Re­act­ive oxy­gen im­pacts car­bon cyc­ling in tidal sands

The Wad­den Sea, stretch­ing along 500 kilo­met­ers of the North Sea shore along the coasts of Den­mark, Ger­many and the Neth­er­lands, mostly con­sists of so-called in­ter­tidal per­meable sed­i­ments – i.e. sea­floor that is flushed by sea­wa­ter in the change of tides. It is fre­quently vis­ited by seabirds, mar­ine mam­mals and re­cre­ation­ists. But this highly dy­namic hab­itat is also home to a pleth­ora of mi­crobes. They pro­cess car­bon and nu­tri­ents from the sea­wa­ter and flu­vial in­flows, mak­ing the sand a cru­cial site for or­ganic mat­ter re­min­er­al­iz­a­tion and trans­form­ing it into an enorm­ous puri­fy­ing fil­ter.

The fre­quent fluc­tu­ation between oxic and an­oxic con­di­tions (at high tide and low tide, re­spect­ively) in the sed­i­ments makes them a per­fect spot for the pro­duc­tion of re­act­ive oxy­gen spe­cies (ROS). ROS are mo­lecules that con­tain oxy­gen and are chem­ic­ally very act­ive. Their en­vir­on­mental role is mul­ti­fa­ceted: ROS can be dan­ger­ous to or­gan­isms and dam­age cell com­pon­ents, but they can also be be­ne­fi­cial for mi­cro­bial growth. Be­cause of their high activ­ity, ROS are very im­port­ant agents in the trans­form­a­tion and cyc­ling of car­bon and other sub­stances in the en­vir­on­ment and can thus have a great im­pact on the func­tion­ing of eco­sys­tems. Nev­er­the­less, they re­main poorly stud­ied in many hab­it­ats – amongst them the sandy flats of the Wad­den Sea.

A group of sci­ent­ists from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men now took a closer look at ROS in a sand­flat called Jans­sand in the Ger­man Wad­den Sea off the is­land of Spieker­oog. Olivia Bourceau, Marit van Erk and their col­leagues from the Mi­cro­sensor Group in­vest­ig­ated the ROS hy­dro­gen per­ox­ide. “We wanted to know if there is any de­tect­able hy­dro­gen per­ox­ide in the in­ter­tidal sands”, says Bourceau. “And if so, we wanted to know how this hy­dro­gen per­ox­ide im­pacts the min­er­al­iz­a­tion pro­cesses, the re­cyc­ling of or­ganic mat­ter, in these sands.”

In­deed, the team around Bourceau and van Erk de­tec­ted high con­cen­tra­tions of hy­dro­gen per­ox­ide in the in­ter­tidal sands. “We found that there is a fine bal­ance between the pro­duc­tion and de­grad­a­tion of hy­dro­gen per­ox­ide”, says co-au­thor van Erk. When the sci­ent­ists changed the in­put of oxy­gen or re­moved hy­dro­gen per­ox­ide in ex­per­i­ments, that massively im­pacted the sand-dwell­ing mi­crobes. ROS in­hib­ited the mi­croor­gan­isms in the sand, thus its re­moval boos­ted mi­cro­bial activ­ity. “The amount of ROS nat­ur­ally present in the sands re­duced the rates of the main min­er­al­iz­a­tion pro­cesses, both aer­obic res­pir­a­tion and sulfate re­duc­tion, sub­stan­tially.”

El­ev­ated ROS levels can be ex­pec­ted par­tic­u­larly dur­ing dis­turb­ance events and at oxic–an­oxic in­ter­faces – both fre­quently oc­cur­ring in in­ter­tidal per­meable sed­i­ments. The high rates of car­bon and ni­tro­gen re­min­er­al­iz­a­tion make these sed­i­ments into huge biocata­lytic fil­ters. Any changes in ROS con­cen­tra­tions thus have the po­ten­tial to dir­ectly im­pact the ef­fect­ive­ness of sands as such fil­ters and the func­tion­ing of shal­low wa­ter eco­sys­tems.

Con­sequently, ROS may play an im­port­ant and yet un­ap­pre­ci­ated role in the biogeo­chem­istry of dy­namic coastal sed­i­ments. “From our find­ings, we can con­clude that ROS have the po­ten­tial to sub­stan­tially im­pact car­bon cyc­ling in the sed­i­ments. Un­der­stand­ing the con­trols on car­bon cyc­ling is very im­port­ant for study­ing eu­troph­ic­a­tion and the im­pact of hu­man activ­ity on coastal sys­tems“, Bourceau con­cludes.

van Erk, M.R., Bourceau, O.M., Mon­cada, C. et al. Re­act­ive oxy­gen spe­cies af­fect the po­ten­tial for min­er­al­iz­a­tion pro­cesses in per­meable in­ter­tidal flats. Nat Commun 14, 938 (2023).

DOI: 10.1038/s41467-023-35818-4

• Max Planck Institute for Marine Microbiology, Bremen, Germany
• Woods Hole Oceanographic Institution, Woods Hole, MA, USA