INVESTIGATING STOICHIOMETRIC CONSTRAINTS AMONG STROMATOLITES AND GRAZERS IN THERMAL SPRINGS OF CUATRO CIENEGAS, MEXICO

Microbial stromatolite communities in thermal springs at Cuatro Cienegas (Mexico) have extremely high C:P and N:P ratios, potentially indicating stoichiometric constraints on herbivores in these food webs.

In three experiments during 2001, 2002 and 2003 we manipulated the P content of stromatolite biomass by incubating stromatolites with added PO4 and comparing the P content, RNA:DNA ratio, and growth of snails (Mexithauma sp.) grazing on P-enriched stromatolites relative to unenriched controls. In a 2-week experiment in 2001, snails feeding on P-enriched stromatolite biomass had higher P-content and RNA:DNA ratio than control snails, consistent with the existence of a stoichiometric constraint on their growth. However, during longer-term experiments (2002) snails on P-enriched stromatolites grew more slowly and suffered significantly higher levels of mortality than controls. In an effort to reconcile these results, a third experiment in 2003 examined the effect of different levels of P-enrichment on snails. It is possible that this ecosystem evolved under such low phosphorus conditions that they are now very sensitive to increased nutrient levels and are on a "stoichiometric knife edge."