Marine Microbial Food Web Networks During Phytoplankton Bloom and Non-bloom Periods: Warming Favors Smaller Organism Interactions and Intensifies Trophic Cascade.

Francesca Vidussi,Thomas Trombetta,Marco Scotti,Behzad Mostajir,Cécile Roques

doi:10.3389/fmicb.2020.502336

Abstract

Microbial food web organisms are at the base of the functioning of pelagic ecosystems and support the whole marine food web. They are very reactive to environmental changes and their interactions are modified in response to different productive periods such as phytoplankton bloom and non-bloom as well as contrasted climatic years. To study ecological associations, identify potential interactions between microorganisms and study the structure of the microbial food web in coastal waters, a weekly monitoring was carried out in the Thau Lagoon on the French Mediterranean coast. The monitoring lasted from winter to late spring during two contrasting climatic years, a typical Mediterranean (2015) and a year with an extreme warm winter (2016). Correlation networks comprising 110 groups/taxa/species were constructed to characterize potential possible interactions between the microorganisms during bloom and non-bloom periods. Complex correlation networks during the bloom and dominated by negative intraguild correlations and positive correlations of phytoplankton with bacteria. Such pattern can be interpreted as a dominance of competition and mutualism. In contrast, correlation networks during the non-bloom period were less complex and mostly dominated by tintinnids associations with bacteria mostly referring to potential feeding on bacteria, which suggests a shift of biomass transfer from phytoplankton-dominated food webs during bloom to more bacterioplankton-based food webs during non-bloom. Inter-annual climatic conditions significantly modified the structure of microbial food webs. The warmer year favored relationships among smaller group/taxa/species at the expense of large phytoplankton and ciliates, possibly due to an intensification of the trophic cascade with a potential shift in energy circulation through microbial food web. Our study compares a typical Mediterranean spring with another mimicking the prospected intensification of global warming; if such consideration holds true, the dominance of future coastal marine ecosystems will be shifted from the highly productive herbivorous food web to the less productive microbial food web.

Highlights

The microbial food web encompasses several microorganisms and plays a pivotal role in marine ecosystems as it controls energy as well as organic and inorganic matter transfer either to higher trophic levels or to the water-dissolved pool
Previous works investigated microbial food web interactions analyzing single networks representative of the bloom period while the present study aims at detecting differences between bloom and non-bloom periods of two distinct climatic years which one was warmer
(2016) than the other (2015), taking into account the consequences of this warming through the comparison of spring networks of these years. It focuses on the comparison between several networks resulting from the interactions among 110 groups/taxa/species of planktonic microorganisms, including virioplankton, bacterioplankton, phytoplankton, and protozooplankton grazers. It analyses the structure of microbial food webs components during bloom periods in 2015, a typical Mediterranean climatic year, and during bloom and non-bloom in 2016 which was characterized by an exceptionally warm winter

Summary

Introduction

The microbial food web encompasses several microorganisms (e.g., virio-, bacterio-, phyto-, and protozooplankton) and plays a pivotal role in marine ecosystems as it controls energy as well as organic and inorganic matter transfer either to higher trophic levels or to the water-dissolved pool (e.g., dissolved organic carbon: DOC). Heterotrophic protists are the main consumers of both bacteria and phytoplankton, and they can actively transfer energy to higher trophic levels (Weisse et al, 1990; Calbet and Landry, 2004; Calbet and Saiz, 2005) They contribute significantly to the dissolved pool through excretion as well as egestion and dissolution of fecal material (Fuhrman, 1999). Viruses often play an underestimated role in carbon cycling They regulate matter circulation through cell lysis, supplying energy for bacterial production. The transfer of energy through the microbial food web is controlled by a multiple combination of interactions and associations between different microorganisms called microbial network

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