Nancy Rabalais’s team has been able to process some of the data and issued a press release on this year’s bottom water hypoxia. As I mentioned in the last post, the zone of hypoxia was actually two zones, which you can see below. The total estimated square mileage of bottom water at or below 2 mg/L dissolved oxygen was 5,052 square miles/14,785 square kilometers, which is almost three times larger than the goal proposed by the Mississippi River Gulf of Mexico Watershed Nutrient Task Force in 2001 and 2008.
Bottom water dissolved oxygen measured on the 2014 shelfwide cruise. Source: Nancy N. Rabalais, LUMCON, and R. Eugene Turner, LSUSurface water chlorophyll a measured on the 2014 shelfwide cruise. Source: Nancy N. Rabalais, LUMCON and R. Eugene Turner, LSU
I can provide some additional thoughts with pretty HD video to boot. The eastern stations, as seen in the chlorophyll map, were predominantly green water, with considerable phyotoplankton mass present in the water column. We could observe significant green-colored biomass both on the GF/D pre-filters and the 0.22 µm Sterivex filters. This is also what you see if you are actually in the water, and the video from the green-water CTD cast at station B6 confirms what was seen with the CTD instrumentation and the filters. Convenient, eh? There is dense, murky greenness at the surface. Deeper, the visibility improves as we get below the highest biomass concentration, but towards the bottom, where hypoxia was observed, we again see increased turbidity, but of a different sort. It’s much whiter than that at the surface. On the return trip, considerable marine snow can be seen (along with a jelly or two and other marine invertebrates).
The western stations, as you might imagine by looking at the surface chlorophyll data, were blue water, with very little phyotplankton mass compared to the eastern stations. The cast at station K3 shows beautiful blue water with high visibility (diver’s paradise), but as you descend, you again pick up the whitish turbidity at the bottom layer where hypoxia was observed.
Sterivex filters from this section were light pink, a phenomenon we observed last year as well. The 16S rRNA and metagenomic data will, among other things, help us uncover a bit more about the variant prokaryotic taxa seen in these contrasting zones of hypoxia.
Let’s face it. There are a lot of different ways to skin a microbe, and get the DNA and RNA out for whatever purposes you might deem necessary or useful. If you ask one or two people you’ll get one or two answers. What if you ask more than a thousand? Here is some insightful crowd-sourced feedback from Twitter on favorite and least-favorite methods of nucleic acid extraction, and we’ve done our best to include references with real comparative data, protocols, and applications:
There are some exciting developments in the world of SAR11 microbiology, and I’d like to take a moment to summarize recent research I’ve been involved in. All three papers have come out this year in The ISME Journal.
The single-cell genomes from Thrash et al., 2014, aligned linearly according to scaffold size, showing (outside-in) GC content, scaffold boundaries, coding regions according to pan-genome status, and shared elements unique to the subclade Ic with orange lines
The final main project of my postdoc with Steve Giovannoni was a characterization of a deep-water subclade of the larger SAR11 group using single-cell genomics and metagenomics. This was a collaboration between our lab and that of Ramunas Stepanauskas and Ed DeLong, and although it began at Oregon State, the work carried over (like many projects do) into my appointment at LSU. The main findings of the paper were that this “subclade Ic” dominated deep-water metagenomic reads compared to extant surface-originated genomes, and that a few gene-content specific variations could be identified that may distinguish these organisms from the surface strains, but that, on the whole, the main observable differences were in global genomic footprints such as expected average genome size, average coding region size, intergenic spacer size, and preferential amino acid substitutions. The deep water ecotype, subclade Ic, has obviously been evolving separately from the surface subclades for some time based on these accumulated variations and phylogenomic branch lengths, however, it also appears these organisms share a similar metabolic niche with the surface strains: they are predicted to be obligate aerobic organisms with a particular appetite for common metabolic intermediates found in the cellular milieu of most organisms: simple carboxylic and amino acids and C1 and methylated compounds.
Steve was also invited to write a Winogradski Review article in coordination with him recently being given the prestigious Jim Tiedge Award from ISME. Ben Temperton, who is also a former postdoc of Steve’s, and who worked closely with me on the subclade Ic study, above, was a co-author. Steve took the opportunity to review many of the observed variations between streamlining selection on free-living, small genomes like SAR11 and Prochlorococcus, and the small genomes of obligate intracellular symbionts like Rickettsia. Importantly, the review also pointed out that examples of streamlining selection are becoming more prevalent as data from single-cell genomics and assembled metagenomes continues to pour in. Indeed, the odd nutrient requirements that are sometimes a result of streamlining selection may be at the heart of the difficulty in cultivating many of these organisms in the lab.
Dovetailing nicely with the theoretical concepts brought up in Steve’s review was a third piece from our lab, a detailed study on an unusual vitamin requirement in SAR11 that was discovered by Paul Carini, currently postdoc-ing with Alyson Santoro. This work identified that members of many SAR11 subclades are unable to make the vitamin thiamine by typical synthetic means, because they lack a single gene in an otherwise complete pathway, thiC. As a result, they are unable to synthesize 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP), a direct precursor to thiamine. However, Paul also discovered that SAR11 has the capacity to use free HMP in media to relieve thiamine limitation, and identified a putative transporter for HMP in several SAR11 genomes. Additionally, HMP measurements from the Sargasso Sea and cultivation experiments pointed to a possible natural source of HMP from cyanobacteria, thus providing an explanation for how SAR11 can remain successful without the key thiC gene.
I’m giving the final #LSUCompBio talk for this semester (so I won’t be live tweeting it, most likely) tonight at 5:30 in LSA A101. The title of my talk is “We’re not in a petri dish anymore: pulling back the curtain on microbial genomics using cultivation-independent techniques.” The talk will be broadcast live and archived afterwards. I’ll be giving an introduction to microbial metagenomics and single-cell genomics, with an example of combining the two from a recent project of mine.
