What is limiting marine heterotrophic microorganisms ?

Johnny Berglund

Marine ecology

Department of Ecology and Environmental Science

Umeå University

Aquatic food web

by Kristina Wiklund, UMF

Some marine protists...

Rhynchomonas nasuta

Parvicorbicula sp.Plagioselmis prolonga

Bodo saliens

Copyright ©, Mats Kuylenstierna & Bengt Karlson

Mesodinium rubrum5m

5m

Heterotrophic nanoflagellates (HNF)

Ciliates

18E 20E 22E 24E 26E

U m eå

Lu leå

Turku

Vaasa

N orrbyn

Sweden

F in land

A5

A1 3

B3B7

C 1C 3

C 1 4

60N

62N

64N

66N

18E 20E 22E 24E 26E

60N

62N

64N

66N

Study area:Gulf of Bothnia

Gulf of Bothnia• Low salinity from 3 psu to 7 psu• Mean depth of 40 m in B.Bay and 66 m in B.Sea• Strong seasonality, ice-cover of 120 and 60 days• High river runoff

Annual average, Kuparinen et al. 1996

0

1

2

3

4

5

Bacterial biomass Phytoplankton biomass Zooplankton biomass

mm

ol

C m

-3

Bothnian Bay

Bothnian Sea

Seasonal pattern in bacterial and HNF abundance

HNF <5 ym

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N m

l-1

Bothnian Bay

Bothnian Sea

Bacteria

0.0E+00

1.0E+06

2.0E+06

3.0E+06

4.0E+06

5.0E+06

0 10 20 30 40 50

N m

l-1Bothnian Bay

Bothnian Sea

week

week

Question:

What is limiting growth of heterotrophic nanoflagellates (HNF)?- especially small flagellates <5m

Why?Because small flagellates (1-3 m) are most important predators on heterotrophic bacteria (Wikner & Hagström 1988 and others)

Hence, small flagellates are an important link to higher trophical levels

Definition (Osenberg & Mittelbach 1995):

Limitation is the extent to which a population’s, or trophic level’s, per capita growth

rate is depressed by the action of a particular factor, for example resource availability or predator risk.

Bothnian Sea

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1.50E+06

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Bac

t. m

l-1

Bothnian Bay

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Bac

t. m

l-1

HNF < 5 Bact.

Factor 1:

~1.5*106 ml-1

~2.5*106 ml-1

Natural threshold for HNF growth

Bacteria as resource

f.w. <90 <10 <5 <0.80.00

0.02

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volu

me

(mm

3 l -1)

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Treatment

f.w. <90 <10 <5 <0.80.0

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wth

ra

te (

d-1)

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A. Heterotrophic bacteria

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Initial growth rate Maximal biovolume

B. Small flagellates

C. Medium flagellates

D. Large flagellates

E. ChoanoflagellatesSamuelsson, K. & Andersson, A. 2002Predation limitation in the pelagic microbial food web,AME in press

• 0.8 – 5 m flagellates most important predator on bacteriaHeterotrophic

bacteria

HNF

PROBLEM...to distinguish effect of predation and effect of reduced competition for bacteria (resource)

Medium HNF

f.w

.

<90

<10

<5

<0.

8

f.w

.

<90

<10

<5

<0.

8

Factor 2:

• thightly size structured food-chain

Predation by large flagellates and ciliates

Hence, there are two factors limiting the

heterotrophic nanoflagellates

1. resource

2. predation

Lets setup an experiment to evaluatethe importance of the factors

•Limitation can be measured by quantifying the increase in a population’s per capita growth rate following the removal of a particular form of limitation, for example after the addition of a surplus resource or removal of predators.

 (Osenberg & Mittelbach 1995)

2 after removal of limitation

1 in natural conditions

Timet1t0

lnN2

lnN1

LnN

0

1 = (lnN1 –lnN0) / (t1-t0) 

Limitation L = 2 -1

+ bacteria

no addition

< 90 m < 10 m < 5 m

Experimental setup to determine limiting factors for HNF

Treatment Resource limitation Predator 5-10 m limitation

Predator 10-90 m limitation

< 90 m + + + < 90 m + bakt. - + + < 10 m + + - < 10 m + bakt. - + - < 5 m + - - < 5 m + bakt. - - -

Dialysis bags incubated in situ at 4 m

depth

Duration: 4 days with sampling every day

Performed in June 2001

Results

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

+ pred.90 -bakt.

+ pred.90 +bakt.

+ pred.10 -bakt.

+ pred.10+bakt.

- pred.-bakt.

- pred.+bakt.

HN

F g

row

th r

ate

h-1

Highly significant (p<0.001) effect of adding bacteria (resource)

Weak effect of predation (p<0.05) by large protozoans (10-90m)

(No effect of predation by large flagellates (5-10 m))

Results...

Relativ importance (change in pop. Growth)

resource 62 % and predation 38 %

Ls (h-1) Lp (h

-1)

B2 - B1 0.0092

B4 - B3 0.0110 B5 - B1 0.0062

B6 - B5 0.0084

B6 - B2 0.0054 Mean 0.0095 Mean 0.0058 Stdev. 0.0013 Stdev. 0.0006

To evaluate possible seasonal variations a second experiment was performed in September 2001

-same experimental setup

Experiment 2

Results...

0

0.005

0.01

0.015

0.02

+ pred.90 -bakt.

+ pred.90 +bakt.

- pred.- bakt. - pred. +bakt.

HN

F gr

owth

rat

e

Again significant (p<0.005) effect of adding bacteria

Relativ importance of resource 81 % and predation 19 %,

though effect of predation not significant

Conclusions

• HNF growth mostly resource limited

microbial food web bottom-up regulated

Modelling still to come....

Thank you for your attention!

Acknoledgements:

Agneta Andersson

Kristina Samuelsson

Thomas Kull

Umeå Marine Science Center