Happy Birthday SAR11
SAR11 turns 33 this month
As many of you know, my dissertation research focused on understanding the growth and physiology of SAR11 marine chemoheterotrophic bacteria. These microbes numerically dominate marine waters globally, where the consume (mostly) low molecular weight organic carbon compounds for energy and carbon. And, they are the archetypal oligotroph—performing exceptionally well in extremely low-nutrient waters.
Today is SAR11’s scientific birthday—the first paper describing the bacterial 16S rRNA clone from SARgasso Sea clone number 11 (SAR11) was published 33 years ago today.
Most of our current knowledge about the physiology and ecology of SAR11 is summarized in a recent review paper. We now know that these marine heterotrophic bacteria numerically dominate marine waters from the surface to the deep ocean globally. And they are the most numerically abundant cells on the planet.
SAR11 are masters of minimalism, coding for only ~1,300 genes, and their genomes are still among the smallest for a free-living organism. They lack abundant transcriptional regulators and as such minimally regulate transcription of their genome.
Many of the foundational discoveries on SAR11 were initiated in the 1990’s well before the deluge of YouTube videos. However, we are extremely lucky that in the late 1990’s PBS published a series of videos titled “Intimate Strangers: Unseen Life on Earth.” In one of these episodes—Oceans of Microbes—they described what is effectively the discovery of SAR11. The video can be seen here (at the 25:23 time stamp). This is a rare contemporaneous snapshot of a foundational discovery in microbiology. [Note: this video used to be much easier to find. It appears that the link above is the only instance…and who knows how long it will be posted]
More recently (2019), Dr. Giovannoni was a guest on the ASM “Meet the Microbiologist” podcast discussing SAR11 and SAR202 with Dr. Julie Wolf.
As a doctoral student, I stumbled upon a defined artificial medium to culture SAR11. SAR11 had been cultured, but was dependent on natural seawater as a base for growth. That meant every few months researchers would need to hop on a ship with 30 20L carboys and head off the Oregon coast to collect seawater. Occasionally, we’d have Seawater shipped to Oregon from the Sargasso Sea in the Atlantic. Identifying the best “batches” of seawater for SAR11 cultivation was somewhat of a competition. Every student or postdoc would cordon off a set of carboys so they could conduct reproducible experiments without variability introduced by the seawater batch. That is, certain ‘vintages’ of seawater performed better than others.
At the time, I was trying to identify “the missing nutrient” that would allow us to culture SAR11 on synthetic medium. The prevailing theory was that although a handful of essential requirements had been identified—reduced sulfur, glycine, and some low molecular weight organic acids— there was something still “missing” from their growth medium that was preventing us from cracking the 1.0 × 10^7 cells per ml threshold. I thought it might have to do with nucleotide metabolism. Specifically nucleotide monophosphates. So I set up a bunch of experiments testing these ideas.
I set up three experiments: one with 100% artificial seawater (ASW), one with 50% natural Sargasso seawater + 50% ASW, and one with 50% natural Oregon coastal seawater + 50% ASW, the full complement of known requirements, and assorted combinations of nucleotide monophosphates. I started the experiment on 12-Nov. 2008: below is a snapshot from my lab notebook.
I called this experiment the “3 Waters” experiment because I used 3 waters (ASW, Sargasso water and Oregon water). I measured the cell densities of the flasks for a bit over a month. While I can’t find the growth curve from these experiments, I do recall that the cell densities did not change at all over those 4-5 weeks. But cells were not dying and that stood out to me as important. Then I gave up. See my 18-Dec 2008 notebook entry:
I placed the flasks back in the incubator and forgot about them until I needed some incubator space. I remember gathering the flasks and was right at the sink, about to dump them down the drain. When I thought “I’ll check these one more time”.
In the intervening time between my “final counts” and me getting ready to dump them, SAR11 had started growing in the 50% seawater 50% ASW flasks. These cultures were passed to new 100% ASW flasks and they grew. We verified it was SAR11 via RFLP:
The rest is history.
Over the years, dozens of scientists around the world have contributed to the now vast knowledge base on SAR11 microbes—too many to list here. While I haven’t kept up with all of the ongoing SAR11 research lately, I feel very fortunate to have contributed a small slice to the ever expanding SAR11 knowledge base.
Happy Birthday SAR11!