''Crocosphaera watsonii'' (strain WH8501) is an isolate of a species of unicellular (2.5-6 µm diameter), diazotrophic marine cyanobacteria which represent less than 0.1% of the marine microbial population.
They thrive in offshore, open-ocean oligotrophic regions where the waters are warmer than 24 degrees Celsius. ''Crocosphaera watsonii'' cell density can exceed 1,000 cells per milliliter within the euphotic zone; however, their growth may be limited by the concentration of phosphorus.
''Crocosphaera watsonii'' are able to contribute to the oceanic carbon and nitrogen budgets in tropical oceans due to their size, abundance, and rapid growth rate.
''Crocosphaera watsonii'' are unicellular nitrogen fixers that fix atmospheric nitrogen to ammonia during the night and contribute to new nitrogen in the oceans.
They are a major source of nitrogen to open-ocean systems.
Nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
is important in the oceans as it not only allows phytoplankton to continue growing when nitrogen and ammonium are in very low supply but it also replenishes other forms of nitrogen, thus fertilizing the ocean and allowing more phytoplankton growth.
Diel rhythm
Diazotrophic cyanobacteria have evolved a number of mechanisms to ensure that oxygen doesn't interfere with the oxygen-sensitive enzyme complex responsible for
nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
, since these enzymes are sensitive to oxygen.
Oxygen is especially toxic to nitrogenase. One way in which nitrogen-fixing cyanobacteria, such as those in the genera "
Nostoc
''Nostoc'', also known as star jelly, troll’s butter, spit of moon, fallen star, witch's butter (not to be confused with the fungi commonly known as witches' butter), and witch’s jelly, is the most common genus of cyanobacteria found in vari ...
" and "
Anabaena
''Anabaena'' is a genus of filamentous cyanobacteria that exist as plankton. They are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanob ...
", avoid oxygen toxicity is through specialized cells called
heterocyst
Heterocysts or heterocytes are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as '' Nostoc punctiforme'', ''Cylindrospermum stagnale'', and ''Anabaena sphaerica''. They fix nitrogen fr ...
s that lack
photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
and hence do not have oxygen evolution. As a diazotrophic cyanobacterium that has oxygen-evolving photosystem II, one of the ways by which ''Crocosphaera watsonii'' protects its
nitrogenase
Nitrogenases are enzymes () that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the Organic redox reaction, reduction of nitrogen (N2) to ammonia (NH3). Nitrog ...
from oxygen is through the adaptation of a diel rhythm.
This temporally separates carbon and nitrogen metabolism between day and night, respectively.
Since an array of enzymes are involved in the regulation of carbon and nitrogen metabolism, this separation entails temporal regulation of many enzymes and processes in these bacteria. Consistent with this, many genes involved in
nitrogen fixation
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmo ...
and
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
show variation in transcription between day and night.
This difference in gene expression between light and dark periods for genes involved in nitrogen fixation was about ten times bigger than for genes involved in photosynthesis, suggesting more stable photosystems compared to nitrogenase in these bacteria.
The expression of many genes involved in nitrogen fixation and photosynthesis have been individually monitored. These include genes encoding the proteins nifH, nifX,
glutamate synthetase, ntcA, glnB-like proteins and glgP, all of which have essential roles in nitrogen fixation.
The expression of all these genes were at their peak at the beginning of dark period and were at their minimum in the early light period.
Furthermore, genes encoding for photosynthetic proteins such as
photosystem I
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses ...
subunit VII (psaC), the
photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
D1 protein D1, D01, D.I, D.1 or D-1 can refer to:
Science and technology Biochemistry and medicine
* ATC code D01 ''Antifungals for dermatological use'', a subgroup of the Anatomical Therapeutic Chemical Classification System
* Dopamine receptor D1, a prote ...
(psbA1) and the photosystem II oxygen-evolving complex protein (psbO), and glgA showed diel variation in expression, with the highest and lowest expression at daytime and nighttime, respectively.
About 30% of the genome of ''Crocosphaera watsonii'' showed diel variation in expression,
indicating its importance. All of these patterns indicate a temporal separation of photosynthesis and nitrogen fixation in ''Crocosphaera watsonii.''
Photosystem II blockage
Another method by which ''Crocosphaera watsonii'' protects its nitrogen fixing enzyme complexes, which is unique to this species, is through its ability to shut down
photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
.
When cultures of ''Crocosphaera watsonii'' are exposed to varying light regimes the diel light:dark cycle regulating nitrogen fixation is disrupted.
Because diazotrophs are able to fix nitrogen in the presence of light, especially during the times of transition from light to dark periods, there must be other methods by which they protect themselves in the presence of oxygen. ''Crocosphaera watsonii'' does this by tightly constraining oxygen evolution in several ways. It has the ability to shut down photosystem II mechanisms in dark periods by reducing its own plastoquinone pool.
Plastoquinone
Plastoquinone (PQ) is an isoprenoid quinone molecule involved in the electron transport chain in the light-dependent reactions of photosynthesis. The most common form of plastoquinone, known as PQ-A or PQ-9, is a 2,3-dimethyl-1,4-benzoquinone mol ...
(PQ) is an essential protein in the electron transfer system, and by limiting the amount of plastoquinone available, Photosystem II is essentially shut down. The cell accomplishes this by changing the redox state of plastoquinone in the dark.
''Crocosphaera watsonii'' also slows its electron transfer rate closer to the dark period, and the cell transitions itself to shut down its photosystem.
Phycobilisomes
Phycobilisomes are light harvesting antennae of photosystem II in cyanobacteria, red algae and glaucophytes. It was lost in the plastids of green algae / plants (chloroplasts).
General structure
Phycobilisomes are protein complexes (up to 600 ...
, which are light harvesting complexes for
photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
are also shown to attach to photosystem I during the night period, adding a further layer of protection to its nitrogen fixing machinery.
Another method of temporarily disabling photosystem II that ''Crocosphaera watsonii'' employs is the deactivation of the D1 protein. The D1 protein forms the reaction center of photosystem II and is important as it binds chlorophyll.
By mutating and thus deactivating D1, photosystem II is also shut down, allowing nitrogen fixation to occur.
Regulating photosystem II in ''Crocosphaera watsonii'' is a fast process, and it can be inactivated or activated immediately when needed. The recycling of the same iron molecules in both photosynthesis and nitrogen fixation also helps enforce the diel cycle of photosynthesis and nitrogen fixation.
Iron salvage
''Crocosphaera watsonii,'' which contributes significantly to oceanic nitrogen fixation,
is limited by iron as is also the case for other marine
diazotroph Diazotrophs are bacteria and archaea that fix gaseous nitrogen in the atmosphere into a more usable form such as ammonia.
A diazotroph is a microorganism that is able to grow without external sources of fixed nitrogen. Examples of organisms that d ...
s.
The reason for this is that the enzyme responsible for nitrogen fixation in diazotrophs, nitrogenase, uses iron as a cofactor.
To overcome the challenge of iron scarcity, ''Crocosphaera watsonii'' has come up with a unique mechanism that involves reduction of
metalloenzyme
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains al ...
inventories.
In this mechanism, the metalloenzyme will be synthesized only when needed and will be degraded as soon as they are no longer needed, and in the process the associated iron is recycled.
In this way, less iron is taken up unnecessarily by the metalloenzyme when it is inactive, and the iron can be used in other processes such as nitrogen fixation by nitrogenase.
This mechanism reduces the iron requirement of ''Crocosphaera watsonii'' by up to 75%, but it costs energy.
This energy is provided by high-energy carbon molecules produced during the light period via photosynthesis.
Moreover, it has been demonstrated that ''Crocosphaera watsonii'' adapts to iron deprivation by reducing its biovolume and its growth rate.
Phosphorus limitation
Another limitation that ''Crocosphaera watsonii'' encounters is phosphorus limitation. Because ''Crocosphaera watsonii'' lives in oligotrophic waters, it can be limited by a variety of nutrients, including dissolved inorganic phosphate.
Phosphate is required in many cellular processes including photosynthesis where it is an integral part of its machinery.
Phosphorus is also a key requirement for nitrogen fixation. By looking at the response of ''Crocosphaera watsonii'' to phosphorus limitation, the effects on nitrogen fixation can also be examined.
''Crocosphaera watsonii'' showed a decrease in photosynthetic efficiency when limited by phosphorus. Unlike most cyanobacteria, ''Crocosphaera watsonii'' does not regulate its phosphate use and metabolism inside the cell during low phosphate conditions.
Instead, ''Crocosphaera watsonii'' uses pstS, a protein that has a high affinity to bind to phosphorus.
During times of phosphorus stress, ''Crocosphaera watsonii'' is shown to have increased levels of pstS transcription.
This is the defensive response of ''Crocosphaera watsonii'' to lower levels of phosphorus, for more efficient uptake. While increasing pstS levels correspond to a lower concentrations of phosphorus around cells, pstS transcript concentrations have no relationship to phosphorus concentrations. PstS transcript levels in ''Crocosphaera watsonii'' instead correspond to the diel cycle, peaking at the end of the light phase.
Another method to measure phosphorus stress in ''Crocosphaera watsonii'' is to look at arsenic concentrations within the cell.
As phosphorus is limited, arsenic is also accumulated inside the cell. As a response, ''Crocosphaera watsonii'' will upregulate arsB levels, which will remove the excess arsenic from within the cell.
These, when looked at together can tell us the condition of the cells in a given ''Crocosphaera watsonii'' population.
References
{{Taxonbar, from=Q29887660
Chroococcales