Kelp deposition changes mineralization pathways and microbial communities in a sandy beach

Kelp deposition on sandy beaches transports enormous amounts of marine organic material to land. We investigated the impact of regular kelp deposition on the geochemistry and microbial community composition of beach sands on the island of Helgoland (North Sea). The microbial community at a beach with regular kelp deposition showed significantly higher proportions of aerobic degraders, fermenters, and sulfur cycling microorganisms. Rapid degradation of deposited kelp by this community leads to high levels of dissolved organic and inorganic carbon and nutrients, as well as a lower pH and anoxia. Aerobic respiration, fermentation, Fe(III)- and SO42- reduction and methanogenesis were very strongly enhanced, with SO42- reduction being the main process in kelp degradation. SO42- reduction rates were about one order of magnitude higher than aerobic respiration rates and increased 20-25 fold immediately upon addition of kelp, showing that the microbial community was prepared for rapid kelp degradation. The route of electrons from kelp to SO42- was not via CO and H2, as expected, but via dissolved organic compounds. O2 supply by the tides was not sufficient and reduced substances escaped from the sediment with tidal water retraction. The resulting extremely high levels of free sulfide (>10 mM) lead to abundant filamentous growth of sulfur-oxidizing bacteria largely composed of a rare oxygen-adapted Sulfurovum species lacking the expected denitrification genes. Our results show that kelp deposition profoundly disturbs the equilibrium of a beach sand ecosystem by the sudden input of organic carbon, with particularly dramatic enhancement of the sulfur cycle.