by Roberto
Altiarchaeum hamiconexum is a relatively recent addition to the growing list of microbial primary producers, i.e. those that fix CO2. Its first sightings date back to only a little more than twenty years and its ability to fix carbon was recognized only the last decade. And, as so many microbes, it has thus far resisted cultivation. Sensu stricto, I should be writing Candidatus Altiarchaeum hamiconexum. But I won't be so strict here, I'll forego the Candidatus business and simply call it A. hamiconexum.
Fig. 1. Biofilm droplets sticking to polyethylene nets. Bar = 0.5 cm. Source
The history of its discovery is a wonderful example of microbial naturalists observing a beautiful phenomenon and pursuing it with a clever approach. While studying outlets of cold water streams that feed a marsh in Southern Germany, where microbial mats and streamers abound, investigators noticed striking "string-of-pearls" structures. Microscopic and molecular phylogenetic analyses allowed them to conclude that the pearls contain predominantly one archaeon (named SM1 at first) and one bacterium. They were naturally curious, could they find the archaeon deeper in the spring waters? To try to trap it, they placed polyethylene nets at various depths. When the nets were in zones with little or no oxygen, what grew on the nets was fascinating. Within minutes, millimeter-sized droplets stuck to the nets. And after a few days in the spring, the droplets on the nets grew to almost centimeter sizes (Fig. 1). Further analyses showed that the droplets were biofilms composed almost exclusively of the archaeon. About as close to pure culture as you get in natural settings! And fast growth in situ. But when taken into the lab, no go.
In the absence of cultivation, it made perfect sense to go all out with metagenomics. The published results from sequencing analyses and experiments to characterize predicted physiological attributes, provide a clear picture of the ecophysiology of the archaeon. The cold spring archaeon found in Germany is a member of a novel and widespread lineage of archaea abundant in subsurface waters. It can fix CO2 using the Wood–Ljungdahl pathway. Given all that knowledge of the microbe, the authors proceeded to name this lineage as order Altiarchaeales within the Euryarchaeota phylum. The root "Alti" is from the Latin altus. (Interestingly, altus can have two, almost opposite meanings: high or deep. In this case Alti is used because of the deep sites of isolation.) What about the species name, hamiconexum?
Fig. 2. Top panel: scanning electron micrograph of evenly spaced Altiarchaeum hamiconexum cells showing a typical hexagonal pattern. Note the thin appendages, the "hami," that connect the cells. Bar = 1 μm. Source. Bottom panel: 3D model of the hamus structure. Length is ~300 nm and the hook ~60 nm. Source
There are several striking structural features of A. hamiconexum within its biofilm that explain the species name. Individual coccoid cells are rather distant from each other. Yet they are connected by thin cellular appendages (Fig. 2 top panel). It seems that the constant structure and size of these appendages define the distance between cells. The three-dimensional reconstruction of the appendage reveals a remarkable hook structure (Fig. 2 bottom panel), termed a "hamus" (plural hami, from Latin for hook). A hamus protruding from one is thought to hook up with another hamus from an adjacent cell. Thus, the biofilm cells are "hami-connected." The species name makes perfect sense. Such a fabulous structure for an appendage. It's no surprise that it caught Elio's attention in the early days of STC (see here)!
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