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Principal results

Samples were analyzed for evidence of microbial activity, possibly related to the formation of microbialites. To date, onboard measurements have shown a certain degree of microbial activity, directly attached to mineral surfaces, which could be related to microbialite formation. According to activity measurements taken along the drill cores, the uppermost subsurface, 0–4 mbsf, is the most active zone. This is a common trend in reef environments because of the closeness to the photic zone inhabited by primary producing eukaryotes such as algae. Pure microbiological activity was only observed in reef cavities where prokaryotic biofilms have the right conditions to develop (Fig. F1H, F1I).

Preliminary results show that biofilms are diverse in structure and color. Figure F1H shows an association between a brown iron/​manganese crust and biofilm, and Figure F1I shows an unusual blue biofilm, which exhibited the highest degree of ATP activity (20,600 RLU). In this sample, it was possible to define spherical assemblages of carbonate minerals embedded into the microbial exopolymeric substances (EPS). Figure F3L shows DAPI-stained cells in high densities in this biofilm in conjunction with mineral precipitation, which is assumed to be calcium carbonate.

Biofilms appear to have high diversity in macroscale observations, and they are equally diverse and heterogenous in microscale resolution, as observed by SEM microscopy. Carbonate minerals appear to be closely associated with claylike minerals (Hole M0007C; Fig. F4D), carbonate microfossils (Hole M0015B; Fig. F4E), and oxidized and reduced Fe minerals, such as pyrite (Hole M0023A; Fig. F4B). The metabolic processes responsible for the precipitation of these minerals cannot be defined as yet because of the complex and diverse conditions in the microenvironments where these biofilms were found.

Some evidence for heterotrophic metabolic activity is shown by exoenzyme measurements, which vary in different biofilm samples. For instance, samples from Holes M0020A, 4.51 mbsf, and M0009D, 3.64 mbsf, show high phosphatase activity, which suggests that a heterotrophic community preferentially degrades organic-bound phosphate compounds, such as phospholipids or nucleic acids. In contrast, samples from Hole M0007B, 6.28 mbsf, show only glucosidase and aminopeptidase activity, which is evidence for the degradation and metabolization of polysaccharides and proteins.

Isolation of microorganisms from biofilm samples was performed on agar plates using a medium that is selective for heterotrophic bacteria. After 2 weeks incubation time, 10 different heterotrophic colonies were isolated (Figs. F5, F6; Table T2). From the anaerobic experiments, only one isolation was successful. Distinct groups of microorganisms are associated with the biofilm and may represent aerobic to anaerobic metabolism. SEM investigation of microbialite samples has shown evidence that anaerobic conditions must have prevailed at times. The occurrence of framboidal pyrite, well distributed in the sediment, supports a certain degree of anoxia in the environment. Some sediment samples also show a close spatial association between the mineral phase and microbes (e.g., Figs. F3J, F4E).

As a biogeographical summary of microbial activity in the Tahiti reefs, northwestern Faaa Hole M0020A and southwestern Maraa Holes M0005C, M0007B, M0007C, M0015B, and M0018A were more active than the northeastern Tiarei sites, where in many cases no living biofilm were detected in cavities along the cores.