|Microbial Mat Twin Cayes, Belize|
How and why we found unusual patterns in dwarf mangrove tissues continued to challenge us. Later we read a publication by Erskine et al. (1998). They found similar unusual nitrogen isotope signals in plants collected downwind of a bird rookery on a subantarctic island. They postulated that the isotope signals in ammonia volatilized from bird guano were incorporated into plant tissues. Wooller and I found a nifty way to measure ammonia concentrations in the air. We purchased disposable “badges” that could be tied to tree branches to absorb ammonia.
|Black dots mean lots of ammonia in the air|
As we worked through the puzzle of interpreting the complex isotope signal data, our group settled in on a strategy of nightly meetings in the Carrie Bow library. Armed with large pieces of fresh paper, sharpies, and a bottle of One Barrel rum, we argued and laughed our way through a myriad of ideas and hypotheses as to why we found such a range in the nitrogen isotopes of mangrove leaves. Wooller and I were the ring leaders. Myrna Jacobson, Babs Smallwood, Quinn Roberts, John Cheeseman, and Isabel Romero joined in. I saved one of these pieces of paper from a particularly fruitful conversation. Our original mission was to collect specimens of the major plants, microbes, and animals of Twin Cayes and measure their isotopic and elemental compositions. We had yet to sample the lichens. Wooller dramatically penned in “Lichens--Not important” on the corner of the page. As you can see, the lichens provided the smoking gun to understanding nitrogen patterns in leaves.
There are large differences in the nitrogen isotope signals between the two chemical phases of ammonia (the gas you smell near animal feed lots) and the dissolved ammonium (the solid chemical in fertilizer). The nitrogen isotope signals of ammonia in the air in Twin Cayes was very similar to what we measured in the dwarf mangrove leaves. Our final experiments showed that mangrove leaves can, indeed, take up gaseous ammonia when their leaves take up carbon dioxide for photosynthesis. Last, we found the unusual nitrogen isotope “signals” in lichens growing on the bark of mangroves throughout the islands. Lichens, microbial symbioses with no roots, take up any available nitrogen from the air. The nitrogen isotope signals of lichens had the same signals as the mangrove leaves collected in the different zones.