Es­pe­cially the mi­cro­bial part of the car­bon cycle is af­fected

Deep-seabed mining is considered a way to address the increasing need of rare metals. However, the environmental impacts are considered to be substantial but remain largely unknown and clear regulatory standards are lacking. Researchers at the Max Planck Institute for Marine Microbiology in Bremen, Germany, together with colleagues from The Netherlands, Belgium, Portugal, Germany and the UK, now describe that mining-related disturbances have a long-term impact on carbon flow and the microbial loop at the deep seafloor. They present their results in the journal Pro­gress in Ocean­o­graphy.

The deep sea is far away and hard to en­vi­sion. If ima­gined it seems like a cold and hos­tile place. However, this re­mote hab­itat is dir­ectly con­nec­ted to our lives, as it forms an im­port­ant part of the global car­bon cycle. Also, the deep sea­floor is, in many places, covered with poly­metal­lic nod­ules and crusts that arouse eco­nomic in­terest. There is a lack of clear stand­ards to reg­u­late their min­ing and set bind­ing thresholds for the im­pact on the or­gan­isms liv­ing in af­fected areas.

Min­ing can re­duce mi­cro­bial car­bon cyc­ling, while an­im­als are less af­fected

An in­ter­na­tional team of sci­ent­ists around Tanja Strat­mann from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in Bre­men, Ger­many, and Utrecht Uni­versity, the Neth­er­lands, and Daniëlle de Jonge from Heriot-Watt Uni­versity in Ed­in­burgh, Scot­land, has in­vest­ig­ated the food web of the deep sea­floor to see how it is af­fected by dis­turb­ances such as those caused by min­ing activ­it­ies.

For this, the sci­ent­ists trav­elled to the so-called DISCOL area in the trop­ical East Pa­cific, about 3000 kilo­metres off the coast of Peru. Back in 1989, Ger­man re­search­ers had sim­u­lated min­ing-re­lated dis­turb­ances in this man­ganese nod­ule field, 4000 metres un­der the sur­face of the ocean, by plough­ing a 3.5 km wide area of seabed with a plough-har­row. “Even 26 years after the dis­turb­ance, the plough tracks are still there”, Strat­mann de­scribed the site. Previous studies had shown that microbial abundance and density had undergone lasting changes in this area. “Now we wanted to find out what that meant for car­bon cyc­ling and the food web of this deep ocean hab­itat.”

“We looked at all dif­fer­ent eco­sys­tem com­pon­ents and on all levels, try­ing to find out how they work to­gether as a team”, de Jonge ex­plained who car­ried out the pro­ject as part of her Mas­ter’s Thesis at the NIOZ Royal Neth­er­lands In­sti­tute for Sea Re­search and the Uni­versity of Gronin­gen, The Neth­er­lands. The sci­ent­ists quan­ti­fied car­bon fluxes between liv­ing and non-liv­ing com­part­ments of the eco­sys­tem and summed them up as a meas­ure of the “eco­lo­gical size” of the sys­tem.

They found sig­ni­fic­ant long-term ef­fects of the 1989 min­ing sim­u­la­tion ex­per­i­ment. The total through­put of car­bon in the eco­sys­tem was sig­ni­fic­antly re­duced. “Es­pe­cially the mi­cro­bial part of the food web was heav­ily af­fected, much more than we ex­pec­ted”, said Strat­mann. “Mi­crobes are known for their fast growth rates, so you’d ex­pect them to re­cover quickly. However, we found that car­bon cyc­ling in the so-called mi­cro­bial loop was re­duced by more than one third.”

The im­pact of the sim­u­lated min­ing activ­ity on higher or­gan­isms was more vari­able. “Some an­im­als seemed to do fine, oth­ers were still re­cov­er­ing from the dis­turb­ance. The di­versity of the sys­tem was thus re­duced”, said de Jonge. “Over­all, car­bon flow in this part of the food web was sim­ilar to or even higher than in un­af­fected areas.”

Plough tracks are still clearly visible on the seafloor of the DISCOL area 26 years after the disturbance. (Credit: ROV-Team/GEOMAR)

Poly­metal­lic nod­ules and crusts cover many thou­sands of square kilo­metres of the world’s deep-sea floor. They con­tain mainly man­ganese and iron, but also the valu­able metals nickel, co­balt and cop­per as well as some of the high-tech metals of the rare earths. Since these re­sources could be­come scarce on land in the fu­ture – for ex­ample, due to fu­ture needs for bat­ter­ies, elec­tro­mo­bil­ity and di­gital tech­no­lo­gies – mar­ine de­pos­its are eco­nom­ic­ally very in­ter­est­ing. To date, there is no mar­ket-ready tech­no­logy for deep-sea min­ing. However, it is already clear that in­ter­ven­tions in the seabed have a massive and last­ing im­pact on the af­fec­ted areas. Stud­ies have shown that many sessile in­hab­it­ants of the sur­face of the sea­floor de­pend on the nod­ules as a sub­strate, and are still ab­sent dec­ades after a dis­turb­ance in the eco­sys­tem. Also, ef­fects on an­im­als liv­ing in the seabed have been proven.

A mined sea­floor might be more vul­ner­able to cli­mate change

The sim­u­lated min­ing res­ul­ted in a shift in car­bon sources for an­im­als. Usu­ally, small fauna feed on de­tritus and bac­teria in the sea­floor. However, in the dis­turbed areas, where bac­terial dens­it­ies were re­duced, the fauna ate more de­tritus. The pos­sible con­sequences of this will be part of de Jonge’s PhD Thesis, which she just star­ted. “Fu­ture cli­mate scen­arios pre­dict a de­crease of the amount and qual­ity of de­tritus reach­ing the sea­floor. Thus this shift in diet will be es­pe­cially in­ter­est­ing to in­vest­ig­ate in view of cli­mate change”, she looks for­ward to the up­com­ing work.

“You also have to con­sider that the dis­turb­ance caused by real deep-seabed min­ing will be much heav­ier than the one we’re look­ing at here”, she ad­ded. “De­pend­ing on the tech­no­logy, it will prob­ably re­move the up­per­most 15 cen­ti­meters of the sed­i­ment over a much lar­ger area, thus mul­tiply­ing the ef­fect and sub­stan­tially in­creas­ing re­cov­ery times.”