Reductive microbial dissolution of manganese nodules as a possible hazard of deep-sea mining

Helgoland Marine Research, May 2019

The microbial lysis of deep-sea nodules as a possible result of large-scale, deep-sea mining is considered. It is assumed that the Mn (IV) and Fe (III) compounds of the manganese nodules are reduced by the numerous aerobic bacteria at the sediment/water interface as soon as the adjacent nodule area is buried by sedimentation of the disturbed deposits and the organic-rich debris from the blooming surface plankton. Intensive mineralization processes in the resettled sediments cause oxygen depletion. Subsequently, the aerobic (and anaerobic) microorganisms will switch to Mn (IV) and Fe (III) oxides as alternative electron acceptors in order to continue their energy-conserving (ATP synthesis) reactions (anaerobic respiration). The higher the amount of decomposable organic matter, the more intensive are these processes. Consequently, buried manganese nodules may be dissolved, thereby liberating mobile Mn (II), Fe (II) and several trace elements (Ni, Cu, Co and others). This possible hazard and its ecological consequences should be evaluated carefully before deep-sea mining is started on a large scale.

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Reductive microbial dissolution of manganese nodules as a possible hazard of deep-sea mining

HELGOLANDER MEERESUNTERSUCHUNGEN Helgol~nder Meeresunters. C. Schiitt & J. C. G. Ottow Universit~t Hohenheim Stuttgart-Hohenheim- Federal Republic of Germany The microbial lysis of deep-sea nodules as a possible result of large-scale, deep-sea mining is considered. It is assumed that the Mn (IV} and Fe (III) compounds of the manganese nodules are reduced by the numerous aerobic bacteria at the sediment/water interface as soon as the adjacent nodule area is buried by sedimentation of the disturbed deposits and the organic-rich debris from the blooming surface plankton. Intensive mineralization processes in the resettled sediments cause oxygen depletion. Subsequently, the aerobic (and anaerobic) microorganisms will switch to Mn (IV) and Fe (III) oxides as alternative electron acceptors in order to continue their energy-conserving (ATP synthesis) reactions (anaerobic respiration). The higher the amount of decomposable organic matter, the more intensive are these processes. Consequently, buried manganese nodules may be dissolved, thereby liberating mobile Mn (II), He (II) and several trace elements (Ni, Cu, Co and others). This possible hazard and its ecological consequences should be evaluated carefully before deep-sea mining is started on a large scale. - I N T R O D U C T I O N A p p r o x i m a t e l y 100 years ago, the first m a n g a n e s e n o d u l e s w e r e d i s c o v e r e d by the British r e s e a r c h v e s s e l " C h a l l e n g e r " on the botton of the Atlantic O c e a n at d e p t h s b e t w e e n 4000 and 5000 m. H o w e v e r , not until the mid-sixties was the e c o n o m i c i m p o r t a n c e of these - in fact p o l y m e t a l l i c - concretions r e c o g n i z e d (Mero, 1962) . B e c a u s e of the r e l a t i v l y h i g h content of nickel, copper, cobalt and m a n g a n e s e (Table 1), these m i n e r a l accretions m a y b e c o m e i n c r e a s i n g l y important, since the terrestrial sources of t h e s e e s s e n t i a l e l e m e n t s m a y run short w i t h i n a v e r y l i m i t e d p e r i o d of time. At present, the r e s e r v e s of C u h a v e b e e n e s t i m a t e d 38 times, those of Ni e v e n 200 times h i g h e r t h a n the actual c o n t i n e n t a l supplies (Schneider, 1977) . Based on c o n s e r v a t i v e calculations the resources r e c o v e r a b l e from the Pacific n o d u l e b e l t h a v e b e e n e s t i m a t e d to account for 170 m i l l i o n tons of nickel, 155 m i l l i o n tons of copper, 30 m i l l i o n tons of cobalt and e v e n 3.7 b i l l i o n tons of m a n g a n e s e (Pellerer, 1975). T h e e c o n o m i c signific a n c e of t h e s e p o t e n t i a l sources of r a w m a t e r i a l has b e e n r e c o g n i z e d and i n t e r n a t i o n a l associations h a v e b e e n e s t a b l i s h e d (Hubred, 1975; Mero, 1977; Schneider, 1977) . Such consortia are r e q u i r e d to d e v e l o p a p p r o p r i a t e m i n i n g t e c h n o l o g i e s as w e l l as to share the c o n s i d e r a b l e e c o n o m i c a l risks. M e a n w h i l e , different pilot projects for t e s t i n g the d e e p sea m i n i n g systems w i t h m i n i n g ships such as " S e d c o 445" w e r e u n d e r t a k e n successfully d u r i n g 1978 { Summerer, 1978 ). T h e r e s e e m s to be no doubt that w e m a y e x p e c t d e e p - s e a m i n i n g on a l a r g e scale w i t h i n the n e x t 10-30 years. C. Schfitt & J. C. G. Ottow DISTRIBUTION AND C O M P O S I T I O N M a n g a n e s e n o d u l e s are w i d e l y d i s t r i b u t e d (25-80 % coverage) on the bottom of the d e e p sea, p a r t i c u l a r l y i n the Pacific Ocean. G e n e r a l l y s p e a k i n g , they m a y occur at almost all depths a n d l a t i d u d e s both i n the oceans a n d i n certain fresh-water lakes. However, the most a b u n d a n t deposits are f o u n d on the surface of the p e l a g i c s e d i m e n t s i n the d e e p sea (Hubred, 1975; Cronan, 1978) . These areas of g e n e s i s are characterized by (a) a n e x t r e m e l y low s e d i m e n t a t i o n rate, (b) a relatively low biological productivity a n d (c) b y a c o n t i n u o u s s u p p l y of o x y g e n b y (arctic) bottom waters. These features are u s u a l l y restricted to the central regions of oceans far from t e r r i g e n i c (organic) supply. It is only at a low m i n e r a l i z a t i o n rate, a restricted s u p p l y of M n a n d Fe, a n d a p e r m a n e n t aerobic e n v i r o n m e n t {reflected b y the relatively h i g h Eh of + 400 to + 565 mV) that n o d u l e s m a y develop at a slow, b u t steady accretion rate (a few ram/106 years). These ecological prerequisites m a y e x p l a i n w h y the most a b u n d a n t (and e c o n o m i c a l l y interesting) deposits h a v e b e e n recorded north a n d south of the equator i n the Pacific Ocean. So far, the m i n i m u m a b u n d a n c e of n o d u l e s r e q u i r e d to start m i n i n g has b e e n e s t i m a t e d as Dissolution of manganese nodules 8000 tons/km 2 with metal contents (weight % of dry material) of 1.25-1.40 % Ni, 0.05-1.35 % Cu, 0.18-0.25 % Co and 25-28 % Mn (Pellerer, 1975~ Cronan, 1978). These trace e l e m e n t s are intimately intermixed with the concentric layered amorphous and (crypto)-crystalline Mn (IV) and Pe (III) oxides as well as with other Mn and Be phases (Crerar & Barnes, 1974; Hubred, 1975; Priedrich et al., 1977) . The interior of the manganese nodules is characterized by high porosity (Halbach et al., 1975) and a great variety of small-scale columnar cups orientated with their long axes in the radial plane of the nodules (Hubred, 1975~ Cronan, 1978) . This high porosity and internal surface area makes post-depositional redissolution and migration of metals possible. Because of the lower activation energy required to reduce Mn (IV) compounds (mainly birnessite = delta-MnO 2 and todorokite) compared with the amorphous and crystalline Fe (III) oxides (McKenzie, 1972~ Crerar & Barnes, 1974) , we may expect the Mn (IV) components to be dissolved much more rapidly and earlier than the corresponding Fe (III) structures. Such a selective dissolution of different constituents would disintegrate the nodule considerably. Indeed, such internal "erosional features" have been observed repeatedly (Hubred, 1975; Cronan, 1978) . ROLE OF MICROORGANISMS IN GENESIS In general, areas with high nodule densities have lower metal contents and vice versa. However, nodules of a single restricted region may also differ considerably in their chemical composition, although some remarkable relationships between certain elements have been observed (Friedrich et al., 1973~ Marchig and Gundlach, 1976~ Friedrich et al., 1977~ Halbach et al., 1979) . Thus, Fe, Mg (and Co) on the one hand, and Mn, Ni, Cu and Co on the other, are usually well correlated. The highly significant correlation between Mn and Ni (with a Mn/Ni ratio of 23) indirectly supports the view that positively charged ions (such as Ni +) are adsorbed physico-chemically onto the primarily negatively loaded Mn colloids by extracting these cations from the sea water. Although this scavenging theory of Goldberg {1954) has been generally accepted (Crerar & Barnes, 1974) , the mechanism of Mn (IV) oxide formation has not yet been completely elucidated. This is caused by the fact that Mn (II) is both undersaturated (with an average concentration of 2 ppb) and chemically stable (at pH 7.7-8.1 and Eh values between +400 to 565 mV) (Crerar & Barnes, 1974~ Hartmann et al., 1975; Miiller, 1977) at the pelagic sediment/water interface. Even if Mn (II) is extracted from the undersaturated sea water by (weakly) positively charged Fe {III}colloids and mineral-sorbed particles, the oxidation and precipitation of Mn (II) may be explained only by assuming an autocatalytic surface effect (caused by an increased Eh) (Crerar & Barnes, 1974) and/or by the biocatalytic activity of certain bacteria (Ehrlich et al., 1972~ Kuznetsov, 1975~ Schfitt & Ottow, 1977; Nealson, 1978) . The possible role of microorganisms in the formation of iron-manganese concretions (nodules) has been a point of considerable discussion ever since the suggestions of Butkewitsch (1928) . These arguments can be ascribed to the poor knowledge of the microflora in the sediment/water interface of the pelagic sediments. Differentiated quantitative and qualitative microbiological population studies with freshly taken, non-contaminated manganese nodules and sediment samples showed that this area is inhabited abundantly by a specific, mainly aerobic bacterial flora, lacking actinomycetes and fungi (Table 2) {Schiitt & Ottow, 1977, 1978~ Schiitt, 1979). These studies further clearly d e m o n s t r a t e d the p r e s e n c e of a great n u m b e r of bacteria, p o t e n t i a l l y c a p a b l e of p r e c i p i t a t i n g M n (II) to M n (IV}, both i n a n d on the n o d u l e s as well as i n the s u r r o u n d i n g w a t e r a n d sediments. However, n o n e of the isolated o r g a n i s m s d e v e l o p e d w i t h M n (II) only, b u t all p r e c i p i t a t e d M n (IV) r e a d i l y i n the p r e s e n c e of a low a m o u n t of glycerol as the c a r b o n source (Schiitt, 1979}. A portion of the m a n g a n e s e - p r e c i p i t a t i n g b a c t e r i a (such as Vibrio spp.) e v e n dissolved n o d u l e material b y u s i n g M n (IV) as a n alternative electron acceptor if the a m o u n t of organic c a r b o n was i n c r e a s e d significantly. Obviously, it is the a m o u n t of d e g r a d a b l e organic matter rather t h a n the actual p r e s e n c e of these bacteria that d e t e r m i n e w h e t h e r M n (II} p r e c i p i t a t i o n or M n (IV) r e d u c t i o n will occur. ROLE OF ORGANIC SUPPLY In F i g u r e 1 the conditions a n d processes i n the neritic a n d p e l a g i c s e d i m e n t s are s u m m a r i z e d a n d compared. There is no doubt that the differences are c a u s e d p r i m a r i l y by the significant v a r i a t i o n in the s u p p l y of d e g r a d a b l e organic matter. In the neritic zone, a c o n t i n u o u s s u p p l y of organic m a t t e r (due to c o n t i n e n t a l erosion a n d h i g h p r i m a r y (Low sedimentation rates) +o,. in , + EO.UATOR-~ I~reLhigh prim, production (reLhigh sedlmentatlonrates} . + + .~J=LQRE~ NERITIC ~ l l h , J high primary production depth (m) ~ 4 1 m ~ = increasing ~eote~ sedimentation rates % 6 L ' N . b "+ ~176 HEM]PELAGIC j rel. Low prim.productionPELAGIC ~O~ @~'Aer~ 9t ~' "~,~.:",'~./'/t"~,~~OO,of r~;a~t~i"on- ~"~'o~te-~~t.%& 1~ ~ i I ~ Fe2? Mn2. (te:igenic supp[y) Fig. 1. Fate of the Mn and Fe compounds in the neritic zone compared with the pelagic sediments. In the neritic zone Mn (IV) and Fe (III) oxides (and hydroxides) are reduced microbiologically by acting as alternative electron acceptors. In the pelagic sediments far from the continental slope Mn (If) and Fe (II, III) compounds are immobilized (nodule genesis) by chemical and/or microbial activity9 These opposite processes are essentially caused by the significant differences in organic supply (energy source for mineralization) * Population densities (highest a n d l o w e s t values recorded) are e x p r e s s e d in Most Probable N u m b e r p e r g o v e n - d r y material (MPN/g) ~for details see Schfitt & Ottow (1977) a n d Schiitt (1979) "* N e i t h e r fungi a n d actinomycetes, nor s u l f a t e - r e d u c i n g b a c t e r i a could be detected; nitrifying b a c t e r i a w e r e irregularly distributed **" E/S-Type = ellipsoidal to s p h e r o i d a l m o n o - n o d u l e ; Kg-Type = ellipsoidal to s p h e r o i d a l poly-nodulel for classification consult M e y e r (1973) + Fe(III)-NH4-citrate w a s s e l e c t e d as a m o d e l c o m p o u n d for the capacity to m i n e r a l i z e Fe-organic c h e l a t e s ( = F e - p r e c i p i t a t i n g bacteria) + + Bacteria, potentially c a p a b l e of p r e c i p i t a t i n g M n (II) (estimated semi-quantitatively) ~most a b u n d a n t r e p r e s e n t a t i v e s w e r e i d e n t i f i e d as m e m b e r s of Pseudomonas spp., A1callgenes spp. a n d Vibrlo spp. {see Schiitt & Ottow, 1978; Schiitt, 1979} production) will l e a d to i n t e n s i v e m i n e r a l i z a t i o n processes. As soon as o x y g e n has b e e n r e s p i r e d , a e r o b i c a n d a n a e r o b i c b a c t e r i a s w i t c h to nitrate, M n (IV) a n d Fe (III) as a l t e r n a t i v e e l e c t r o n a c c e p t o r s in o r d e r to c o n t i n u e t h e i r e n e r g y c o n s e r v i n g r e a c t i o n s (ATP synthesis) (Trimble & Ehrlich, 1968; O t t o w & G l a t h e , 1973; M u n c h & Ottow, 1977; M u n c h et al., 1978) . T h e s e p r o c e s s e s m a y b e s u m m a r i z e d as follows. B i o l o g i c a l o x i d a t i o n (-- d e h y d r o g e n a t i o n ) : o r g a n i c m a t t e r m e t a b o l i c activity d e h y d r o g e n a s e s ~ H+ 4- e 4- b i o m a s s 4- A T P 4- p r o d u c t s M n (IV) or Fe (III) o x i d e s as H - a c c e p t o r s ( h y d r o g e n a t i o n b y reductases); M n O 2 4- 2e ? 4H + ~ M n (II) ? 2I-I20 F e ( O H ) 3 ? e 4 - 3 H + ) F e ( I I ) ? O x y g e n d e p l e t i o n a n d t h e a c c u m u l a t i o n of r e d u c e d p r o d u c t s a r e r e s p o n s i b l e for the r a p i d drop in Eh (in the r a n g e of ? 190 to 220 mV). It is i m p o r t a n t to r e a l i z e that M n (IV} a n d Fe (III) are r e d u c e d d i r e c t l y b y m e t a b o l i c activity r a t h e r t h a n i n d i r e c t l y b y a l o w e r e d r e d o x p o t e n t i a l . A drop in Eh is the r e s u l t of m i c r o b i a l r e d u c t i o n p r o c e s s e s a n d not a m e c h a n i s m of r e l e v a n t c h e m i c a l transformations. Its s i g n i f i c a n t role is r e s t r i c t e d to the c r e a t i o n of a c e r t a i n r e d o x l e v e l that m a y s u p p o r t the s t a b i l i t y of M n (II) or Fe (II) c o m p o u n d s . B e y o n d a t r a n s i t i o n a l a r e a (with m i n o r or m o d e r a t e o r g a n i c s u p p l y a n d thus r e l a t i v e l y low m i c r o b i o l o g i c a l activity) w e u s u a l l y find the central p e l a g i c zone (red oozes) w i t h e x t r e m e l y low o r g a n i c s e d i m e n t a t i o n rates (Volkov et al., 1975) . T h e s e a r e a s w i t h e x t e n d e d fields of m a n g a n e s e n o d u l e s are r e l a t i v e l y poor in o r g a n i c m a t t e r ( 0 . 1 - 0 . 3 % o r g a n i c carbon) (Mfiller, 1977), strictly a e r o b i c a n d c h a r a c t e r i z e d b y e x t r e m e l y low o x y g e n - c o n s u m p t i o n rates. T h e r e s t r i c t e d m i c r o b i a l activity is s u p p o r t e d b y low t e m p e r a t u r e s (nearly -- 1 ~ a n d the e n o r m o u s h y d r o s t a t i c p r e s s u r e (5000-m d e p t h c o r r e s p o n d s to 500 bar). Due to the low b a c t e r i a l activity, the r e d o x p o t e n t i a l r e m a i n s at l e v e l s of 4-400 a n d 4-565 m V (Miiller, 1977) . Such c o n d i t i o n s s h o u l d b e c o n s i d e r e d as the m a i n p r e r e q u i s i t e for n o d u l e formation. T h e e q u a t o r r e g i o n , however, is c h a r a c t e r i z e d b y r e l a t i v e l y h i g h p r i m a r y p r o d u c t i o n a n d thus h i g h e r s e d i m e n t a t i o n rates of o r g a n i c m a t t e r w h i c h result in e n h a n c e d microb i a l activity a n d r e d u c t i o n processes. C o n s e q u e n t l y no n o d u l e s m a y b e g e n e r a t e d in this p a r t i c u l a r a r e a of the c e n t r a l ocean. NODULE LYSIS A S A N E N V I R O N M E N T A L I M P A C T In F i g u r e 2 the o r i g i n a l d e e p - s e a situation (A) is c o m p a r e d w i t h the c o n d i t i o n s (B) that m a y r e s u l t from d e e p - s e a mining. A p a r t from the direct i n f l u e n c e of d i s c h a r g e d b y products, p r o l o n g e d d e e p - s e a m i n i n g m a y i n d u c e at l e a s t two side effects. Firstly, the d i s t u r b e d s e d i m e n t s m a y p o l l u t e the u p p e r w a t e r c o l u m n with c l a y s a n d p a r t i c u l a t e o r g a n i c m a t t e r (POM), w h i c h settle a g a i n only v e r y slowly. T h e t u r b i d i t y w i l l d e s t r o y m a r i n e p h y t o - a n d z o o p l a n k t o n as w e l l as the sessile f a u n a in the a r e a close to the h a u l i n g p l a n t (Thiel, 1975, 1978; A m o s et al., 1977) . H o w e v e r , t h e r e m a y b e a s e c o n d p r o b l e m that s h o u l d b e o u t l i n e d here. O n c e d i s t u r b e d , the s u s p e n d e d o r g a n i c m a t t e r will b e m i n e r a l i z e d r a p i d l y a n d the l i b e r a t e d nutrients w i l l p r o b a b l y s u p p o r t a p l a n k t o n b l o o m i n the surface regions (increased p r i m a r y production). U n m i n e d m a n g a n e s e n o d u l e s w i l l b e b u r i e d b y s e t t l i n g of the s u s p e n d e d material, w h i c h is significantly e n r i c h e d b y p h y t o p l a n k t o n i c debris. This a d d i t i o n a l e n e r g y s u p p l y will i n c r e a s e the activity of the m i c r o o r g a n i s m s a n d the rate of o x y g e n c o n s u m p t i o n . Because of o x y g e n depletion, the b a c t e r i a will switch to other electron acceptors such as NO~, M n (IV) a n d Fe (III) c a u s i n g denitrification as w e l l as the reductive dissolution of m a n g a n e s e n o d u l e s (Fig. 2B). The h i g h e r the a m o u n t of d e c o m p o s a b l e organic debris, the more i n t e n s i v e a n d complete is the lysis of b u r i e d m a n g a n e s e n o d u l e s i n the s e d i m e n t . T h e s e i n t e n s i v e m i n e r a l i z a t i o n processes cause the Eh to drop rapidly. At this stage, the processes a n d conditions i n the u p p e r layers of the d i s t u r b e d s e d i m e n t s are c o m p a r a b l e w i t h the neritic situation at the c o n t i n e n t a l slope. A l t h o u g h the m o b i l i z e d M n (II) a n d Fe (II) c o m p o u n d s m a y b e r e p r e c i p i t a t e d i n the aerobic area n e a r the h a u l i n g plant, the fate of the l i b e r a t e d trace e l e m e n t s is u n k n o w n . A low primary production nutrient B increased primary production h ~ . . , ~ . tow sedimentation rotes hi " " tow decomposition of organic sources Eh +400-500mV, aerobic Fe2* e Nn2. 2e [CH20]+ H20 ManinoeerroolibzJoretiospn. 2H? +2e+mO~ass+CO2--*dropEh(,~d?OmV) ~. no nodule [ysis nodule [ysis undisturbed sediments disturbed sediments caused by d e e p - s e a m i n i n g A l t h o u g h our hazardous i n t e r p r e t a t i o n is largely hypothetical, its ecophysiological b a c k g r o u n d is d e r i v e d from w e l l e s t a b l i s h e d processes that occur i n organic-rich, h y d r o m o r p h i c soils (Ottow & Glathe, 1973; M u n c h & Ottow, 1977) . We therefore wish to attract a t t e n t i o n to the possible e c o n o m i c a l a n d ecological risks that m a y arise from d e e p - s e a m i n i n g performed on a large scale. The risks o u t l i n e d here should be experim e n t a l l y e x a m i n e d b e f o r e d e e p - s e a m i n i n g is s t a r t e d . 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C. Schütt, J. C. G. Ottow. Reductive microbial dissolution of manganese nodules as a possible hazard of deep-sea mining, Helgoland Marine Research, 443, DOI: 10.1007/BF02414769