Acidification increases abundances of Vibrionales and Planctomycetia associated to a seaweed-grazer system: potential consequences for disease and prey digestion efficiency

Abstract

Ocean acidification significantly affects marine organisms in several ways, with complex interactions. Seaweeds might benefit from rising CO₂ through increased photosynthesis and carbon acquisition, with subsequent higher growth rates. However, changes in seaweed chemistry due to increased CO₂ may change the nutritional quality of tissue for grazers. In addition, organisms live in close association with a diverse microbiota, which can also be influenced by environmental changes, with feedback effects. As gut microbiomes are often linked to diet, changes in seaweed characteristics and associated microbiome can affect the gut microbiome of the grazer, with possible fitness consequences. In this study, we experimentally investigated the effects of acidification on the microbiome of the invasive brown seaweed Sargassum muticum and a native isopod consumer Synisoma nadejda. Both were exposed to ambient CO₂ conditions (380 ppm, pH 8.16) and an acidification treatment (1,000 ppm, pH 7.86) for three weeks. Microbiome diversity and composition were determined using high-throughput sequencing of the variable regions V5-7 of 16S rRNA. We anticipated that as a result of acidification, the seaweed-associated bacterial community would change, leading to further changes in the gut microbiome of grazers. However, no significant effects of elevated CO₂ on the overall bacterial community structure and composition were revealed in the seaweed. In contrast, significant changes were observed in the bacterial community of the grazer gut. Although the bacterial community of S. muticum as whole did not change, Oceanospirillales and Vibrionales (mainly Pseudoalteromonas) significantly increased their abundance in acidified conditions. The former, which uses organic matter compounds as its main source, may have opportunistically taken advantage of the possible increase of the C/N ratio in the seaweed under acidified conditions. Pseudoalteromonas, commonly associated to diseased seaweeds, suggesting that acidification may facilitate opportunistic/pathogenic bacteria. In the gut of S. nadejda, the bacterial genus Planctomycetia increased abundance under elevated CO₂. This shift might be associated to changes in food (S. muticum) quality under acidification. Planctomycetia are slow-acting decomposers of algal polymers that could be providing the isopod with an elevated algal digestion and availability of inorganic compounds to compensate the shifted C/N ratio under acidification in their food. In conclusion, our results indicate that even after only three weeks of acidified conditions, bacterial communities associated to ungrazed seaweed and to an isopod grazer show specific, differential shifts in associated bacterial community. These have potential consequences for seaweed health (as shown in corals) and isopod food digestion. The observed changes in the gut microbiome of the grazer seem to reflect changes in the seaweed chemistry rather than its microbial composition.

Keywords: Algae microbiomes; Grazer microbiomes; Invasive seaweeds; Metabarcoding; Ocean acidification; Sargassum muticum; Synisoma nadejda.

Figure 1. Schematic representation of the mesocosms experiment.

(A) Ambient (380 ppm) and (B) acidified (1,000 ppm) conditions each with four 3 L experimental units only containing 1 g wet weight (WW) S. muticum and four 3 L experimental units containing 1 g WW S. muticum along with the grazer S. nadejda were placed randomly in each CO₂ treatment. Each unit represented a replicate from which sample(s) (seaweed or seaweed and grazer) were taken.

Figure 2. Community structure.

Plot of canonical analysis of principal coordinates (CAP) based on Bray–Curtis distances calculated on square-root transformed bacterial abundances, showing the axes that best discriminate the bacterial assemblages across CO₂ levels (blue-ambient versus red-acidified), grazing by S. nadejda on S. muticum (open squares-grazed seaweed vs filled squares-non-grazed seaweed) and the gut of the isopod on a diet of S. muticum (circles).

Figure 3. Host and treatment effects on associated bacteria phyla.

Relative abundance and distribution of the bacteria phyla associated to the brown seaweed Sargassum muticum, without (No grazing) and with (grazing) Synisoma nadejda isopods, and the gut of the isopod after three weeks on a Sargassum muticum diet, under ambient (380 ppm) and elevated/acidified (1,000 ppm) CO₂ conditions.

Figure 4. Relative abundance of the genera belonging to the main bacterial phyla.

(A) Bacteroidetes, (B) Proteobacteria, and (C) Planctomycetes, associated with the brown seaweed Sargassum muticum grazed by Synisoma nadejda isopods (left side), and the gut microbiome of the isopod on a Sargassum muticum diet (right side), after three weeks under ambient (380 ppm; −CO₂) and elevated/acidified (1,000 ppm; +CO₂) CO₂ conditions.

Figure 5. Mean relative abundances of bacterial classes, and respective orders (A, Flavobacteriales and Ricketsiales; B, Bdellovibrionales and Oceanospirillales; C, Acidithiobacillales; D, Alteromonadales; E, Non-ID SJA-4 and AKAU3564 Phycisphaerae), significantly more abundant in either grazed Sargassum muticum or the gut of Synisoma nadejda.

After three weeks under ambient (380 ppm) and elevated/acidified (1,000 ppm) CO₂ conditions. Alpha = 0.05, error bars show standard error per treatment (n = 4).

Figure 6. Core communities.

Venn diagram representing the number of bacterial genera (present in at least 75% of samples) shared between the different CO₂ treatments (ambient CO₂₋ ↓ CO_2; elevated/acidified CO₂₋ ↑ CO₂) and associated to grazed Sargassum muticum (Sm + G), and the gut microbiome of the isopod Synisoma nadejda on a Sargassum muticum diet. The bar plots show the distribution of Phyla of selected intersections.

Figure 7. Mean relative abundances of associated bacterial orders.

(A and B) Sargassum muticum under grazing/non grazing influence after three weeks under ambient (380 ppm) and elevated/acidified (1,000 ppm) CO₂ conditions and (C–E) grazed Sargassum muticum and the gut of Synisoma nadejda after three weeks under ambient (380 ppm) and elevated/acidified (1,000 ppm) CO₂ conditions, that responded to acidification, but for which a significant interaction between acidification and type of sample (seaweed or grazer gut) was observed. Alpha = 0.05, error bars show standard error per treatment (n = 4).