Projected near-future levels of temperature and pCO2 reduce coral fertilization success

Abstract

Increases in atmospheric carbon dioxide (pCO2) are projected to contribute to a 1.1-6.4°C rise in global average surface temperatures and a 0.14-0.35 reduction in the average pH of the global surface ocean by 2100. If realized, these changes are expected to have negative consequences for reef-building corals including increased frequency and severity of coral bleaching and reduced rates of calcification and reef accretion. Much less is known regarding the independent and combined effects of temperature and pCO2 on critical early life history processes such as fertilization. Here we show that increases in temperature (+3°C) and pCO2 (+400 µatm) projected for this century negatively impact fertilization success of a common Indo-Pacific coral species, Acropora tenuis. While maximum fertilization did not differ among treatments, the sperm concentration required to obtain 50% of maximum fertilization increased 6- to 8- fold with the addition of a single factor (temperature or CO2) and nearly 50- fold when both factors interact. Our results indicate that near-future changes in temperature and pCO2 narrow the range of sperm concentrations that are capable of yielding high fertilization success in A. tenuis. Increased sperm limitation, in conjunction with adult population decline, may have severe consequences for coral reproductive success. Impaired sexual reproduction will further challenge corals by inhibiting population recovery and adaptation potential.

Figure 1. Representative images of unfertilized and fertilized Acropora tenuis embryos.

Unfertilized eggs (a) are spherical and show no signs of cleavage while fertilized embryos (b) show distinct cell division.

Figure 2. Theoretical and actual fertilization curves for Acropora tenuis.

Schematic (a) showing the graphical representation of Fert_max and Sperm EC50, the sperm concentration at which 50% of Fert_max is achieved. The shading represents the region of sperm limitation resulting in less than maximal fertilization success (adapted from Marshall [15]). Fertilization curves (b) of Acropora tenuis by temperature and CO₂ treatments. Dashed black lines indicate 95% confidence intervals for each curve. Parameter estimates are given in Table 3.

Figure 3. Respiratory alterations of target pH levels by sperm concentration and treatment (Mean ± 1 SD).

Despite the rapid changes in pH at higher sperm concentrations (10⁶ and 10⁷ sperm ml⁻¹), high levels of fertilization (≥95%) were achieved. See text for details.