Under-studied organisms impact coral reefs
Coral reefs are endangered around the world. These complex ecosystems are anchored by corals and algae, the main producers of dissolved organic matter (DOM). DOM is a variable and complex mixture of thousands of organic substances and is not bioavailable to most marine animals. The DDMS project, undertaken with the support of the Marie Sklodowska-Curie Actions Programme, investigates how seawater bacteria and sponges process DOM.
Microbialisation of reefs
Overfishing and nutrient pollution cause reef microbialisation, a process that destabilises the food chain. Overfishing contributes to the imbalance by reducing the number of fish that feed on algae. At the same time, nutrient pollution further stimulates algal growth, at the expense of corals. With more algae in reef ecosystems, a vicious circle occurs. Algae produce more DOM than corals, and the increase attracts more microbes. With increased DOM biomass, the energy in reefs is directed towards microbes. This reduces the biodiversity of reefs and contributes to their decay.
Sponges, bacteria and the coral reef
While coral reefs around the world are in jeopardy, marine ecosystems are still poorly understood. Until recently it was unclear that sponges, a ubiquitous organism in many ocean environments, play a role in processing DOM and are essential to the carbon cycle. New knowledge about food web mechanisms helps explain threats to coral reefs. According to project fellow Benjamin Mueller: “Since DOM constitute the largest pool of organic carbon in the ocean and sponges dominate benthic communities in many coastal and deep-sea habitats, the ability to process refractory substances may have a significant effect on carbon cycling locally and potentially even on a global scale.” Data suggest that while seawater bacteria select certain components of DOM to process, sponges are less selective. Mueller says: “This may point towards a fundamental difference in the uptake mechanism of dissolved organic substances by sponges and bacteria and has major consequences to our understanding on how bacteria and sponges alter the quantity and quality of dissolved organic matter in our oceans.”
DOM uptake experiments
All Mueller’s field work occurred at the CARMABI Research Station in Curaçao in the Southern Caribbean where coral and algae species were incubated to produce DOM. Water analysis was performed using fluorescent spectroscopy, total dissolved amino acid analysis and untargeted metabolomics to help characterise differences between the DOM types. The DOM types were then fed either to encrusting sponges living inside the reef or to sea bacteria to discover how these organisms interact with coral-produced and algae-produced DOM. In recent decades many coral reefs shifted from coral- to algal-dominance, which likely also affected the composition of DOM present. Mueller tested the hypothesis that changes in DOM composition have processing consequences for sponges and bacteria by incubating coral and algae in situations that mimic past, present and future scenarios, with coral-dominant, equal ratio and algae-dominant conditions. Finally, Mueller explored the possibility of competition between sponges and bacteria. Sponges and bacteria were each fed water previously processed by the other organism to see if they were able to utilise the pre-processed water. Coral reefs are complex systems and they are under threat. With 90 % of the sea floor still unexplored, a better understanding of marine environments is needed. Investigating the DOM uptake mechanisms of sponges and bacteria is important to understanding and protecting reef ecosystems.
Keywords
DDMS, sponges, algae, coral reefs, microbialisation, seawater bacteria, , coral, DOM uptake, CARMABI, dissolved organic material
