Filtering Out the Bad From the Good: Sponge Effects on Coral Reef Microbes

The local and global decline in coral reefs is concurrent with the growth of algae. Algal-dominated coral reefs are characterized by an increase in microbes, including opportunistic pathogens. The flow of organic matter in... [ view full abstract ]

The local and global decline in coral reefs is concurrent with the growth of algae. Algal-dominated coral reefs are characterized by an increase in microbes, including opportunistic pathogens. The flow of organic matter in coral reefs is controlled by the “sponge-loop,” whereby sponges consume and transform microbes into complex forms of carbon that become available to benthic organisms. Here we investigate whether sponges determine the structure and abundance of microbial communities in reefs with differing coral cover and subsequent microbial abundance. We compared the microbial community entering the sponge with the water exiting the sponge to show the differential consumption microbes by the sponge. We identified the effect sponges have on microbial communities by comparing the number and type of microbes outside and inside caves on coral reefs with either high or low coral cover. Microbial cell abundances in the caves of algae-dominated and coral-dominated coral reefs were 1,029,050 (± 48,982) and 748,853 (± 127,234) microbes per milliliter respectively, with an average difference of 280,197 (± 27,862) microbes per milliliter, revealing the amount of microbes sponges are capable of filtering possesses an upper limit. After visualizing the metagenomics data using an MDS plot, sponges in algal-dominated reefs were shown to filter microbes selectively, propagating lower microbial diversity. Our findings suggest sponges cannot reverse the initial shift from coral to aglae-dominated coral reefs and subsequent increase in microbial abundance. Therefore, maintaining the integrity of the whole ecosystem is important and conservation efforts should focus on preventative measures for coral reef preservation.