Which physiological mechanisms explain the different sensitivities and synergistic effects observed?

Correspondence: Felix Mark, Hans Pörtner | Integrative Ecophysiology | Alfred Wegener Institute for Polar & Marine Research, 27570 Bremerhaven, Germany | e: fmark@awi.de, hans.poertner@awi.de | p: +49-471-4831-1015, -1307

The concept of oxygen and capacity limitation of thermal tolerance (OCLT) provides a matrix integrating the synergistic effects of environmental stressors including ocean acidification. The thermal window is narrowed through CO

²

specific effects on molecular to whole organism functions. Available evidence suggests that the OCLT concept closely defines the sensitivity and response of individuals to climate change at ecosystem level. It also provides causality and quantifies the levels and changes of organism performance and resistance as a reason for changes in species interactions

¹

. An understanding of ecosystem-level processes results as needed to achieve more realistic estimates of species and ecosystem sensitivities to environmental change.

Which physiological mechanisms explain the different sensitivities and synergistic effects observed?

References: 1) Pörtner (2010) J Exp Biol 213:881-893 2) Walther et al. (2009) Biogeosciences 6:2207-2215 3) Walther et al. (2010) Mar Ecol Prog Ser 417:159-170 4) Deigweiher et al. (2008) Am J Physiol 295:R1660-1670

5. Ecosystem effects involve multiple stressors:

General Model

Genetic level: mRNA expression (x-fold) of gill Na⁺/HCO₃^- cotransporter (NBC1) is reduced in long-term warm acclimated Antarctic eelpout P. brachycephalum. Expression was normalized to β-actin and given relative to the expression of the respective control group animals.

CO₂ may reduce the capacity of the warm acclimation response in Antarctic fish gills and thereby contribute to an earlier onset of thermal stress.

Acclimation condition

0°C 5°C 5°C + CO₂

NBC1 expression (x-fold)

0.0 0.2 0.4 0.6 0.8 1.0 1.2

* **

* * *

#

Mitochondrial level: Mitochondrial capacities that generally are in excess of whole organism functional capacities and energy turnover are thermally less responsive under elevated P^CO2 in Antarctic notothenioids.

At thermal extremes, mitochondria may not display sufficient capacity to meet whole organism energy demand causing an earlier onset of thermal stress.

7. Synergistic effects: Mechanisms underneath

Q₁₀: 3.2 Q₁₀: 2.1 Q₁₀: 2.1 Q₁₀: 1.9

ComplexII state III respiration

0 6 12 0 6 12 0 6 12 0 6 12 0

1 2 3 4 5 6 7

8 #

assay temperature [°C]

[nmol O 2*min-1 *mg-1 ]

0°C 0°C, 0.2kPa 7°C 7°C, 0.2kPa

Pachycara brachycephalum

Notothenia rossii

Strobel et al., in prep.

Sun et al., in prep.

6. Confirmation of general model

Whole organism level: CO₂ sensitivity is temperature dependent, and, vice versa, temperature sensitivity is CO₂ dependent.

The thermal window of the spider crab Hyas araneus is progressively narrowed by elevated CO₂ levels, indicated by the shift in upper critical temperature (Tc) to lower values².

Hyas araneus Walther et al., 2009

Working model for acid-base regulation under hypercapnia in the gills of the marine teleost Zoarces viviparus⁴. Red circles: gene expression during short-term hypercapnia (24 to 96 h), green circles: long-term response (6wks). mRNA levels were found to be up-regulated (+), down-regulated (-) or unchanged (=). HA: H⁺-ATPase, NHE1/2/3: Na⁺/H⁺-exchanger isoforms, CA: Carbonic anhydrase, AE1: Cl^-/HCO₃^- -exchanger, NKA:

Na⁺/K⁺-ATPase, NBC1: Na⁺/HCO₃^— co-transporter, NKCC: Na⁺/K⁺/2Cl^— co-transporter. Open arrows: changes in substrate concentrations.

environment fish gill epithelium blood

CO₂

NBC1 HCO₃

NHE2/3

H⁺ Na⁺

2 K⁺ K⁺

H₂O

HCO₃ H⁺

CA +

Cl^- AE1

HCO₃

Cl^- Na⁺

+

NKA ATP

ATP

NKCC

Na⁺ H⁺

Na⁺ K⁺ 2 Cl^- Na⁺

3 Na⁺

Na⁺

NHE1

CO₂

Cl^--channel HA

K⁺channel Na⁺channel

Na⁺

-

+

+

=

=

+ -

-

+ +

+

-

+

Net H⁺ extrusion H⁺

-

-

-

4. Specific effects:

Gene expression in fish gills

S^TIRRER

T^EMPERATURE C^ONTROLLED C^HAMBER

3. Specific effects:

Energy budget of fish gills

V-type ATPase (Bafilomycin)

Cl^-/ HCO₃^- exchanger

(DIDS)

Na⁺/ K⁺ ATPase (Ouabain) Na⁺/ H⁺

exchanger (EIPA)

With full compensation of acid-base status, some ion transporters in perfused cod gills display enhanced turnover rates indicating elevated energy demand and a shift in gill energy budget elicited by changes in P^CO2.

control

inside (perfusate)outside (seawater)

Gadus morhua

Schaum et al., in prep.

7,4 7,5 7,6 7,7

7,3 7,8

extracellular [HCO 3- ] (mM)

0 2 4 6 8 10 12 14

2,26 1,86 1,60 1,33 1,07

0,80

0,53

0,27 0,13 PCO₂ (kPa)

β_NB = 8.95 mmol/pH/l

pH

PCO2 (kPa) 0.04

0.2 0°C 7°C

Bivalves like the Greenland Smoothcockle (Serripes groenlandicus) only passively regulate extracellular pH regardless of temperature.

2. Specific effects: Acid-base Regulation in Polar Organisms

0,0 0,5 1,0 1,5 2,0 2,5

7 7,1 7,2 7,3 7,4 7,5 7,6 7,7 7,8 7,9 8 8,1

0,0 0,5 1,0 1,5 2,0 2,5

7,0 7,1 7,2 7,3 7,4 7,5 7,6 7,7 7,8 7,9 8,0 8,1

2,5 2,0 1,5 1,0 0,5 0,0

2,5 2,0 1,5 1,0 0,5 0,0

2,5 2,0 1,5 1,0 0,5

0,07,0 7,2 7,4 7,6 7,8 8,0

extracellular pH CH ralullecartxeO 3- ]Mm[

~4°C

~1°C

~7°C

~380 ppm

~750 ppm

~3000 ppm ~1120 ppm

CH ralullecartxeO 3- ]Mm[ CH ralullecartxeO 3- ]Mm[

β_NB=0.4 mmol/pH/l

P^CO ₂(sea water):

1330 660

2000 2660

4000 5330

6670 :extracellular

P^CO2(ppm)

1330 660

2000 2660

4000 5330

6670

1330 660

2000 2660

4000 5330

6670

:extracellular P^CO2(ppm)

:extracellular P^CO2(ppm)

β_NB=0.4 mmol/pH/l β_NB=0.4 mmol/pH/l

Stark et al., in prep.

Strobel et al., in prep.

Serripes groenlandicus

Notothenia rossii

Antarctic fish (Notothenia rossii) show a compensation of blood pH by active HCO₃^- uptake at low temperatures and even overcompensation at warm temperatures.

Organisms from polar areas are exposed to the largest changes in P^CO₂ and may be most sensitive due to low capacities of physiological functions and low metabolic rates.

Hyas araneus larvae

In Megalopae and post larvae of the spider crab Hyas araneus from Arctic and temperate regions, hypercapnia leads to developmental delay and enhanced mortality, illustrating the high sensitivity of early life stages³.

1. Whole Organism Synergistic Effects (e.g. crab larvae)

0 20 40 60 80 100 120 140 0

20 40 60 80

100 9°C

Survival (%)

0 10 20 30 40 50 60 70

0 20 40 60 80

100 15°C

0 20 40 60 80 100 120 140 0

20 40 60 80

100 9°C

Developmental time (d)

Survival (%)

0 10 20 30 40 50 60 70

0 20 40 60 80 100

Normocapnia 710 ppm 3000 ppm

15°C

Normocapnia 710 ppm 3000 ppm

Developmental time (d)

Helgoland (south)

Svalbard (north)

Megalopa Crab I

(source: Schiffer/Harms)

Walther et al., 2010

Introduction

Atmospheric CO

₂

accumulation elicits climate change and associated impacts on marine ecosystems, emphasizing the need for an integrative understanding of the driving forces and their specific and synergistic effects. Besides indirectly inducing ocean warming, CO

₂

directly causes ocean acidification, but the specific contribution of this process to ongoing ecosystem change is not yet clear. Learning about the principles involved can benefit from the observed organism and ecosystem responses to the warming trend.

Understanding the specific effects of CO

₂

and the synergisms with temperature requires the identification of sensitive physiological mechanisms.

Unifying principles of ocean acidification effects on marine ectotherms?

F C Mark, A Stark, K Walther, A Strobel, C E Schaum, C Kreiss, K Deigweiher, T Sun, M Lucassen, C Bock, F J Sartoris, H O Pörtner

Alfred Wegener Institute

for Polar and Marine Research

Bremerhaven, Germany