal Thau Lagoon

Type Article
Acceptance Date 2021 IN PRESS
Language English
Author(s) Courboulès JustineORCID1, Vidussi Francesca1, Soulié Tanguy1, Mas Sébastien2, Pecqueur David3, Mostajir Behzad1
Affiliation(s) 1 : MARBEC (Marine Biodiversity, Exploitation and Conservation), Univ Montpellier, CNRS, Ifremer, IRD, Montpellier, France
2 : MEDIMEER (Mediterranean Platform for Marine Ecosystems Experimental Research), OSU OREME, CNRS, Univ Montpellier, IRD, IRSTEA, Sète, France
3 : BioPIC Platform (Biology Platform of Imaging and Flow Cytometry) FR3724, Observatoire Océanologique de Banyuls-Sur-Mer Laboratoire Arago - SU/CNRS, Banyuls-sur-mer, France
Source Aquatic Ecology (1386-2588) (Springer Science and Business Media LLC) In Press
DOI 10.1007/s10452-021-09852-7
Keyword(s) Pico- and nanophytoplankton, Bacterioplankton, Virus, Warming, In situ mesocosm, Thau lagoon

To investigate the responses of a natural microbial plankton community of coastal Mediterranean waters to warming, which are still poorly known, an in situ mesocosm experiment was carried out in Thau Lagoon during autumn 2018. Several microorganisms, including virio-, bacterio-, and phytoplankton < 10 µm in size, were monitored daily and analysed using flow cytometry for 19 consecutive days in six mesocosms. Three mesocosms (control) had the same natural water temperature as the lagoon, and the other three were warmed by + 3 °C in relation to the control temperature. The cytometric analyses revealed an unexpected community dominated by picophytoplanktonic cells, including Prochlorococcus-like and Picochlorum-like cells, which had not previously been found in Thau Lagoon. The experimental warming treatment increased the abundances of nanophytoplankton, cyanobacteria, bacteria and viruses during the experiment and triggered earlier blooms of cyanobacteria and picoeukaryotes. Only the abundance of Picochlorum-like cells was significantly reduced under warmer conditions. The growth and grazing rates of phytoplankton and bacterioplankton estimated on days 2 and 8 showed that warming enhanced the growth rates of most phytoplankton groups, while it reduced those of bacteria. Surprisingly, warming decreased grazing on phytoplankton and bacteria at the beginning of the experiment, while during the middle of the experiment it decreased the grazing on prokaryote only but increased it for eukaryotes. These results reveal that warming affected the Thau Lagoon plankton community from viruses to nanophytoplankton in fall, inducing changes in both dynamics and metabolic rates.

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