Periodic Reporting for period 2 - DDMS (Differential DOM-cycling through Microbes and Sponges)
Période du rapport: 2022-08-15 au 2023-08-14
Coral reefs are a hotspot of biodiversity and productivity in the oceans and provide a plethora of ecosystem services including the support of fisheries, tourism, coastal protection, building materials, and pharmacological products with an estimated value of 352,249 $ ha-1 yr-1. Manmade disturbances, including nutrient pollution and overfishing, threaten these services by compromising the health of corals and at the same time stimulating the growth of algae.
Algae release large amounts of dissolved organic matter (DOM). This algal-DOM has been associated with growth of pathogenic bacteria and overall coral reef degradation—the “microbialization of reefs”. Sponges have been proposed to catalyze this reef degradation by feeding on algal-DOM, releasing nutrients, and thereby fertilizing further algal growth at the expense of corals.
The aim of this project is to investigate whether bacteria and sponges compete over DOM as food source. Thereto, the composition of coral- and algal-DOM, as well as mixtures representing past, current, and future DOM-producing communities was determined and the uptake and utilization of these DOM-types/-mixtures by bacteria and sponges were tested.
Algae release large amounts of dissolved organic matter (DOM). This algal-DOM has been associated with growth of pathogenic bacteria and overall coral reef degradation—the “microbialization of reefs”. Sponges have been proposed to catalyze this reef degradation by feeding on algal-DOM, releasing nutrients, and thereby fertilizing further algal growth at the expense of corals.
The aim of this project is to investigate whether bacteria and sponges compete over DOM as food source. Thereto, the composition of coral- and algal-DOM, as well as mixtures representing past, current, and future DOM-producing communities was determined and the uptake and utilization of these DOM-types/-mixtures by bacteria and sponges were tested.
Two field trips to the CARMABI research station on Curaçao were successfully carried out. In 2020, the composition of DOM released by two coral and three algal species, as well as general reef water was determined. Tested DOM-types differed considerably in the C and N content as well as fluorescent DOM (fDOM) components and total dissolved amino acid (TDAA) composition. Subsequently, the availability and processing of these DOM-types by sponges and bacteria were assessed in 72 incubations and 36 bioassays, respectively. Obtained results showed that depending on the DOM type and therefore DOM composition, different components were removed. Moreover, the data suggest that sponges and bacteria largely remove similar components from the DOM pool, yet while bacteria tend to take up certain components selectively (and thereby change the DOM composition), sponges appear to remove DOM less selectively (and change the DOM composition to a lesser degree). This may point towards a fundamental difference in the uptake mechanism of DOM by sponges and bacterioplankton and could have major consequences to what extent this processing alters the composition and thereby quality of the processed DOM. This difference in uptake mechanism may further explain why sponges tend to be better capable to process seawater without the extra addition of coral- or algal-DOM compared to bacterioplankton.
In nature, the available DOM is unlikely to be originating from one coral or algal species, but to be a mixture of different DOM types reflecting the presence of DOM producers in the environment. Therefore, three DOM mixtures mimicking past (coral-dominated), current (balanced), and future (algal-dominated) benthic DOM-producing communities were used in the second field trip in 2021. Moreover, the ability of sponges to utilize bacteria-processed DOM mixtures and vice-versa was assessed in a total of 64 sponge incubations and 32 bioassays with bacterioplankton. As with the individually tested DOM types before, there were pronounced differences in the response (e.g. bacterial growth rates and change in bacterial community composition) to the DOM mixtures and sponges and bacteria removed similar components from the DOM mixtures. Interestingly, the change in bacterial community composition during bioassays were similar irrespective whether the DOM mixture was pre-procced by sponges or not. On the other hand, pre-processing can have an effect on bacterial growth. Depending on the respective DOM mixture, pre-processing by sponges can suppress (seawater without addition of coral-algal DOM mixture), stimulate (current DOM mixture), or not affect growth rates (past and future DOM mixture). This suggest that DOM composition can have a considerable effect on the interaction between sponges and microbes.
Despite considerable limitations during the outgoing phase due to the global COVID-19 pandemic, project activities and results were disseminated through, social media channels, the interactive live broadcast lecture series AXA coral live, invited lectures, participation at international conferences, features in two TV documentaries, as well as press releases and blog posts (see photo).
In nature, the available DOM is unlikely to be originating from one coral or algal species, but to be a mixture of different DOM types reflecting the presence of DOM producers in the environment. Therefore, three DOM mixtures mimicking past (coral-dominated), current (balanced), and future (algal-dominated) benthic DOM-producing communities were used in the second field trip in 2021. Moreover, the ability of sponges to utilize bacteria-processed DOM mixtures and vice-versa was assessed in a total of 64 sponge incubations and 32 bioassays with bacterioplankton. As with the individually tested DOM types before, there were pronounced differences in the response (e.g. bacterial growth rates and change in bacterial community composition) to the DOM mixtures and sponges and bacteria removed similar components from the DOM mixtures. Interestingly, the change in bacterial community composition during bioassays were similar irrespective whether the DOM mixture was pre-procced by sponges or not. On the other hand, pre-processing can have an effect on bacterial growth. Depending on the respective DOM mixture, pre-processing by sponges can suppress (seawater without addition of coral-algal DOM mixture), stimulate (current DOM mixture), or not affect growth rates (past and future DOM mixture). This suggest that DOM composition can have a considerable effect on the interaction between sponges and microbes.
Despite considerable limitations during the outgoing phase due to the global COVID-19 pandemic, project activities and results were disseminated through, social media channels, the interactive live broadcast lecture series AXA coral live, invited lectures, participation at international conferences, features in two TV documentaries, as well as press releases and blog posts (see photo).
Coral reefs provide a plethora of ecosystem services and are therefore relevant in the light of the UN Sustainable Development Goals (1) No Poverty, (2) Zero Hunger, and (3) Life below Water. However, combination of local and global stressors threaten coral reefs and the services they provide. Understanding how declines in coral abundances and concomitant increases in algae affect the composition of DOM, the largest reservoir of organic nutrients in the oceans, and how it is utilized and recycled by its main consumers, microbes and sponges, is essential to develop suitable management tools and conservation strategies. Obtained results are expected to considerably advance our knowledge on the utilization of different DOM components by sponges and bacteria, potentially shedding light on fundamental differences in the mechanisms involved in the uptake and processing of DOM by these key DOM consumers and how they alter the local DOM pool. Results generated in this MSCA will not directly provide solutions for the aforementioned challenges, but they can contribute to lay the foundation for integrating microbes and sponges in coral reef food web models and in our general understanding of the functioning of these valuable ecosystems. Moreover, DOM cycling by microbes and sponges is not restricted to coral reefs but is of relevance in most benthic ecosystems where sponges are abundant, from coastal areas to the deep-sea and from the tropics to the polar regions. To what extend results can be extrapolated to these ecosystems remains to be tested.