Transcript Detritus chains, Decomposers, microbial loop

Detritus chains, Decomposers,
Microbial loop
Wetzel 489-525, 731-783
The organic carbon cycle
Allochthonous
DOC, POC
Outflow DOC, POC
CO2
Littorial flora, Phytoplankton, Bacteria
Heterotrophic
organisms
Detritus
CO2
Detritus
Detritus
is all dead organic matter,
distinguishable from living organic and
inorganic matter.
. POM constitutes only about 10% of the
total detrital organic matter. DOM is about
90% (see Figure 17-9).
Detritus (Con.)
• Nonhumic substances:
Carbohydrates, proteins, peptides, amino acids, fats, waxes,
resins, pigments, and other low-molecular-weight organic
substances.
Labile, easily to be utilized and degraded by
microogranisms, exibit rapid flux rates; low instantaneous
concentrations in water
• Humic substances (80% of the organic matter):
Humic acids, fulvic acids, humin; the formation of humic
substances occurs during the degradation of higher aquatic
and terrestrial plant material (celluloses, hemicelluloses,
and lignin) by fungi and bacteria.
Loss: photolysis, microbial decompositon, aggregation and
sedimentation, and outflows
Sources:
. Autochthonous– dead organic
carbon is synthesized and cycling
within the system.
. Allochthonous– inputs of dead
organic carbon from sources external
to the ecosystem that enter and cycle
in the system
Detritus food chain
• Is any route by which chemical energy contained
within detrital organic carbon becomes available
to the biota.
• Includes the cycling of detrital organic carbon,
both dissolved and particulate, to the biota by
direct heterotrophy.
• Emphasizes the actual trophic linkage between the
non-living detritus and living organisms and
recognizes the metabolic activities of bacteria
attached to detrital substrates as a trophic transfer.
Detritus food chain (Con.)
fermentation
Hydrolysis
proteolysis
Protein
Alcohol,
simple sugars
Fatty acids
Carbohydrate
lipid
deamination
Amino acids
CO2 , CH4
nitrification
NH3
NO2+NO3
denitrification
N2
Factors affecting decomposition:
• Quality and quantity of organic matters
• Physical parameters: temperature,
stratification, basin parameter, size of
particles
• Chemical parameters: oxygen level
Detritus food chain (con.)
Table 17-13
Oligotrophic lakes
1) organic inputs are small
2) organic matter is exposed during sedimentation to oxic
conditions for long distances
3) degradation of sedimenting organic matter is relatively
complete
4) organic sediment accumulation is slow
Eutrophic lakes
1) Massive inputs of organic matter
2) sedimentation is rapid
3) less volume of aerobic water
4) rapid accumulation of organic matter in anaerobic
hypolimnia and sediments.
In streams or rivers
Fig. 23-2, Fig. 8-8
In rivers
• The role of animals in comparison to
microflora in decomposition of POM varies
widely, but is certainly small.
• DOM (>90%) drives the metabolism of
streams and rivers.
In lakes
• The decomposition of carbon by microflora of the
littoral and the sediments, probably dominates in
most lakes of the world since a vast majority of
lakes are small to very small.
• In large lakes or in hypereutrophic lakes, the
inputs from littoral sources are proportionately
low, and animals may assume a somewhat greater
importance in the degradation of POC.
• Decomposition of DOC is almost completely by
bacteria and fungi and by physical photolysis.
Annual Organic Carbon budget estimates in a 500-m segment
of the Kogesawa River, Uratakao, Japan (table 23-19)
Organic matter kg C yr-1
Percent (%)
Primary production
100
5.1
Litterfall into stream
90
4.6
Lateral movement to stream
38
1.9
Fine particulate organic matter
480
24.5
Dissolved organic matter
Groundwater inflow, DOC
1170
80
59.8
4.1
1958
100.0
Community respiration
77
4.1
Fine particulate organic matter
620
32.7
Dissolved organic matter
Subtotal
1200
1897
63.2
100.0
Inputs
Subtotals
Outputs
Total annual budget of Carbon fluxes for Lawrence Lake, Michigan. (table 23-16)
Components
g C m-2 yr-1
Inputs
Autochthonous
Phytoplankton
43.4
Submersed macrophytes
87.9
Epiphytic algae
37.9
Algal secretion and autolysis
14.7
Littoral plant secretion
5.5
Heterotrophy
2.8
Dark CO2 fixation
7.1
Allochthonous
stream and groundwater DOC
21
Stream
4.1
Shoreline litter POC
0.01
226.4
Outputs
Respiration
Benthic respiration
117.5
Bacterial respiration of DOC
20.6
Bacterial respiration of POC
8.6
Algal respiration
13
Permanent sedimentation
14.8
Coprecipitation of DOC with CaCO3
2
Outflow
dissoved
35.8
particulate
2.8
215.1
Percentage
19.1
38.8
16.8
6.5
2.5
1.2
3.1
9.3
1.8
0
100
54.6
9.6
4
6.1
6.9
1
16.5
1.3
100
Detritus food chain (conclusions)
• The central pool of organic carbon is DOC
• Detrital metabolism occurs principally in the benthic
region (lakes and rivers) and the pelagic area during
sedimentation (lakes).
• Most organic detritus is metabolized by the bacteria and
the fungi.
• Mineralizing rate is influenced by the geomorphology of
the ecosystem and the type of organic matter.
• Organic matter entering rivers and lakes from the
landscape is mostly in dissolved form and is chemically
modified by the flora and microflora of streams and
wetlands before reaching the lake.
Decomposers
• Abundance and distribution
Aquatic bacteria are typically nutrient limited the
numbers and biomass of bacteria increase with increasing productivity
and concentrations of inorganic and organic compounds in lakes.
In
eutrophic lake we usually expect a higher bacteria
biomass than oligotrophic lake. Bacteria are high
in epilimnion, decreases in hypolimnion and
increases sharply at water and sediment interface,
but decline again below the interface. (table 17-1)
Decomposer (con.)
• Seasonal: Genereally lower during winter than
during summer. This is because autochthonous and
allochthonous orgaic matters are low in low
temperature. Bacteria biomass generally increases
with organic matter loading. A close relation often
exists between seasonal changes in biomass of
phytoplankton and bacteria. Often bacteria
increase lag behind pulses of phytoplankton 5-10
days. The reason probably is that phytoplankton
organic matter which facilitating the development
of bacteria.
Seasonal changes in bacterial numbers and
production (Fig. 17-2)
The role of bacteria
• Decomposers
• Food
• Fix nitrogen
Traditional view of food web
Fish
Zooplankton
Phytoplankton
nutrients
Microbial loop—Stone & Weisburd, 1992
Fish
Protozoa
Zoop.
Bacteria
Phytoplankton
nutrients
The microbial loop is simply a model of
the pathways of carbon and nutrient cycling
through microbial components of pelagic
aquatic communities. (Wetzel, pp409)
Life at small size scales
1. Picoplankton (less than 2 m)
. In the North Atlantic, 60% of primary
production
. In oligotrophic seas, 80-90% of primary
production
2. Bacteria
. Consume 20-60% of primary production
. Three major ways: faeces and sloppy feeding
of ZP, release of exudates from algae, hydrolysis
of organic particles from bacteria.
Life at small size scales (con.)
3. Virus
4. Protozoa (dominant bacterivores)
. Microflagellates and protozoan ciliates
. High intrinsic growth rates
. Active bacterivores.
. Graze on picophytoplankton
. Mineralize nutrients efficiently.
Control of bacterioplankton by biota (Fig 17-6)
Functions of Microbes
• Small, high surface area to biomass ratios,
permitting a more intimate contact with the
environment, a greater uptake potential for
nutrients and a more rapid turnover of nutrients
and organic matter than larger organisms
• Slow sinking rates and tend to remain in the upper
waters of lakes and oceans for long periods before
settling out to greater depths. So the nutrients
enters the microbial loop have a greater likelihood
of remaining in the photic zone longer than those
incorporated directly into larger metazoans with
faster sinking rates.
Conclutions (microbial loop)
• The microbial loop is a model of pathways
of carbon and nutrient cycling through
microbial components of pelagic aquatic
communities.
• Protistan zooplankton are the most
important microbial consumers and have
major functions in organic carbon
utilization and nutrient recycling.