The Ecological Society of America (ESA) annual conference, once dominated by discussions of lions, tigers and bears (or so I’ll pretend), has become a convergence point for microbial ecologists. From amphibian skin microbiomes to biological soil crusts and more, the diversity of microbial talks this year was astounding.
As the conference winds down, I wanted to share just a few of the coolest microbial findings that stuck with me from the last couple days at ESA. What was your favorite microbial talk of the week? Post about it in the comments, or better yet, tweet about it! And don’t forget to tag me (@themadstone) as well as the official ESA microbial ecology twitter (@ESAmicrobe), if you do ;)
Can dormancy explain microbial biogeography?
Dr. Jay Lennon gave a great talk on dormancy; a subject that’s particularly fascinating to me in light of the vast (and probably rather dormant) microbial populations living in deep, subterranean environments.
Dormancy, otherwise known as metabolic inactivity, is an important strategy for surviving stressful environments. Conditions such as extreme drought, cold, or lack of food can cause microbes to enter a dormant state. Dormant microbes are fascinating not only for their ability to survive extreme conditions, but to do so for thousands to millions of years.
Lennon’s research group is particularly interested in applying principles of biogeography- how diversity scales across space- to microbial ecology. But understanding spatial patterns in microbial diversity is tricky because: 1) microorganisms are enormously diverse compared with macroorganisms and 2) unlike, say, mountain lions, which may be very genetically similar across kilometer scales but diverse over thousands of kilometers, genetically distinct populations of microbes can exist at the millimeter scale. Scientists find that microbial communities in soil samples taken 1 centimeter apart from each other can be up to 90% genetically distinct.
Here’s where dormancy comes in. Lennon’s group can separate the active from dormant members of microbial communities, by looking at the number of RNA transcript copies for a ubiquitous bacterial gene. RNA transcripts, the intermediate product between a DNA “blueprint” and an active protein, are well correlated with metabolic activity. Lennon finds differences in how diversity scales with space when examining the dormant versus active portions of a microbial community. Simply put, dormant microbial communities are more homogenous across space than their active counterparts. Furthermore, active microbial communities tend to become dominated by select taxa, probably those with some fitness advantage in the local environment. Dormant communities are less likely to be dominated by a few individual taxa, but rather represent a well-distributed “seed-bank” of genetic diversity.
The difference in how dormant versus active microbial communities scale with space may seem like a nuance, but this information could go a long way toward helping ecologists understand the distribution of microbial diversity across our planet. Lennon’s results suggest dormant microbes may represent up to 40% of a community. That’s no small portion of the total genetic diversity. Furthermore, dormant microbes, as a potentially long-term seed-bank, may help replenish the diversity of a community following extinction events.
Pine needle bacteria that soak up nitrogen
Did you know that nearly all leaves on Earth host their own microbiomes? Neither did I! But perhaps it’s not too surprising, given we’re now studying the microbiomes of cell phones and sewers. Anyway, it’s pretty cool. It’s extra cool in alpine ecosystems, where scientists are finding that these leaf-dwellers may play an important role in making nitrogen available to plants.
Dr. Carolin Frank’s lab studies nitrogen cycling in high, cold alpine ecosystems dominated by coniferous trees. Alpine ecosystems have long puzzled ecologists, because their soils and vegetation often contain more nitrogen than can be easily explained. Dr. Frank and colleagues hypothesized the “missing nitrogen” might come from nitrogen fixers- bacteria that carry a special enzyme which allows them to convert atmospheric N2 gas to ammonia- found in an unlikely place.
The team went out to a high-altitude forest in Colorado, collecting pine needles and bringing them back to the lab. In the lab, they placed the needles in sealed jars and added a chemical called acetylene. Acetylene, a simple carbon compound, can be chemically converted by nitrogenase, the same enzyme used to fix dinitrogen gas. This is a quick and cheap way of assessing whether a sample contains the nitrogenase enzyme, and hence nitrogen-fixing bacteria.
After finding that the pine needles did indeed host nitrogen fixers, the group used genetic techniques to look more closely at who exactly is there. The most prominent bacteria in all pine needle samples was genetically similar to Gluconacetobacter -a nitrogen fixer found in sugarcane.
So, nitrogen fixers are living in pine needles and helping their host trees take up nitrogen. If replicated in other coniferous forests, these findings could transform our understanding of alpine and boreal ecosystems, helping explain how coniferous trees persist on very nitrogen-limited soils.
Ocean acidification and marine calcifiers: it’s more complicated than you thought.
Or at least, it’s much more complicated than an ignorant terrestrial scientist like myself thought. See, my understanding of ocean acidification went something like this: more CO2 in the atmosphere means more CO2 dissolving in the surface ocean. When this CO2 dissolves, it reacts with water to produce acid, lowering the pH of the ocean. This is bad news for organisms that build exoskeletons from calcium carbonate (CaCO3), because the protons that create acidity get in the way of calcium carbonate formation. Sound good?
Well, like most natural phenomena when examined closely, the answer is quite a bit more complicated. Dr. Deborah Iglesias-Rodruigez enlightened me today as to some of that complexity. Iglesias-Rodruigez studies Coccolithophores, a common group of marine phytoplankton (photosynthetic organisms) that precipitate a beautiful, calcium carbonate exoskeleton. But in order for these little guys to create their exoskeletons, they must first take up some form of carbon into their cells.
It turns out Coccolithophores have evolved to take up the predominant form of carbon found in seawater: bicarbonate, or HCO3-. And it also turns out this is the particular form of carbon that becomes more abundant during ocean acidification. So- does that mean acidification is good for calcium carbonate formation, after all?
Well, not quite. Although Coccolithophores seem to take up more bicarbonate under more acidic conditions, this isn’t exactly a good thing. Iglesias-Rodruigez finds that, in acidic conditions, Coccolithophores can start over-calcifying: precipitating more calcium carbonate than they need to, which is energetically wasteful. Moreover, once that calcium carbonate is precipitated, it gets slowly eaten away by the more-acidic seawater.
There was more detail to be found in this presentation, including some interesting stuff about differences in microbial cell volume mediating ocean acidification effects, but I’ll leave that for another day. One ocean chemistry lesson today was enough for my brain to juice on.