This is the second of two articles written by Elizabeth Sargent on the marine nitrogen cycle; the first article covered some of the basics and can be found here
There are three main groups of nitrogen fixing organisms in the ocean: filamentous, heterocystous, and unicellular diazotrophs. The unicells are relatively new to the research scene (discovered in 1998), and are difficult to image due to their size, distribution, and our lack of knowledge about their lifestyle. I have some epifluorescent microscope images of unicellular diazotrophs, but you can get a good idea of what that looks like by picturing the night sky; it’s a whole lot of darkness and some tiny yellow dots. These are pretty interesting organisms, and current research suggests they are important contributors of new nitrogen in the ocean, but I’ll be focusing on the diazotrophs I can see and quantify more readily in my samples: Trichodesmiuim and Richelia intracelluaris.
The most comprehensively studied marine diazotroph is (and I apologise for the number of adjectives here but, trust me, I could use a lot more!) the conspicuous, buoyant, colonial, non-heterocystous cyanobacterium Trichodesmium (hereafter Tricho). Tricho is a filamentous diazotroph consisting of long chains of cells called trichomes. These trichomes frequently form large colonies of varying morphologies, but are also known to float freely in the water column. Colony morphologies are most often split into two categories, and are described as tufts or puffs. Tufts are longitudinal groupings of trichomes aligned parallel to one another that are often twisted in the centre, while puffs are spherical grouping of trichomes that are interlaced (pictured). Unfortunately, “tufts & puffs” is one of the only things most people remember about Tricho, so commit that to memory, and then check back in; there are plenty of other diazotroph facts worth knowing. Tricho cells also contain gas vesicles, which provide them with buoyancy, mostly confining them to the surface layer. As well as being diazotrophic, these organisms are also autotrophic, and require access to light for photosynthesis; thus, maintaining their position in the euphotic zone is important. These organisms can become so buoyant that they can get ‘stuck’ at the surface earning them the nickname ‘sea sawdust,’ as was first described by Captain James Cook. I have yet to encounter this phenomenon, but if we all keep our fingers tightly crossed, maybe I’ll get the chance on my next research voyage…
Heterocystous cyanobacteria make up the second group of marine nitrogen fixers that I study. Richelia intracellularis (hereafter Richie. No? Okay, hereafter just Richelia) is an endosymbiotic, heterocystous diazotroph, and is often found in consortia with the diatoms Rhizosolenia and Hemiaulus. Richelia possesses a single terminal heterocyst attached to a chain of vegetative cells. The heterocyst is a specialised cell for nitrogen fixation, and is noticeably larger than the vegetative cells. Unlike Tricho, Richelia does not contain gas vesicles meaning it cannot regulate its position in the water column. Luckily it can rely on its diatom host for this and in return supplies the host with newly fixed nitrogen. It’s like if you were to live with someone in return for cooking them 3 meals a day; it might get a bit cramped, but they’re not going to complain if you’re a master chef. Richelia is the most widely distributed marine heterocystous cyanobacteria, and is likely a major contributor to global marine nitrogen fixation. Crucially, the host diatoms of Richelia are known to aggregate into mats and rafts when in high concentrations thus increasing their sinking rate; during large blooms, this can result in mass export of these Richelia-diatom associations, and can have important implications for nitrogen and carbon cycling in the ocean. A great example of this can be seen in some recent research out of the Hawaii Ocean Time-series data revealing an annual export pulse consisting of these diazotrophic diatom associations (http://www.hawaii.edu/news/2012/02/07/summer-export-pulse/).
Slowly, we are coming to understand the mechanisms & intricacies of marine nitrogen cycle, and specifically those of marine diazotrophs, but the environmental factors that control the abundance and distribution of marine nitrogen fixers are still not entirely understood. A more thorough comprehension of the life cycles and fate of marine diazotrophs would be a vital tool in expanding the description of the role they play in marine biogeochemical cycles. Hold your horses, I’m working on it.
PS. It is a little known fact that this Coldplay song is actually dedicated to nitrogen fixation (Chris Martin is all about the diazotrophs)
Elizabeth Sargent is currently a PhD student at the National Oceanography Centre, Southampton studying nitrogen fixation and its role in fluxes of carbon and nitrogen to the deep sea