Hello and welcome back to Microbe Monday. I hope this November day finds you well.

When we think about bacteria, one of the principal assumptions we make is that they are microscopic. This doesn't always hold true. Many bacterial communities harbor various "field marks" that can indicate their presence. The book "A Field Guide to Bacteria" by Betsey Dexter Dyer goes into detail about that sort of thing if you're interested.

That's not the only way they can be visible to the naked eye, though. Today I want to talk about Thiomargarita, with particular attention to Thiomargarita namibiensis since that's the specific species I am most familiar with even though bigger ones exist.

T. namibiensis is found in low oxygen environments and was first discovered, as one might imagine, off the coast of Namibia. It grows almost exclusively in diatomaceous ooze due to the very high levels of sulfides.

In essence, the sulfur pearl needs both nitrates and sulfides to power its metabolism, but is also immobile. They can swap in nitrate to fulfill the last electron acceptor in their chemolithoautotropic ETC. One of the particularly insane things about its metabolism is that it employs a reverse Krebs cycle to make carbon. It runs the same complex reaction we all use to breathe in reverse and it garners biological building material from it.

When it takes sulfides, it transforms them into little grains of sulfur, which make it look opalescent like nacre, hence the name. It relies more on the nitrates, though, which come more from seawater than their muddy habitat. There's massive seasonal, climactic, and current-dependent variation in nitrate levels in the environment, and these little guys can't move around to chase it. Instead, they become very large and round.

One of the things that stops bacteria from being super big is surface area. Most things in biology are down to surface area. Unlike cells of so-called eukaryotes, they lack a lot of the specialized infrastructure used to get stuff where it needs to go (although they have a sort of cytoskeleton analogue). Still, for bacteria, they largely need to rely on sheer probability to get stuff where it needs to be. It's a gamble. Stuff moves around randomly but if you're small enough, they'll slot into where they belong most of the time. Bacteria are chaotically-aligned beings, you understand.

T. namibiensis is not like that. They are very large (0.1-0.75 mm diameter). They are larger than most of your cells. You can see them with the naked eye. This doesn't seem to align well with the probabilistic mechanism they use, does it?

What fits everything together is that they are big because of a massive central vacuole. Vacuoles are kinda just a membrane and don't require much upkeep. Think of them like a big sack that holds stuff. For T. namibiensis, it holds nitrates during "feast" periods so it can survive "famine" periods. Concentration gradients keep it together. The actual inner-cell space is still fairly small, it just has a big ol blob inside of it.

Thank you for reading this microbe monday. Please have a good day and remember there's really weird metabolisms happening all around you.