Course:EOSC 475/ResearchProject/GlobalSignificanceTrichodesmium
“Trichodesmium, a Globally Significant Marine Cyanobacterium" was published May 23, 1997 in Science by Douglas G. Capone, Jonathan P. Zehr, Hans W. Paerl, Birgitta Bergman and Edward J. Carpenter, and is an impactful piece that presents the significance of Trichodesmium in global oceans. This paper focuses on the genus Trichodesmium, which are species defined as cyanobacteria commonly found in oligotrophic tropical and subtropical oceans. Its unique adaptation of fixing nitrogen (N2) gas while maintaining photosynthesis has challenged many of the preconceived notions on nitrogen fixation. Nitrogen fixers, also known as diazotrophs, are highly abundant in western currents, and high light environments. This is likely why Trichodesmium forms colonies commonly found in transparent waters, in the mixed layer. They also favour tropical low productive regions, constituting a significant portion of overall primary production and nitrogen balancing in these areas. With adaptations that have fundamentally changed our understanding of nitrogen fixation Trichodesmium are cyanobacteria that have far reaching impacts and global significance.
Background & Research Leading to Publication
Trichodesmium was first discovered in 1830 by Ehrenberg who observed a bloom that discoloured the water in the Red Sea (Ehrenerg 1830). Modern interest in Trichodesmium dates back to the 1960s, when it was determined that a large portion of the ocean is nitrogen limited. In 1961, when Trichodesmium was discovered as a light dependent nitrogen fixer, the scientific community was skeptical, as it was extremely unique from anything observed previously (Dugdale et al., 1961). In the 1990s, research stems from an increased understanding of global carbon dioxide (CO2) and the potential of using Trichodesmium as a solution. Trichodesmium species were also found to be extremely diverse, which led to questions surrounding its classification as a solo genus. DNA analysis and sequencing proved not only that Trichodesmium is in fact a nitrogen fixer, but also that it is its own genus (Capone et al. 1997).
One of the major signifiers that Trichodesmium is a nitrogen fixer was the discovery of the nitrogenase gene sequence within its genome. The nitrogenase enzyme gene is a highly conserved sequence through all nitrogen fixers, as nitrogenase, made up of two dinitrogenase functional units, is the catalyst for fixation (Zehr & Capone 1996). Since dinitrogenase and its iron (Fe) protein dinitrogenase reductase are so highly conserved, these are great gene sequences to use to identify species that can fix nitrogen, but it is almost impossible to use to contrast fixers (Zehr & Capone 1996). With this very conservative gene in mind it was in the late 1980’s that led researchers to look into PCR techniques to gain more insight into this gene and the answers it may hold (Zehr & McReynolds 1989).
Prior to Capone et al.’s research in 1997, it was known that Trichodesmium is more resistant to oxygen (O2) degradation than other cyanobacteria. Also, it is able to photosynthesize and fix nitrogen in the same cell simultaneously. However, it is not immune to the impact of oxygen and is found to respond negatively to elevated levels of the gas (Dugdale 1961; Zehr et al. 1993). Even the transcription of nitrogenase genes and synthesis of the nitrogenase enzyme complex are sensitive to oxygen and inactivated by it (Zehr et al. 1997). It was also found that Trichodesmium has a relatively low turnover rate of nitrogenase during the active period of nitrogen fixation (Zehr et al. 1993). To further complicate the situation, nitrogenase was only found active in Trichodesmium samples collected during daylight (Saino & Hattori 1978) dispelling the belief that Trichodesmium may be fixing nitrogen at night and photosynthesizing during the day. Another major discovery was found that nitrogen fixation by Trichodesmium occurs mostly in the spring when photosynthesis would also be highly active, unsurprisingly it was found most dominate in the Indian ocean and the Atlantic due to its unique temperature requirements of waters over 20°C (Capone et al. 1982).
Prior to this research, it was known that Trichodesmium has relatively low growth rates compared to eukaryotic phytoplankton (Carpenter & Capone 1983). Though during an event known as a bloom it is not uncommon for Trichodesmium to form large microbial mats that rapidly decay after favourable conditions pass providing large concentrations of organic carbon and nitrogen for other species (Zehr & Capone 1996).Trichodesmium is distributed by wind stress on the water surface, which plays a role in the distribution across the ocean. In low wind stress, the buoyancy of Trichodesmium is maintained by gas vesicles, leading to extensive surface blooms called red tides (Carpenter & Capone 1983). With these blooms providing large quantities of nutrients to the tropical and subtropical waters it is not surprising that they are crucial species for their ecosystems. Some studies have found that cyanobacteria fuels half of all new nitrogen production; over a seven year study in the North Pacific Gyre it appears to becoming even more important with significant increases over these seven years in Trichodesmium at shallow depths (0-45m) (Letelier et al. 1997). Early research in the Sargasso Sea and the Caribbean though proved that this importance is not homogenous as the Trichodesmium in the Sargasso appears to have a negligible input into the total nitrogen balance, whereas in the Caribbean it’s importance was great (Carpenter & McCarthy 1975).
Content of Paper
“Trichodesmium, a Globally Significant Marine Cyanobacterium” by Douglas G. Capone, Jonathan P. Zehr, Hans W. Paerl, Birgitta Bergman, and Edward J. Carpenter is a snapshot in our understanding of the genus Trichodesmium in the year 1997. The goal of the paper was to present the current knowledge available on Trichodesmium as well as discuss some of the directions that needed to be further researched or even to suggest new possibilities. A lot of the information that was discussed in the paper and where Trichodesmium stood in the literature can be found in the Background & Research Leading to Publication section on this page.
There were 7 key questions that the paper focussed on as being unresolved or topics that could still be considered open for discussion and hypothesis at that time:
1. Where does Trichodesmium fit into the broader scheme of cyanobacterial phylogeny?
There is currently only one complete genome sequence for a Trichodesmium species which causes obvious challenges when it comes to placing the genus into a broader phylogeny. Another challenge presented by any diazotroph is that the DNA sequence of the unique enzymes for nitrogenase are highly conserved - meaning they are extremely poor for creating a phylogeny of nitrogen fixers as there is not enough difference to separate them at nodes. Also, Trichodesmium has many unique characteristics from other cyanobacteria. One, is the size of Trichodesmium, with its ability to form colonies (commonly referred to as puffs), the species can be seen with the naked eye. Another is the very warm waters needed for Trichodesmium to survive. Cyanobacteria’s two other prominent photosynthetic species Synechococcus and Prochlorococcus both require far less heat to function. Also, Trichodesmium is not an easy species to grow in laboratory conditions, this makes creating its phylogeny even more difficult.
2. How does Trichodesmium sustain simultaneous photosynthetic oxygen evolution with nitrogenase activity?
This is a question that has followed Trichodesmium since it was discovered. At the time of this paper the possibilities were broken down into four options beyond the standard belief that photosynthesis occurs during the daytime and nitrogen fixation occurs during the night:
a. Nitrogenase is unique and not poisoned by oxygen
Along with question 1 this would have answered a lot of the mystery surrounding Trichodesmium. However, this is known to be untrue. The gene sequence for nitrogenase would have had to be fundamentally different than other nitrogen fixers. This, of course, is already known to be false as Trichodesmium’s nitrogenase sequence is very similar to all other diazotroph genes for nitrogen fixation. Likewise, neither the actual enzyme shape nor the amino acid configuration leads to any suggestion that Trichodesmium’s nitrogenase is in any way more oxygen resistant or completely immune to its effects. So another possibility will have to be considered.
b. Oxygen is a poison but nitrogenase is continuously reproduced
The beginning of this theory is sound in that oxygen is in fact poisonous to Trichodesmium’s nitrogenase, however research that has gone into looking at production rates of the enzyme has led to the belief that this theory too, is false. Production rates of nitrogenase are actually relatively slow in Trichodesmium which leads to the assumption that oxygen cannot be killing them every time photosynthesis is occurring. This does make practical sense, if nitrogenase was constantly being poisoned by the oxygen produced during photosynthesis than a large portion of the energy produced by the cell would go into reproducing this lost nitrogenase. This would then require more photosynthesis to occur in order to replace this energy which would result in more oxygen poisoning. It would be a flawed system which would be unlikely for Trichodesmium to be a competitive species.
c. Spatially segregated
This was the most likely option for the combination of photosynthesis and nitrogen fixation in a single species. It is well documented in other nitrogen fixing species that certain cells are specialized to run each process, this way the oxygen never poisons the nitrogenase. It is a system well documented as working in an assortment of other species, unfortunately there is no spatial separation in Trichodesmium. Also, as it is well documented that Trichodesmium fixes nitrogen during sunlight hours this means that there is no temporal separation either, meaning the processes are running together in the same space.
d. Some process removes oxygen before is reaches nitrogenase
This theory is the most likely solution to date. It answers the question about how the two processes can run in the same space and at the same time. Our understanding about what this process is, is still very limited. However, it has to uptake enough oxygen to prevent it from impacting the nitrogenase and cannot interfere with either process. Also, it has to have separate genes for it as there is no evidence of shifted sequencing in either process for Trichodesmium.
3. Why does it fix nitrogen only during daylight periods?
This is a situation that has confused many researchers. As is discussed above, with no spatial separation between photosystems and fixation processes the next logical thought was that there could be a temporal separation. Since photosynthesis requires sunlight, a lifecycle with fixation of nitrogen during the night and photosynthesis during the day is a suitable method to avoiding oxygen poisoning of nitrogenase. Of course, Trichodesmium yet again proved this theory incorrect. Researchers have found only Trichodesmium collected during the daylight hours has active nitrogenase, in the night it appears the process of nitrogen fixation and photosynthesis both shut down. With both these processes running at the same time within the same cell there must be a situation that results in the success of the situation. This is address in How does Trichodesmium sustain simultaneous photosynthetic oxygen evolution with nitrogenase activity?
4. What physiological, morphological, and behavioural adaptations contribute to Trichodesmium's ecological success in the oligotrophic marine environment?
One of the largest advantages Trichodesmium has to other photosynthetic organisms in oligotrophic regions is its ability to fix nitrogen. Oligotrophic zones have a low availability of organic nutrients in comparison to the rest of the ocean. Often species that live there have to adapt to dealing with the extreme conditions. With so few organisms in oligotrophic waters there is a limitation on organic nutrients available to different species. By using nitrogen gas as a source over organic nitrogen there is a distinct advantage due to its abundance in the atmosphere and ability to overcome the lack of organic material in the ocean. Also, Trichodesmium’s ability to produce specialized toxins to avoid predation leads to an advantage. Oligotrophic waters lead to a need for predators to consume many different kinds of prey to satisfy energy requirements so Trichodesmium’s production of toxins could provide some level of protection.
5. What environmental and ecological factors controls production and nitrogen fixation in Trichodesmium in situ?
One of the major controls of nitrogen fixation is the amount of nitrogenase in an individual Trichodesmium. As the main enzyme behind the fixation process, the amount of nitrogen each Trichodesmium individual can fix is relative to the amount of nitrogenase in their cells. Another control is how many diazotrophs are in the water, and how much competition for nitrogen there is. This competition is nowhere near as strong as other photosynthesizers competing for organic nitrogen. The reason for this is two-fold, first there is more nitrogen gas available than organic nitrogen and second there are fewer nitrogen fixers competing than organic nitrogen dependant photosynthesizers.
6. To what extent does it contribute to productivity, nutrient cycling and trophodynamics in tropical and subtropical seas?
Trichodesmium is a critical species to its ecosystem. Not only does it bring new nitrogen into the euphotic zone, it also provides new organic carbon as a photosynthesizer. This addition of new nitrogen into oligotrophic zones is a huge benefit to other photosynthesizers who are working with a deficit due to the lack of life in these zones to resupply organic nitrogen. Also, in many of these regions there is downwelling which leads to a net loss of nitrogen into the deep waters, so addition of nitrogen gas from the atmosphere is highly useful to these species.
7. What is the overall importance of Trichodesmium on the global marine nitrogen and carbon cycles?
As nitrogen is added to eutrophic zone from the atmosphere it will also draw down large amounts of carbon dioxide into the ocean which could result in the oceans being a more successful carbon dioxide sink. This is a positive process as the nitrogen gas is added to Trichodesmium, then into the ecosystem as organically available nitrogen. By drawing more carbon dioxide into water there is an increase in photosynthetic life and an increase in the amount of carbon sinking into the deep ocean. As the deep water cycle in the oceans is over a thousand years the carbon remains in this sink for quite some time before resurfacing into the shallow waters. Nitrogen, as it is a very organically active nutrient, remains in the euphotic zone being taken up for quite some time before it also eventually sinks into the deep waters.
Influence on the Scientific Community
Directions of research were greatly shaped by this paper, as it explored and questioned the gaps in what was known about Trichodesmium at the time. Bergman et al. recently published a paper reiterating the importance of Trichodesmium, claiming it is crucial for organisms in the tropical and subtropical oceans (2013). Trichodesmium is a vital player in the biogeochemical cycles of carbon and nitrogen, it was found that only nitrogen fixers are able to supply enough nitrogen to support the tropical and subtropical regions (Bergman et al. 2013). In response to Capone et al.’s first question regarding Trichodesmium’s place in the broader scheme of cyanobacterial phylogeny, researchers found that the genus Katagnymene is very closely related and the two appear to interweave in each other’s phylogenies (Bergman et al. 2013). It is apparent that there is much room for research in this area still in order to fully understand the history of the two.
It was also found that compared to other nitrogen fixing species, Trichodesmium is able to synthesize in waters with a lower density of inorganic and organic nutrients, adding to their significance in the global nitrogen cycle (Bergman et al. 2013). Aeolian dust leads to an increase in iron in the Atlantic, and iron and phosphorus are critical nutrients. Considering this, it makes sense that Trichodemsium was found to account for up to 50% of the nitrogenase genes in the North Atlantic (Bergman et al. 2013). Trichodesmium erythraeum IMS101 is the only complete genome in the genus that is sequenced, making it quite unique and a good starting point for further research (Bergman et al. 2013).
As much as the research and knowledge surrounding the role of Trichodesmium in global oceans has increased since Capone et al.’s paper in 1997, there is still much to discover. Bergman et al. have opened more doors in the paths of research. For example, it was found that Trichodesmium populations become trapped at the surface of the water and die, resulting in a mass influx of carbon and nitrogen for other species. It is still unclear as to why this occurs. Secondly, Trichodesmium is known to release toxins to deter predation, but the role these toxins play in the ecosystem is largely unknown (Bergman et al. 2013). Despite its long history, Trichodesmium is still unknown at many levels and there are many questions still to be answered.
Changes in technology have greatly improved our understanding of Trichodesmium on the global scale since 1997. Medium Resolution Imaging Spectrometer Satellite has shifted how Trichodesmium is measured on a planetary scale. Using the maximum chlorophyll index at a wavelength of 700nm due to Trichodesmium’s unique red pigment, long-term and seasonal trends are becoming better understood in the marine community (Gower et al. 2014). Recent research has found that blooms typically occur between May and July, but that there is some variation on a longer timescale of 10 years (Gower et al. 2014). Although other species have a similar wavelength to Trichodesmium when looking through a satellite, other species lack the background blooms that Trichodesmium cause, these secondary blooms make it easy to distinguish algal blooms (Gower et al. 2014).
As an attempt to quantify Capone et al.’s discussion of the importance of nitrogen fixation completed by Trichodesmium, an analysis of nine studies with 138 observations was conducted (Galloway et al. 2004). This study resulted in a measurement of nitrogen fixation yield of 128 μmoles/m²/day based on each study’s average yield. This paper is quite significant and well cited in the Trichodesmium research world, and was inspired by the research conducted in 1997 by Capone et al.
In the year following Capone et al.’s research, it was found that Trichodesmium appears to be the dominant nitrogen fixer in the ocean (Falkowski et al. 1998). The main suggestion as to why this may be is because Trichodesmium has a high photosystem I to photosystem II ratio (PSI:PSII). PSI is high in iron, which is a critical nutrient in the nitrogen fixation process, explaining the dominance of Trichodesmium in the global ocean.
During the same year that Capone et al. published their paper, it was confirmed by Carpenter et al. (1997) that Trichodesmium had, compared to any of the marine phytoplankton identified, the lowest nitrogen isotope 15 but the highest carbon isotope 13. This research suggested that Trichodesmium is one of the main reasons that N15 and C13 are so low in marine sediments and suspended particles in the tropical and subtropical oceans (Carpenter et al. 1997).
References
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Carpenter, Edward J., and Douglas G. Capone. Nitrogen in the Marine Environment. New York: Academic Press, 1983.
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