Growing rocks: Implications of lithification to microbial community ecology in a desert stream

Background/Question/Methods

Lithifying microbial communities have left their signature on Earth’s rock record for over 3.4 billion years. Formed by interactions between microbes and minerals, these microbial communities (“stromatolites”), are regarded as important players in paleo-biogeochemical cycles. While not as abundant on Earth’s surface today as in previous eras, extant lithifying microbial communities (“microbialites”) provide unique opportunities to study microbe-mineral interactions and better understand the past environments in which the fossilized stromatolites formed. In this project, we use comparisons of lithifying and non-lithyfying microbial communities to determine how calcium carbonate deposition influences biogeochemical cycling. All microbes need nutrients and energy for growth; indeed, nutrients are often a factor limiting microbial growth. In Cuatro Cienegas, MX, microbes live in environments with extremely low levels of phosphorus (P). We hypothesize that calcium carbonate deposition within lithifying microbial communities can sequester bioavailable phosphorus (P), and therefore expect the growth of microbialites to be P-limited. To test our hypothesis, we first compared nutrient limitation in lithifying and non-lithifying microbial communities in Río Mesquites, Cuatro Ciénegas, using in situmesocosms and bioassays. Then, we directly tested the effects of calcification on oncoid microbialite nutrient cycling.

Results/Conclusions

We found that lithifying microbialites were indeed P-limited, while non-lithifying, benthic microbial communities tended towards co-limitation by nitrogen (N) and P. Using in situ mesocosms with microbialites, we found that photosynthesis and aerobic respiration responded positively to P additions, but that the heterotrophic microbes were co-limited by organic carbon (C) and P (P<0.05). C additions also caused shifts in bacterial community composition based on analysis of terminal restriction fragments of 16S rRNA genes. Unexpectedly, calcification rates increased with C additions (P<0.05), but not with P additions. This suggests that when dissolved organic carbon is available, sulfate reduction may be an important pathway for calcification. Experimental reductions to calcification rates caused changes to microbial biomass carbon and phosphorus concentrations (P<0.01 and P<0.001, respectively), although shifts depended on whether calcification was decreased abiotically or biotically.  These results show that resource availability does influence microbialite formation and that lithification may promote phosphorus limitation, however, further investigation is required to understand the mechanism by which the later occurs.