The Influence of a Pacific Invasive Sponge on Coral Reef Dynamics in Hawai‘i.

Abstract

Sponges are ecologically important components of many benthic ecosystems and are abundant on coral reefs. Many sponges host a diverse consortium of microbes and are known to rely on their symbiotic microbial communities for a variety of functions including nutrition, metabolic waste removal, and the production of secondary metabolites for chemical defense. Mycale grandis is an alien invasive sponge that first appeared in Hawaiʻi in the late 1990s and is found within several partially degraded shallow water coral ecosystems throughout the main Hawaiian Islands. In surveys of south Kāneʻohe Bay, M. grandis benthic coverage was found to range from 2% on fringing coral reefs to 32% in mangrove ecosystems. I report seawater pumping rates (0.016 L seawater s-1 kg-1 sponge (dry mass)) and ammonia oxidation rates (21.2 nM g-1h-1) for M. grandis, which are the first such rates measured in a Pacific sponge. Combining pumping rates, biomass estimates, and nitrogen flux rates for M. grandis with depth and circulation parameters in south Kāneʻohe Bay indicates that it is the most significant benthic source of dissolved inorganic nitrogen to the water column in the environments studied. Individual amino acid 𝛿13C and 𝛿15N values suggest that M. grandis acquires nutrition from its associated bacteria through direct assimilation of bacterially-synthesized amino acids. Statistically indistinguishable ΣV indices and trophic position of microbial and sponge cells also support this dietary strategy. These results strongly suggest that the M. grandis microbial consortia assimilate DOM, resynthesize the organic material, and pass on nutrition to the sponge in the form of amino acids through translocation. The vulnerability of native and endemic species to invasive reef species coupled with the threats of increased anthropogenic activity, ocean acidification, and ocean warming have growing implications for ecosystems throughout Hawaiʻi. It is evident that where sponges are abundant members of the reef community, the sponge holobiont can play important roles in organic matter recycling and can significantly alter nutrient profiles within the water column through their rapid rate of seawater circulation and biogeochemically active microbiome.