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.
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