Ecology and Energetics of Feeding Strategies in eukaryotes
Feeding Strategies of Eukaryotes
Radical evolutionary transition at the dawn of the eukaryotes – with this change in
cellular complexity arose new mechanisms to acquire energy and nutrients
Heterotrophy
a) Intracellular parasite symbiont
Intracellular parasites: usually sit in cytosol or host induced
phagosome (sometimes also nucleus) (example?)
Taking up nutrients from host using transporter proteins
Reproduce in host and burst from cell so that the life-cycle
can repeat
Reliance on host derive metabolites has led to loss of many
biosynthetic pathways (Gardner et al, 2002)
b) Osmotrophy
- Three steps: secretion of degradative enzymes (by golgi and ER) secrete
complex polymers into extracellular space, extracellular digestion, uptake of
nutrients using transporter proteins
- Important to recognise cellular systems like ER, Golgi, cytoskeleton allowed
directed and efficient targeting of osmotrophic function
Which organisms use osmotrophy?
- Using combination of cell structure data and molecular phylogeny to identify
evolutionary relationships (Keeling et al., 2005)
- All eukaryotic (and prokaryotic) cells have some form of osmotrophic function
- But some forms specialise: Fungi (Candida albicans, plant parasites, saprotrophic
– mushrooms break down plant structures) and Oomycetes and
hyphochytriomycetes (plant and salmon parasites – Phyphoteran infestans)
Extracellular Digestion
Key model system for studying this is the secreted enzyme invertase in
Sacharomyces cerevisiae
- Has an N-terminal targeting peptide, that “labels” enzyme so is secreted from
cell through the ER
Catalyses breakdown of sucrose to glucose and fructose
Single hexoses can then be taken up by the cell at higher
efficiency using a range of hexose transporters
In yeast 20 different proteins function in hexose uptake
Includes 6 transporters for glucose, fructose and mannose
, 99% of liberated hexoses are lost to competitors or by diffusion (Gore et al., 2009)
- Highly inefficient feeding strategy
Which leads to public goods games (ie. competitive osmotrophic interactions) –
organisms which want to benefit from others without doing any work
Key model system for experimentation in microbial social interactions
Public Goods Games and Consequences
Worker vs. Cheater (not producing enzymes) –
obtaining sugars that the worker has produced
- Osmotrophic public goods games can lead to
conflict control which organisms can enter
ecosystem/benefit from the public goods games
One more class of genes is important in
osmotrophic interactions: toxin
production/detoxification (often as gene clusters)
- Fungi have huge variety of secondary metabolite pathways – many of them have
unknown function but are toxic
- Often called spiteful traits
- Mechanism for manipulating community composition and public goods
interactions
Extracellular hydrolysis of sucrose allows other cells to share glucose and
fructose leading to selection for “multicellular conglomeration” of
daughter cells
Fungal multi-cellular structures (ie. hyphal networks) can be advantageous
as a consequence of osmotrophic feeding (“maximise surface coverage of
environment and minimise nutrient loss”
Transporter Proteins
What enzymes are secreted and what
transporters are available determines how a
osmotrophic cell feeds and therefore what
environment it can colonise
As transporters determine how osmotrophs
feed, very important
Selection on transporter function is intense –
eg. glycolytic flux in yeast cells is rate limited
by sugar transport (up to a certain threshold)
– therefore the faster a cell can uptake sugar the faster it will metabolise, grow,
reproduce and outcompete others
, Selection experiments on yeast in glucose
limiting conditions for 450 generations
(relatively small scale in evolutionary terms)
- Resulted in improvement in sugar uptake in
the ‘evolved” progeny compared to parental
cell line
Secret to the success of fungi
Rigid cell wall, (composed of chitin and cellulose –
primarily composed of linked sugar monomers =
highly demanding growth strategy) conveys
resistance to high turgor pressure, enables an
osmotrophic lifestyle and high metabolic rate
Loss of phagotrophy (Bartnicki-Garcia, 1987)
Expands at the apex (tip)
- Requires cell material (equipment to make chitin
etc.) to be transported to the tip (eg membranes,
wall forming enzymes, ribosomes)
- Filamentous structures form into multi-cellular
structures (hyphal networks) that improve
efficiency and ecosystem coverage
- Feed as they grow
The Spitzenkoerper: complex machinery that
drives filamentous growth
- Filamentous growth and transport is highly
demanding
- Eg. Neurospora growth requires 38 000
vesicles fusing with the tip per minute
(Steinberg, 2007)
c) Phagotrophy
Ingestion or internalisation of “large”
particles by a cell (usually greater than 0.4um)
Range of important functions
Distinct from general process of endocytosis
Feeding Strategies of Eukaryotes
Radical evolutionary transition at the dawn of the eukaryotes – with this change in
cellular complexity arose new mechanisms to acquire energy and nutrients
Heterotrophy
a) Intracellular parasite symbiont
Intracellular parasites: usually sit in cytosol or host induced
phagosome (sometimes also nucleus) (example?)
Taking up nutrients from host using transporter proteins
Reproduce in host and burst from cell so that the life-cycle
can repeat
Reliance on host derive metabolites has led to loss of many
biosynthetic pathways (Gardner et al, 2002)
b) Osmotrophy
- Three steps: secretion of degradative enzymes (by golgi and ER) secrete
complex polymers into extracellular space, extracellular digestion, uptake of
nutrients using transporter proteins
- Important to recognise cellular systems like ER, Golgi, cytoskeleton allowed
directed and efficient targeting of osmotrophic function
Which organisms use osmotrophy?
- Using combination of cell structure data and molecular phylogeny to identify
evolutionary relationships (Keeling et al., 2005)
- All eukaryotic (and prokaryotic) cells have some form of osmotrophic function
- But some forms specialise: Fungi (Candida albicans, plant parasites, saprotrophic
– mushrooms break down plant structures) and Oomycetes and
hyphochytriomycetes (plant and salmon parasites – Phyphoteran infestans)
Extracellular Digestion
Key model system for studying this is the secreted enzyme invertase in
Sacharomyces cerevisiae
- Has an N-terminal targeting peptide, that “labels” enzyme so is secreted from
cell through the ER
Catalyses breakdown of sucrose to glucose and fructose
Single hexoses can then be taken up by the cell at higher
efficiency using a range of hexose transporters
In yeast 20 different proteins function in hexose uptake
Includes 6 transporters for glucose, fructose and mannose
, 99% of liberated hexoses are lost to competitors or by diffusion (Gore et al., 2009)
- Highly inefficient feeding strategy
Which leads to public goods games (ie. competitive osmotrophic interactions) –
organisms which want to benefit from others without doing any work
Key model system for experimentation in microbial social interactions
Public Goods Games and Consequences
Worker vs. Cheater (not producing enzymes) –
obtaining sugars that the worker has produced
- Osmotrophic public goods games can lead to
conflict control which organisms can enter
ecosystem/benefit from the public goods games
One more class of genes is important in
osmotrophic interactions: toxin
production/detoxification (often as gene clusters)
- Fungi have huge variety of secondary metabolite pathways – many of them have
unknown function but are toxic
- Often called spiteful traits
- Mechanism for manipulating community composition and public goods
interactions
Extracellular hydrolysis of sucrose allows other cells to share glucose and
fructose leading to selection for “multicellular conglomeration” of
daughter cells
Fungal multi-cellular structures (ie. hyphal networks) can be advantageous
as a consequence of osmotrophic feeding (“maximise surface coverage of
environment and minimise nutrient loss”
Transporter Proteins
What enzymes are secreted and what
transporters are available determines how a
osmotrophic cell feeds and therefore what
environment it can colonise
As transporters determine how osmotrophs
feed, very important
Selection on transporter function is intense –
eg. glycolytic flux in yeast cells is rate limited
by sugar transport (up to a certain threshold)
– therefore the faster a cell can uptake sugar the faster it will metabolise, grow,
reproduce and outcompete others
, Selection experiments on yeast in glucose
limiting conditions for 450 generations
(relatively small scale in evolutionary terms)
- Resulted in improvement in sugar uptake in
the ‘evolved” progeny compared to parental
cell line
Secret to the success of fungi
Rigid cell wall, (composed of chitin and cellulose –
primarily composed of linked sugar monomers =
highly demanding growth strategy) conveys
resistance to high turgor pressure, enables an
osmotrophic lifestyle and high metabolic rate
Loss of phagotrophy (Bartnicki-Garcia, 1987)
Expands at the apex (tip)
- Requires cell material (equipment to make chitin
etc.) to be transported to the tip (eg membranes,
wall forming enzymes, ribosomes)
- Filamentous structures form into multi-cellular
structures (hyphal networks) that improve
efficiency and ecosystem coverage
- Feed as they grow
The Spitzenkoerper: complex machinery that
drives filamentous growth
- Filamentous growth and transport is highly
demanding
- Eg. Neurospora growth requires 38 000
vesicles fusing with the tip per minute
(Steinberg, 2007)
c) Phagotrophy
Ingestion or internalisation of “large”
particles by a cell (usually greater than 0.4um)
Range of important functions
Distinct from general process of endocytosis
