Microbiome changes in the sponge Halichondria panicea along the Baltic Sea salinity gradient

Area of Science:

• Marine microbiology
• Ecology
• Symbiosis

Background:

• The Baltic Sea's salinity gradient impacts marine life, but effects on host-associated microbes are poorly understood.
• Marine sponges host complex prokaryotic communities vital for host health and function.
• Investigating how sponge microbiomes respond to decreasing salinity is crucial for understanding their ecological role.

Purpose of the Study:

• To analyze the microbiome of the marine demosponge *Halichondria panicea* along a declining salinity gradient in the Baltic Sea.
• To assess changes in prokaryotic community composition, diversity, and abundance in response to varying salinities.
• To investigate potential impacts on sponge health and symbiont stability.

Main Methods:

• 16S rRNA gene metabarcoding of sponge and seawater microbiomes.
• Absolute quantification of total prokaryotic sponge communities and the key symbiont *Ca. Halichondribacter symbioticus* using ddPCR.
• Sampling of *Halichondria panicea* across a salinity range from 28.3 PSU (Kattegat) to 11.4 PSU (Baltic Sea).

Main Results:

• Sponge microbiomes were location-specific and distinct from surrounding water, dominated by *Ca. Halichondribacter symbioticus*.
• At lower salinities (Eastern Baltic Sea), sponge microbiomes exhibited higher diversity and increased similarity to seawater microbiomes.
• Absolute bacterial abundance increased at low salinities, while *Ca. Halichondribacter symbioticus* remained stable; sponge body volume decreased, suggesting reduced fitness.

Conclusions:

• Decreasing salinity in the Baltic Sea alters the microbiome of *Halichondria panicea*, with increased similarity to surrounding water potentially indicating dysbiosis.
• Reduced sponge fitness observed at the lowest salinities correlates with microbiome shifts.
• This study offers initial insights into salinity-driven microbiome changes in sponges within challenging environmental gradients.