Biological aspects of deep-sea manganese nodule formation.

Abstract

The relative contributions of biogenic and inorganic processes to manganese nodule and crust formation is still a controversial issue.Sorne pertinent activities of bacteria and foraminifera are reviewed and supplemented with new findings concerning foraminifera, xenophyophores and related, non-testate rhizopodean protozoa, as weil as polychaete worms, from manganese nodules, crusts and sediments in the Central Pacifie'.The investigations by the author, using freshly collected material with preserved cytoplasm structures, highlight the special feeding biology of the rhizopodean groups abounding on manganese nodules and crusts and in the associated sediments.Ali these rhizopodea are characterized by the accumulation of buge volumes of faecal pellets (stercomata) containing biogenic and mineral particles.There is evidence that these pellets constitute food residua digested extra-cellularly, and thus in contact with the ambient sea-water.The theory is advanced that by browsing the environment over a wide range, the rhizopodea gather, together with bacteria and other components of their diet, widely dispersed particulate metal oxides.During digestion, the accumulated particles are presumably subject to micro-zones of lowered pH and redox conditions, i.e. factors which govern the chemistry of manganese.While iron (and probably copper) is retained by the rhizopodea, manganese is dissolved but may later be reprecipitated and accreted immediately adjacent to the sites of digestion under and around the sedentary foraminifer or related organism, thereby contributing to nodule growth.According to this model, rhizopodean protozoa (and, additionally, polychaete worms that feed on them) control the concentration, fractionated dissolution and accretion of metals in oligotrophic regions of the deep sea.