Sponge nutritional modes in an Indo-Pacific seagrass ecosystem

Life on earth depends on the constant cycle of nutrients that keep ecosystems in equilibrium and maintain ecosystem functioning, which might be disrupted due to the major and rapid environmental changes we are currently facing. Sponges are a major component of benthic communities in many marine ecosystems worldwide. Most sponges are heterotrophic and gain their nutritional needs from filter feeding dissolved organic carbon and particulate organic carbon from the water column. However, some sponges also form symbioses with photosynthetic symbionts and are autotrophic under some circumstances. These two different nutritional modes may play different roles in the movement of energy flow through benthic systems. Here I studied the sponge assemblages in a seagrass meadow at two spatially separated sites at three tidal zones, located in the Wakatobi National Park, Indonesia, which is representative of shallow-water seagrass ecosystems in the Southeast Asian Indo-Pacific bioregion. My thesis aims to increase our understanding of the importance of these two different sponge nutritional modes in seagrass ecosystems to better understand how environmental change might impact sponge assemblages.

In my first data chapter, I conducted sponge and seagrass surveys to investigate the seagrass and sponge community structure and the potential drivers of the observed distribution patterns. For the identification of the sponges, I combined morphological and molecular techniques, using four DNA markers (18S, 28S, ITS, and CO1-ext). I identified ten sponge species: Spongia sp., Spheciospongia sp, Phyllospongia foliascens, Haliclona koremella, Amphimedon sp., Dactylospongia elegans, Axinella sp., Clathria reinwardti, Rhopaloeides sp., and Siphonodictyon mucosum. Sponges were found in all tidal zones of the studied seagrass meadow, including at the high-shore zone that regularly experiences aerial exposure during low tide. I propose that sponge morphological adaptation is important for sponge survival in the different seagrass zones.

My second data chapter determined the dominant nutritional mode of seagrass sponges by measuring in situ gross primary production (GPP) to dark respiration (P:R) ratios. I measured the P:R ratios for eight sponges out of the ten found in the studied seagrass meadow. I found that six sponges were autotrophic and net oxygen producers over a full 24 hour period. Based on their high biomass, autotrophic sponges contributed considerably to the total sponge assemblage biomass at all tidal zones and sites. While the high-shore and middle-shore zones had more similar abundances of heterotrophic and autotrophic sponges, autotrophic sponges dominated the sponge assemblage in the near-reef-flat zone of the seagrass meadow by contributing up to 98% of the total sponge biomass.

In my third data chapter, I measured the sponge-mediated organic carbon flux in situ to investigate the influence of sponges on the carbon flow from the water column to benthos. The five most abundant sponge species, comprising two heterotrophic sponges and three autotrophic sponges, representing 75.1–99.8% of the total sponge biomass at my studied seagrass meadow, were investigated. I found that in dark incubations (representing night-time), the heterotrophic and autotrophic sponges did not show significant differences in sponge-mediated carbon flux (p > 0.05). However, in light incubations (representing daytime), autotrophic sponges released organic carbon to the water column, while heterotrophic sponges removed organic carbon from the water column. Overall, at the seagrass meadow level, the sponge assemblage was still a net remover of organic carbon over a 24-hour cycle at all tidal zones, including where autotrophic sponges contributed 98% of the total sponge biomass (the near-reef-flat zone).

In my fourth data chapter, I measured the sponge-mediated picoplankton flux of the six most abundant sponges (representing 75.1–99.8% of the total sponge biomass) in situ to investigate the influence of sponge assemblages on picoplankton abundance. I found diel variability in all picoplankton observed and for the sponge-mediated fluxes. I found that heterotrophic sponges removed significantly more picoplankton-derived organic carbon than the autotrophic sponges, removing 40–60 times more carbon than the autotrophic sponges over 24 hours. This chapter supports the hypothesis that autotrophic sponges are gaining supplementary nutrition from their hosted photosynthetic symbionts, reducing their need to suspension feed for their nutritional needs.

In summary, autotrophic sponges dominated the sponge assemblages in the seagrass ecosystem I studied in the Southeast Asian Indo-Pacific bioregion. This thesis has shown that autotrophic sponges can be net oxygen producers, releasing organic carbon to the water column (during daytime) and thus are not so dependent on picoplankton feeding. Variation in bentho-pelagic interactions due to autotrophic sponges compared to heterotrophic sponges will influence the impact of any increase or decline in sponges as a result of changing environmental conditions.