۱- Introduction

۲- Materials and methods

۳- Results

۴- Discussion

References

Abstract

Photosynthesis is an important driver of calcium carbonate deposition on tropical coral reefs largely due to the symbiosis of numerous invertebrates with photosynthetic dinoflagellates in the family Symbiodiniaceae. In bioeroding sponges, however, similar symbioses appear to support the decalcification of carbonate substrates. Compared to its role in calcification, the relative importance of photosynthesis to decalcification processes is less known. Here, the daytime and night-time chemical bioerosion rates of the common Indo-Pacific sponge Cliona orientalis were examined under varying levels of photosynthetic activity and/or varying abundances of intracellular dinoflagellates. Photosynthesis was manipulated either by preconditioning the sponges with the photosynthetic inhibitor diuron (DCMU), or by exposing them to short-term heat stress to achieve bleaching (loss of symbionts). DCMU reduced symbiont numbers and diminished their ability to evolve oxygen. Thermal stress caused a significantly greater loss of symbionts, but photosynthesis was less inhibited. In both cases, decreases in photosynthetic activity and symbiont densities led to proportionately lowered daytime chemical bioerosion rates. Moreover, increased rates of night-time bioerosion were linked to greater daytime rates of photosynthesis, rather than to the night-time respiration of the sponge holobiont. Our findings support the conclusion that photosynthetic products (photosynthates) and/or by-products (oxygen) stimulate sponge bioerosion. This work further reveals the importance of symbionts in the ecology of such sponges and in their ability to sustain high bioerosion activity in otherwise nutrient-poor ecosystems.

Introduction

Tropical coral reefs thrive in shallow clear waters where sunlight powers productive and diverse food chains. Symbioses between invertebrates and a variety of photosynthetic microorganisms enable high rates of productivity on reefs despite low environmental concentrations of inorganic nutrients (Muscatine and Porter, 1977; Seckbach, 2004; Yellowlees et al., 2008). The evolutionary success that results from hosting phototrophic microbes such as dinoflagellates, diatoms, chlorophytes, rhodophytes, or cyanobacteria is seen in a wide range of diverse invertebrates belonging to taxa such as Cnidaria, Mollusca, Foraminifera, Platyhelminthes, and Porifera (Trench, 1993; Stat et al., 2006; Lipps and Stanley, 2016). Presently almost 200 sponge species (Porifera) are known to host microbial photosynthetic symbionts (“photosymbiotic” sponges), which may contribute to the nutrition and health of the sponge (e.g. Wilkinson, 1987; Hill et al., 2011; Thacker and Freeman, 2012). Sponge species that receive substantial contributions in this way are often named “phototrophic” after the nutritional mode of their symbionts, or “mixotrophic” given that they also depend on filter-feeding by specialized food-trapping sponge cells. The nutrition of sponges is often further supplemented by inputs from intimate associations formed with heterotrophic bacterial populations of varying density and diversity (e.g. Hentschel et al., 2003; Taylor et al., 2007).