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Integrated analyses of nitrogen and phosphorus deprivation in the diatoms Phaeodactylum tricornutum and Seminavis robusta

Alipanah, Leila
Doctoral thesis
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URI
http://hdl.handle.net/11250/2375857
Date
2015
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  • Institutt for biologi [1651]
Abstract
Microalgae growth and production is highly dependent on nutrient availability. The

availability of nutrients in the ocean is variable on temporal and spatial scales.

Therefore, nutrient limitation is observed in many regions of the ocean. Nutrient

limitation can strongly affect microalgal metabolism through extensive physiological

and biochemical alterations. Diatoms, an ecologically important group of unicellular

algae, have acquired several acclimation mechanisms to cope with nutrient limitation.

In the present thesis, we utilized the diatoms Phaeodactylum tricornutum and Seminavis

robusta as model organisms. The thesis has been divided into three parts: 1)

Identification of acclimation mechanisms in P. tricornutum under nitrogen deprivation,

2) Uncovering molecular and metabolic modifications to phosphate deprivation in P.

tricornutum, and 3) Analysis of mechanisms underlying S. robusta nitrogen stress

response.

In part one, we found that the photosynthetic capacity and chlorophyll content of the

cells was reduced while neutral lipids increased in nitrogen-deprived cultures. We also

observed reduced biosynthesis and increased recycling of nitrogenous compounds.

Repression of the Calvin cycle and chrysolaminaran biosynthesis, and simultaneous

induction of glycolysis, the tricarboxylic acid cycle and pyruvate metabolism, led to

carbon reallocation. Neutral lipid accumulation was fed by funneling of carbon from

breakdown of carbon stores along with remodeling of membrane lipids and induction of

the de novo triacylglycerol biosynthesis.

In the second part, we looked at acclimation mechanisms used by P. tricornutum under

P deprivation. The strongest transcriptional responses were observed for genes encoding

proteins involved in phosphate acquisition and scavenging. We could also observe

changes in a number of processes involved in photosynthesis, nitrogen assimilation, and

nucleic acid and ribosome biosynthesis. P deprivation resulted in carbon and lipid

restructuring. Carbon metabolism was altered through induction of cytosolic glycolysis

and the pentose phosphate pathway, and suppression of the Calvin cycle. Finally, we

recognized several modifications in cellular lipids. Whereas phospholipid biosynthesis

was repressed, neutral lipid and sulfolipid biosynthesis were induced.

In the third part, we analyzed the acclimation of S. robusta to nitrogen deprivation. We

detected significant drop in cell growth as well as strong increase in neutral lipids

following nitrogen deprivation. While genes related to light absorption and

photosynthetic electron transport were down-regulated, several genes involved in

nitrogen uptake and assimilation were up-regulated. We suggested that the induction of

several genes connected to glycolysis, the TCA cycle and mitochondrial pyruvate

dehydrogenase complex could direct carbon skeleton and energy to lipid biosynthesis.

Despite the suppression of de novo fatty acid biosynthetic pathway, a fatty acid synthase

I gene was induced in N-deprived cells, which might lead to production of fatty acids

for triacylglycerol biosynthesis. We also proposed that accumulation of triacylglycerol

in nitrogen-replete cells mainly occurs via the up-regulation of phospholipases and

phospholipid:diacylglycerol acyltransferase.
Publisher
NTNU
Series
Doctoral thesis at NTNU;2015:245

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