Global warming increases the strength and frequency El Niño events, affecting marine ecosystems along the equatorial pacific region. One of the best-documented examples of the effect of this extreme climatic event on marine biodiversity is found in the Galapagos Islands. There, the rise in water temperatures forces changes in the diet of marine iguanas that have led to massive populations collapses (80-90% of all individuals). The collapse in iguana populations is caused by the shifts in algal species representing the primary diet of this marine herbivore. The warmer and nutrient-poor surface waters that occur during El Niño decrease productivity in green and red algal populations – the main feed for the iguana – and instead enable the growth of brown algae. Classical studies have shown that although iguanas ingest brown algae, they lack the ability to digest it, leading to their death by starvations. Indeed, whereas the gut contents of healthy marine iguanas contain liquefied algae, dead animals studied during El Niño events contain un-digested brown algae.
We suspect that as with other marine or terrestrial herbivores, the capacity to degrade complex plant tissues such as those found in macroalgal cell walls is determined by the intestinal microbes. More specifically the bacteria inhabiting the iguana’s hindgut provide the enzymes that can depolymerize the complex polysaccharides that form algal cell walls into sugar monomers. This breakdown initiates a cascade of fermentation reactions that lead to the production of short chain fatty acids which represent the main energy source for the animal. This in principle the same process that allows animals such as cows or other ruminants to subsist purely on plant material, indigestible to most animals.
This presumed symbiotic relationship between iguanas and their gut microbes suggests that the inability of the animal to digest brown algae is due to the incompatibility of microbiome composition, in particular with respect to the repertoire of hydrolytic and lytic enzymes of the microbes that are required to digest laminarin, cellulose, alginate and fucoidans, the dominant polysaccharides in brown algae.
We, therefore, initiated a collaborative study between our group and the group of Prof Otto Cordero from MIT with the purpose to understand this phenomenon and to help preserve marine iguanas. We are currently looking for additional funding for this important research.