Defining the tipping point: a complex cellular life/death balance in corals in response to stress

Abstract

Apoptotic cell death has been implicated in coral bleaching but the molecules involved and the mechanisms by which apoptosis is regulated are only now being identified. In contrast the mechanisms underlying apoptosis in higher animals are relatively well understood. To better understand the response of corals to thermal stress, the expression of coral homologs of six key regulators of apoptosis was studied in Acropora aspera under conditions simulating those of a mass bleaching event. Significant changes in expression were detected between the daily minimum and maximum temperatures. Maximum daily temperatures from as low as 3°C below the bleaching threshold resulted in significant changes in both pro- and anti-apoptotic gene expression. The results suggest that the control of apoptosis is highly complex in this eukaryote-eukaryote endosymbiosis and that apoptotic cell death cascades potentially play key roles tipping the cellular life/death balance during environmental stress prior to the onset of coral bleaching.

Figure 1.

Light (a) and temperature (b) regimes within the aquaria of the experimental setup on outdoor decking at Heron Island Research station from 7°C below the study organisms physiological bleaching threshold, BT, up to exposure to the bleaching threshold, BT, adjacent to the reef flat and the dark apdapted photosynthetic yield of endosymbiotic dinoflagellates in both control and thermal stress conditions (c) throughout the experimental period.

Figure 2.

Gene expression (represented as fold change) of the identified pro-apoptotic Bcl-2 family members Bok (a), Bak (b) and Bax (c) relative to control, in the coral Acropora aspera in response to daily (8am, 1pm, 6pm) environmental temperature fluctuations within the experimental system from 7°C below the study organisms physiological bleaching threshold, BT, up to exposure to the bleaching threshold, BT.

Figure 3.

Gene expression (represented as fold change) of the identified anti-apoptotic of Bcl-2 (a) Bax inhibitor BI-1 (b) and BIR/survivin (c) relative to control in the coral Acropora aspera in response to daily (8am, 1pm, 6pm) environmental temperature fluctuations within the experimental system from 7°C below the study organisms physiological bleaching threshold, BT, up to exposure to the bleaching threshold, BT.

Figure 4. A conceptual model of the cell death and symbiosis breakdown under temperature and light stress in coral.

Red coloration indicates morphological evidence for apoptotic cell death; blue block coloration indicates no evidence for apoptotic cell death. Red arrow, significant change in gene expression related to cell death. Blue arrow, significant gene expression change related to cell survival. Grey arrow, indicates progression of the apoptosis cascade.