Urchins in a high CO2 world: partitioned effects of body-size, ocean warming and acidification on metabolic rate

Abstract

Body size and temperature are the major factors explaining metabolic rate, and the additional factor of pH is a major driver at the biochemical level. These three factors have frequently been found to interact, complicating the formulation of broad models predicting metabolic rates and hence ecological functioning. In this first study of the effects of warming and ocean acidification, and their potential interaction, on metabolic rate across a broad range in body size (two to three orders of magnitude difference in body mass), we addressed the impact of climate change on the sea urchin Heliocidaris erythrogramma in context with climate projections for southeast Australia, an ocean warming hotspot. Urchins were gradually introduced to two temperatures (18 and 23°C) and two pH levels (7.5 and 8.0), at which they were maintained for 2 months. Identical experimental trials separated by several weeks validated the fact that a new physiological steady state had been reached, otherwise known as acclimation. The relationship between body size, temperature and acidification on the metabolic rate of H. erythrogramma was strikingly stable. Both stressors caused increases in metabolic rate: 20% for temperature and 19% for pH. Combined effects were additive: a 44% increase in metabolism. Body size had a highly stable relationship with metabolic rate regardless of temperature or pH. None of these diverse drivers of metabolism interacted or modulated the effects of the others, highlighting the partitioned nature of how each influences metabolic rate, and the importance of achieving a full acclimation state. Despite these increases in energetic demand there was very limited capacity for compensatory modulating of feeding rate; food consumption increased only in the very smallest specimens, and only in response to temperature, and not pH. Our data show that warming, acidification and body size all substantially affect metabolism and are highly consistent and partitioned in their effects, and for H. erythrogramma, near-future climate change will incur a substantial energetic cost.

Seagrass (Posidonia oceanica) seedlings in a high-CO2 world: from physiology to herbivory

“…Additionally, we performed the feeding experiments only with the sea urchin P. lividus , whereas different herbivore species may have responded differently to CO 2 -driven changes in plant chemical composition4574 or epiphyte abundance or composition since it is expected that fleshy epiphytes may increase93 and calcareous epibionts would decrease their abundance under low pH conditions30. While we only performed the feeding experiments under ambient CO 2 water conditions, studies to date with adult sea urchins do not suggest strong changes in feeding rates9495, nor in preferences (S.R. Fitzpatrick, personal communication) under ambient vs. high p CO 2 conditions.…”

Ocean acidification has little effect on developmental thermal windows of echinoderms from Antarctica to the tropics

“…This can either be in the form of lower seawater pH reducing thermal tolerance and narrowing thermal windows (as in Metzger et al ., ; Walther et al ., ), or alternatively, warming may counteract effects of reduced pH through stimulation of metabolic rates (as in Byrne et al ., ; Sheppard Brennand et al ., ). Furthermore, it is possible that the metabolic costs associated with the stressors may be additive (Byrne, ; Carey et al ., ). Among these outcomes, the present study found that for all the developmental stages and species examined, the effect of pH on thermal windows was additive, with increased abnormality and growth reductions under reduced pH being consistent across thermal gradients.…”

“…Similar results were observed in a recent study on the effects of warming and reduced pH on metabolic rates of juvenile and adult Heliocidaris erythrogramma (Carey et al ., ) in which urchins were exposed to two temperature treatments (18 °C and 23 °C) and two pH treatments (7.5 and 8.0) for a period of 2 months. Thermal stress incurred a 20% increase on the metabolic rate, while reduced pH increased the metabolic rate by 19%.…”

Risk and resilience: variations in magnesium in echinoid skeletal calcite

“…In addition, sea urchins exhibit considerable capacity to adjust the acid−base balance of their coelomic fluid in response to increased acidification and thus can maintain body fluid homeostasis , although living with increased acidification incurs an energetic cost (Carey et al 2016). It may be that spines, teeth and test plates, although lower in Mg and thus less soluble, will prove to be more vulnerable to dissolution effects.…”