Microbial Surface Biofilm Responds to the Growth-Reproduction-Senescence Cycle of the Dominant Coral Reef Macroalgae Sargassum spp.

Bettina Glasl,David G Bourne,Pedro R Frade,Tania Aires,Nicole S Webster,Jasmine B Haskell,Ester A Serrão

doi:10.3390/life11111199

Bettina Glasl, David G Bourne + Show 5 more

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https://doi.org/10.3390/life11111199

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Macroalgae play an intricate role in microbial-mediated coral reef degradation processes due to the release of dissolved nutrients. However, temporal variabilities of macroalgal surface biofilms and their implication on the wider reef system remain poorly characterized. Here, we study the microbial biofilm of the dominant reef macroalgae Sargassum over a period of one year at an inshore Great Barrier Reef site (Magnetic Island, Australia). Monthly sampling of the Sargassum biofilm links the temporal taxonomic and putative functional metabolic microbiome changes, examined using 16S rRNA gene amplicon and metagenomic sequencing, to the pronounced growth-reproduction-senescence cycle of the host. Overall, the macroalgal biofilm was dominated by the heterotrophic phyla Firmicutes (35% ± 5.9% SD) and Bacteroidetes (12% ± 0.6% SD); their relative abundance ratio shifted significantly along the annual growth-reproduction-senescence cycle of Sargassum. For example, Firmicutes were 1.7 to 3.9 times more abundant during host growth and reproduction cycles than Bacteroidetes. Both phyla varied in their carbohydrate degradation capabilities; hence, temporal fluctuations in the carbohydrate availability are potentially linked to the observed shift. Dominant heterotrophic macroalgal biofilm members, such as Firmicutes and Bacteroidetes, are implicated in exacerbating or ameliorating the release of dissolved nutrients into the ambient environment, though their contribution to microbial-mediated reef degradation processes remains to be determined.

Highlights

A consistent feature of many degraded coral reefs is the shift from a coral- to an algal-dominated benthos [1,2,3], which is associated with a dramatic reduction in coral biodiversity and loss of reef ecosystem resilience [4,5]
A total of 201,818 reads were obtained for the 16S rRNA amplicon sequencing dataset of the 30 Sargassum spp. biofilm samples corresponding to 8483 zero-radius operational taxonomic units
Alpha diversity estimates of the bacterial community associated with the Sargassum spp. biofilm remained stable throughout the three different life stages of Sargassum spp. (Figure 1a)

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Introduction

A consistent feature of many degraded coral reefs is the shift from a coral- to an algal-dominated benthos [1,2,3], which is associated with a dramatic reduction in coral biodiversity and loss of reef ecosystem resilience [4,5]. Algal-derived DOC stimulates an increase in the planktonic microbial biomass and energy usage through the growth of copiotrophic microbes [8,9,11], which in return can lead to an increase in coral mortality [6,7,12,13]. This microbe-mediated coral mortality frees space on the reef benthos, facilitating further algal growth [9]. While the interplay between macroalgae proliferation and the microbialization of coral reef seawater is well understood, less is known about the compositional and functional dynamics of the macroalgal-associated microbial biofilm and its interrelations to changes in the reef ecosystem

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