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).
