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This is a bit convoluted because I'm still working on how to pack meaningful parts of the information below into the leafcutter manuscript (or how to avoid having to pack them in).

A challenging part of writing about the leafcutter manuscript involves how different groups of people think about and write about nutrition. From a broad ecological perspective, it makes the most sense to use elements as a fundamental currency, and that's the perspective I started from originally. For those interested in animal nutrition, however, it makes more sense to think about elements in nutrient form, and to distinguish between things that can be readily digested and assimilated or not. So I have to be more nutrient-explicit than just talking about elements.

But where do fungi fit in? Many kinds of fungi can break down cellulose and lignin, which is why fungi are key for decomposition. I learned yesterday that in many ecosystems, decomposition often occurs in a two-stage process, with small soil organisms initially breaking down leaves into small particles, and then fungi can come in and colonize things: when tough leaves are put into a mesh bag with pores that are so small that only microbes can enter, the leaves don't break down. As soon as the pores are large enough to permit entry of soil arthropods, the leaves get decomposed. On the other hand, if leaves that are easy to break down are put into those same mesh bags (e.g. kale), there's no difference in decomposition rates between mesh bag types. But the jury's out with regards to the extent to which cellulose digestion is important in the leafcutter system specifically.

Anyway. Over the course of further extensive literature searching, I also determined that next to nothing is known about the ability of fungi to ingest and utilize lipids as an energy source. The vast majority of work has focused on the utilization of different carbohydrate sources. Typically, glucose is prioritized, but in mixtures, fungi (in general) will use different blends of things. This corresponds well with some of the studies that have been conducted on enzyme activities in leafcutter fungus gardens - gardens will shift enzyme production in response to changes in fungal substrate.

I got to thinking about all of this over the course of trying to determine why, from a biochemical standpoint, there are large differences in the carbon and nitrogen content of palo verde leaves as compared to polenta. I think the main reasons are because corn is mostly full of stored carbohydrates, whereas leaves are full of photosynthetic machinery. In addition, leaves of desert plants often have more waxy cuticles to prevent water loss. Waxes and and chlorophyll are high in carbon, and chlorophyll is somewhat high in nitrogen, so these are reasonable explanations for the differences.

It took a while to think this through and find good references for it, though, because I wasn't sure about how large the differences in N content were for leguminous vs. non-leguminous plants, or whether the differences are strongly tied to the production of plant secondary compounds. I also had to remember which kinds of plants use C3 versus C4 versus CAM photosynthesis. The literature informs me, however, that at least ~75% of the protein in plant leaves in general is associated with chlorophyll, so on a coarse level this explanation seems like it will hold unless a reviewer informs me otherwise.

I'm still not sure about how to justify my diet treatments, though, or if I even need to do much in the way of justification. I do, however, need to be able to talk about why it is that colonies in the wild collect foods that have more protein-biased ratios than the high-protein polenta treatment I used.

The answer really lies in the waste material. Unfortunately, I haven't been able to analyze the waste material for anything other than overall elemental composition. I also need to go back to working on comparisons of throughput rates for leaves versus polenta. Just knowing elemental composition doesn't reveal the full story in terms of nutrient utilization; I need to know things about both input rates and output rates. The main thing I know at the moment is that they aren't exactly steady-state in smaller colonies, which complicates things - there's variation in retention times within the fungus garden. If there was even a hint of steady-state rates, I could say, "Here's what goes in, here's what comes out, so here's what the ants and fungus and microbial community have used up."

I could design and conduct some really good experiments with what I know now, but I really don't have that luxury at the moment.


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