2 25 Abstract 26 Animals feeding on plants (herbivorous) may have nutritional deficiencies and 27 use bacterial nitrogen fixation in guts to compensate unbalanced diets with high carbon 28 and low nitrogen. Using the acetylene reduction assay we searched for nitrogen fixation 29 in the feces from several herbivorous animals in captivity. We detected acetylene 30 reduction in feces from two African spurred tortoises, Centrochelys sulcata and in feces 31 from six Gopherus berlandieri tortoises and isolated nitrogen-fixing klebsiellas from 32 them. Additionally, we performed a gut metagenomic study with Illumina sequencing 33 from a healthy Mexican G. berlandieri tortoise, and the nif genes identified in the feces 34 microbiome matched those from Klebsiella variicola. Fecal bacterial composition from 35 tortoises was similar to that reported from other reptilian guts. 36 3 37 Introduction 38 Nitrogen fixation is a unique biochemical process carried out by nitrogenases 39 encoded by nif genes, which are found in only a few prokaryotes (1). Nitrogen fixation 40 is energetically expensive and nitrogen-fixing bacteria are in many cases minor 41 components of the microbial community that nevertheless provide a valuable ecological 42 service (2). Nitrogen-fixing bacteria are called diazotrophs (3). 43 When associated with nitrogen-fixing bacteria, insects or plants may inhabit in 44 nitrogen poor conditions. Termites contain nitrogen-fixing bacteria in their guts that 45 allow them to grow in wood, similarly wood eating beetles contain nitrogen-fixing 46 bacteria (4). Nitrogen fixation by Klebsiella variicola occurs in the fungal garden of 47 ants (5). A novel betaproteobacterium capable of fixing nitrogen is a recently described 48 symbiont that is transmitted by eggs of carmine cochineals which feed on cacti as the 49 Gopherus berlandieri tortoise does (6). Cacti have high sugar content and are nitrogen 50 poor, conditions that would be favorable for nitrogen fixation in animals that use cacti 51 for food, such as the carmine cochineal or the G. berlandieri tortoise. 52 G. berlandieri, is distributed from North Mexico (states of Coahuila, Nuevo 53 Leon and Tamaulipas) to South-central Texas in United States (7). This species 54 generally maintains restricted mobility by living for prolonged times in the same 55 burrow (8) and its diet is mainly composed of grasses, weeds and cacti. On the other 56 hand, the African spurred tortoise, Centrochelys sulcata, which is also vegetarian, lives 57 in West Africa (9). 58 Metagenomic analyses have contributed to gain a better insight on taxonomic 59 composition and functions of the gut microbiota of various species, including non-60 model organisms, such as sea turtles (10). Gut microbiota may confer adaptability to 61 herbivorous tortoises such as G. berlandieri and C. sulcata naturally exposed to low 4 62 nitrogen-content diets. The aim of this research was to identify nitrogen-fixing species 63 in the gut microbiota of G. berlandieri and C. sulcata. In addition we present one G...