Sugars and the first greening

Gardeners know (or at least intuit) the first principle of plant growth. Known as Leibig’s Law, it holds that plants will continue to grow until they exhaust the least available nutrient. It’s also known as the Law of the Minimum because what stops growth is the least available input, regardless of the availability of other nutrients.

Unless you cultivate true aquatic plants, you would probably never consider carbon as a limiting factor. After all, terrestrial plants get their carbon from the air and as we know there is more than enough carbon in the atmosphere. But evidently carbon was the limiting factor in the early development of land plants, and according to a new paper by Linda E. Graham of the University of Wisconsin and others, that limitation was overcome by the use of sugars in the water.

The paper,” 97 Amer . J.  Botany 1485 (2010) (abstract with full text open access), explains the experiments Graham and associates performed growing the emergent peat moss Spahnum compactum and two unbranched (zygnematalean) green algae. They first tested the extent of carbon limitation of the two green algae in their natural low turbulence, low pH habitats by growing them in aerated and non-aerated cultures. Aeration had two effects: it make available more CO2 and it reduces the resistance to the diffusion of CO2 at the boundary of the cells. The result was that measured by mean chlorophyll a concentrations, aerated Cylindrocustis grew more than 13 times than non-aerated, while aerated Mougeotia great six times as much as non-aerated. When 1% glucose was added to the non-aerated cultures chlorophyll a concentrations for Clindrocystis cultures increased 9 times that of control and the same test with Mougeotia caused a 1.7 times increase in chlorophyll a concentrations.

Sphagnum was used to test the relative utilization of glucose against sucrose. The result was that after 9 weeks 1% additional glucose increased the dry mass of the peat moss 28 ties.  The various permutations of the relative growth with different additions of glucose and sucrose is best shown in a chart:

Medium ratio Growth ratio
1% glucose / 0% glucose 28
2% G / 0% G 39
2%G / 1% G 1.4
1%G / 1% sucrose 3.2
1%S / 0% S 11.9
2% S / 1% S 1.8
2% G  1% S 4.4
2% G / 2% S 2.4

You can see from the chart not does Sphagnum show preferential uptake for glucose over sucrose but also that there are saturation levels (or the moss began reaching limits of some other input).

Graham draws two conclusions from these results. The first one concerns the evolution of the first terrestrial plants. Together with her earlier study of glucose utilization in the charophycean green alga Coleochaete orbicularis (Graham, Graham, Russin & Chesnick, “Occurrence and phylogenetic significance of glucose utilization by charophcean algae Glucose enhancement of growth in Coleochaete oribularis,” 81 Amer. J. Botany 423 (1994)) Graham concludes that the less derived charophycean green algae (which are closely related to the ancestors of bryophytes and other land plants) were less able to convert exogenous sugar than the more derived Coleochaete and Sphagnum. And by overlaying the ability to use external sugar on a phylogram showing the ability of the taxons to invest cell walls with polyphenolics she concludes that the two abilities evolved together. This supports the hypothesis that the increased ability for sugar utilization subsidizes the high cost of synthesizing phenolic compunds used to make more resistant cell walls. More resistant cell walls protect the cells from microbial attack, ultraviolet damage from sunlight and drying out.

Graham compares the results obtained here with other studies of peat mosses. Mosses other than Sphagnum have been shown to process sucrose. Another study on a different species of Sphagnum showed that it used sucrose efficiently (more so than other sugars), but the author surmised that it might have used available invertase to cleave sucrose into glucose and fructose, which were then absorbed by the monosaccharide transporters (the same mechanism Graham proposes for S. compactum.

She then speculates on the adaptive value of use of exogenous sugars in peat moss. In addition to subsidizing the cost of synthesizing polymers for cell wall compounds, outside sugars might be used as a substrate for cellulose production by the moss or they might be used when photosynthesis is reduced (because of lack of light, water or other necessary minerals). Exogenous sugars might also help mosses to retrieve their own organic exudates (which are used possibly to attract microbes which fix nitrogen).

As for the ecological implications, Graham notes that peat mosses are known to store a substantial proportion of the Earth’s global soil carbon. When considering the global carbon cycle, in the past mosses were considered to use only atmospheric carbon dioxide. That mosses exhibit mixotrophic behavior by not relying solely on photosynthesis requires that in assessing the impact of peat mosses (which cover substantial areas of the northern hemisphere) on carbon cycling (historically and in the future) use of soil and water organic carbon should be factored in.

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