NATURE GEOSCIENCE | VOL 2 | DECEMBER 2009 | www.nature.com/naturegeoscience 825 news & views M icrobial life lourishes at the bottom of the ocean 1–3 . Yet the microbial communities living in this deep dark environment receive no sunlight and a limited supply of easily degradable organic carbon — both of which are important sources of the energy needed by most organisms to sustain life. Inorganic compounds could theoretically supply the energy needed to support the growth of microbial communities on the sea loor 4,5 , but a clear association between speciic energy sources and biomass production has proved diicult to establish. On page 872 of this issue, Templeton and colleagues 6 show that microbes living on recently erupted basalts on the sea loor may be sustained by chemicals derived from hydrothermal venting. Underwater mountain ranges — known as mid-ocean ridges — and underwater volcanoes act as conduits for the low of magma from the mantle to the sea loor. he crystallized magma forms beds of pillow- shaped basalts (Fig. 1). hese pillow basalts are rich in the reduced forms of elements such as sulphur, iron and manganese. he exposure of these reduced compounds to cold oxygenated sea water promotes redox reactions that release free energy. If this energy is transferred to microbial cells, it can be used in the energy-requiring processes essential for life. Redox reactions involving these inorganic chemicals could therefore support microbial communities living on the basalt surfaces 4,5 . he venting of hydrothermal luids from the crust also expels dissolved and particulate forms of reduced chemical compounds, such as sulphur, iron and hydrogen, into the deep ocean. Previous studies of life on sealoor basalts have focused on establishing the presence and composition of microbial communities, rather than their energy source. hese micro- and molecular-biological studies clearly demonstrate that sealoor lavas harbour a substantial community of phylogenetically and physiologically diverse microorganisms 1–3 . his community is dominated by bacteria, and comprises at least 16 diferent taxonomic groups, including all subdivisions of the Proteobacteria 1 . Furthermore, functional gene analyses indicate that the possible metabolisms of these microorganisms could include carbon ixation, methane oxidation and formation, nitrogen ixation and iron reduction, to name but a few 2 . It has been proposed that the alteration of basalt, in particular the oxidation of reduced iron and sulphur in the basalt, provides the energy necessary for fuelling this sealoor biosphere 1,4,5 . Speciically, it has been observed that microbial biomass and community composition correlate with the age and alteration state of sealoor basalts 7 . And although biomass production was not speciically measured, laboratory experiments have demonstrated that iron-oxidizing bacteria can live on basalt glass, and increase basalt-weathering rates 8 . However, more direct evidence linking these geochemical reactions with biomass production is limited. Templeton and colleagues 6 set out to establish the geochemical processes associated with the growth of microbial communities in basaltic ocean crust. hey examined young fresh basalt surfaces recovered from the Loihi seamount (Fig. 1), a highly active sealoor volcano of the coast of Hawaii. his volcano continually erupts lava as well as vast quantities of dissolved iron, manganese and carbon dioxide. Using a combination of electron microscopy, spatially resolved X-ray microprobe mapping and X-ray absorption spectroscopy, the researchers found extensive microbial bioilms coating the surfaces of these sealoor basalts 6 . hese bioilms were encrusted in iron- and manganese-oxide minerals, which they suggest originated from hydrothermal venting, based primarily on two observations. First, they found little evidence for weathering of the basalts where bioilms formed. Second, the substantial BIOGEOCHEMISTRY Life in the deep sea Volcanic rocks on the sea floor are home to diverse and abundant microbial communities. Microscopic and spectroscopic analyses suggest that iron and manganese derived from hydrothermal venting support microbial colonization of the ocean crust. Cara M. Santelli Figure 1 | Basaltic pillow lavas on the sea floor at Loihi seamount, Hawaii. Templeton and colleagues 6 analysed the surface of recently erupted basalt at Loihi seamount. They found extensive microbial biofilms encased by iron and manganese crusts, and suggest that iron and manganese derived from hydrothermal venting provides the energy necessary for microbial colonization of these young basalt rocks. Image courtesy of the FeMO project funded by the National Science Foundation; Katrina J. Edwards is the principal investigator. © 2009 Macmillan Publishers Limited. All rights reserved