The Crazy Things Plants do for Nitrogen
What’s the big deal with nitrogen, you ask? Nitrogen is the key ingredient in proteins. In biology, if our DNA does the talkin’, proteins do the walkin’. Proteins can be enzymes that help reactions happen. In plants, the sugar-making part of photosynthesis, (aka the dark cycle) is run by an enzyme called RuBisCO, which also happens to be the most abundant enzyme on earth. The remarkable thing about this super important enzyme, responsible for turning CO2 into sugars, is that it is a super slow, clunky enzyme. Plants often compensate for its slowness by making more of it. More enzyme=more protein. Plants need nitrogen to make proteins.
I thought about naming this post: “Nodules, photosynthetic pathways, and carnivorous plants, oh my!” But the number of syllables got in my way. Still, these three evolutionary wonders all help plants deal with limited nitrogen, and deserve to show up in a blog post together.
But why are plants so nitrogen-limited in nature?
The answer includes a terrifying word from Intro to Chemistry: stoichiometry. (Which I recently learned from this fantastic paper: Nitrogen and Nature.)
The number of nitrogen molecules in plants is small among all of the carbon molecules, which drives the ratio of carbon to nitrogen molecules the leaf litter, overwhelming the soil with carbon and making nitrogen hard to come by. Further, nitrogen is typically stuck directly to carbon in a covalent bond, which is harder to break off than ionic bonds that work like magnets. Even when nitrogen is in this “magnetic” form, these ions are negatively charged—just like soil particles—and easily leech from the soil with rainwater.
Yes, that is why nitrogen is limited in the soil, but most of the nitrogen in the world is in the air; isn’t there plenty of it in the air that plants could use? The atmosphere is made up of roughly 79% nitrogen. Unfortunately, that nitrogen is not accessible to plants. Each nitrogen is tightly bound to a second nitrogen atom like this: N≡N, and triple bonds are especially tough to break, even for a plant.
But some bacteria can break all three bonds!! Microbes have crazy metabolic pathways, many of which are crucial to the chemical balance of the world existing as we know it. By turning N≡N into fertilizer in a process called nitrogen fixation, the nitrogen cycle is driven by microbes.
Evolutionary wonder #1: Some plants figured this out (over evolutionary time), and formed a symbiotic relationship with nitrogen-fixing bacteria. These groups of bacteria: rhizobia, Frankia, and cyanobacteria, live in nodules on their plant hosts’ roots and provide a source of house-made fertilizer in exchange for sugars. Plants, which do photosynthesis, that have nitrogen-fixing bacteria in their roots represent the interface of two important geochemical cycles: the nitrogen and carbon cycles.
Not only are plants solar-powered sugar factories, but some plants can make sugars through several different metabolic pathways: C3, C4, and CAM. For perspective- we don’t even have one carbon assimilating pathway, but the plant kingdom has three.
Evolutionary Wonder #2: In “normal photosynthesis”, or C3 photosynthesis, plants lose water through pores called stomata when they take in carbon to turn into sugars. Some plants reduce the amount of water they lose by either opening their stomata at night when the air is cooler (Crassulacean Acid Metabolism or CAM), or by concentrating the CO2 in a separate compartment with those super slow enzymes (C4). Why are these nitrogen-limited adaptations? Because making enzymes more efficient helps plants with the initial problem of needing nitrogen to churn out tons of slow, clunky enzymes.
Yet some plants carry out old fashioned C3 photosynthesis without any N-fixing bacteria in root nodules and are able to live in extremely nitrogen-poor soil. Their secret? Eat the tiny packets of nitrogen buzzing and crawling everywhere around them.
Even Darwin himself was excited by this menacing behavior and laid some groundwork for research on how Venus flytraps close in on their prey.
Despite the diversity of strategy, ranging from pitcher plants that collect bugs that slip into the plants’ digestive juices to sticky plants that catch bugs with glandular hairs, carnivorous plants evolved to deal with stressful environments lacking that not-so-secret ingredient in all proteins:
Nitrogen.
-Adrienne L. Godschalx, June 10th, 2015
