Toxic 3D Prints, Old-School Graphics, and Mud Microbes

Today I Learned:
1) 3D printed parts made using stereolithography may be toxic. They're certainly toxic to zebrafish embryos. Embryos are pretty sensitive, though....

Stereolithographic 3D printing is the version of 3D printing that draws a piece out of a bath of liquid plastic monomer, using some sort of light to cure (polymerize) the monomers into hard plastic. The problem seems to be that the monomer solution is toxic (this has been known for a while) and the printing process doesn't completely polymerize the plastic -- additional curing of a piece for a while with UV light removes the toxicity effect.

Deposition-based 3D printing, which as far as I know is more common (it's what we have in our apartment), uses pre-polymerized plastic that's heated and cooled, so it's not toxic by this mechanism.

2) ...some of the tricks used in old-school (1970s and 1980s) gaming computers to make the graphics work. I had no idea how crunched those old graphics systems were. A typical screen resolution at the time was 320x200. Paltry, I know, but that adds up to 64k pixels. That was at a time when most computers had 16kb or 32kb of RAM -- and no dedicated video RAM! So storing a byte's worth of data for each pixel was totally out of the question. Even black-and-white graphics, with one bit per pixel, would chew up beetween half and a quarter of the RAM on most systems. That's pretty intense.

I won't go into the details of how to get around this issue here, but you can learn more in this guy's video: https://www.youtube.com/watch?v=Tfh0ytz8S0k.

Thanks to Chris Lennox for linking me to this!

3) ...about some really cool mud microbes! Seriously, really cool stuff. I heard a lecture by a guy who studies microbial communities in the surface layers of mud. A lot of the microbiota of mud surfaces are sulfur-metabolizing microbes. Instead of burning metabolizing complex organic compounds like glucose for energy, they do this awesome thing where they store elemental sulfur in solid chunks inside their cells, then burn it with oxygen to generate ATP. Because of this, sulfur and oxygen are really important for these microbes.

Mud has this interesting property where oxygen diffuses in from the top, forming an oxygen gradient, while there's usually a sulfur source near the bottom, which diffuses up to form an opposing gradient. The guy I heard talk today studies microbes that live at a sort of boundary layer where there's a good balance of oxygen and sulfur.

The lecturer talked specifically about two species, a bacteria called Thiovulum majus and a cilliate called Uronemella. Both species build sheets of sticky polysaccharides called "veils", to which they tether themselves with a similar substance in large colonies. Both species also have a ton of flagella, which they use to push themselves out until they strain against their tether. Once at their tethers' ends, the microbes use their flagella to flow water past themselves, drawing in oxygen-rich water from above, kind of like the way rotifers and some hydra stream food-rich water to filter-feed more efficiently.

Uronemella isn't particularly unusual as far as Eukaryotes go. T. majus is a different story. First off, it's a massive bacteria at about 10 microns long (that's closer to a human skin cell than to a typical bacterial cell; there are other microbes which the lecturer mentioned in passing that grow to hundreds of microns long(!), which lets them have one end in oxygen-rich mud and the other in sulfur-rich mud). It also has *way* more flagella than a typical bacteria, averaging around 50 flagella per cell. These flagella are chemically and structurally identical to a typical bacterial flagella, but their sheer quantity lets T. majus exert 40 pN of force on the surrounding water -- much more than most bacteria can exert, and comparable to Uronemella.

T. majus also has a highly unusual chemotaxis system (that is, control that lets it move towards desirable chemicals). Most bacteria use a variant of run-and-tumble chemotaxis, where the bacteria switch between spinning in circles and swimming in a straight line, and spend more time spinning when they're in a high concentration of something they like. T. majus doesn't do this. Instead, it turns smoothly to move up desirable gradients. We don't know how. The speaker speculated that T. majus might be big enough to simply sample the chemical at either end of the cell and determine the direction of the gradient, but it was only mentioned in passing.

Anyway, both Uronemella and T. majus form these big (visible to the naked eye) veils and, as a community, pull in oxygen-rich water from above and consume the oxygen. In a test-tube environment with oxygenated air at the top and a sulfur source at the bottom, they will form a veil, then move their way up the test tube, depleting oxygen as they go. It's kind of like lighting a match at one end of a tube filled with propane -- you get a traveling wave of flame, consuming the propane as it goes. Except instead of converting propane to CO2 and H2O, the microbes convert O2 and sulfur (which they seem to have in excess) into more microbes.

There were more interesting things about T. majus and Uronemella, but this post is getting rather lengthy. Perhaps the most important thing I learned from this lecture was the following general principle: whenever you see life building sheets or films, especially in water, it may be forming sheets at a gradient boundary of something important. I'll have to keep an eye out for more examples (or counterexamples) of this.

You can read more about T. majus and Uronemella here: http://tinyurl.com/pasuobp. I recommend looking through the figures and table if you can get access; don't skip Figure 9, which shows dimples in the veils caused by turbulent forces.