Sponge symbiosis

Sponges are known to harbour microbes within their mesohyl but not all sponge species host similar densities of bacteria. Two distinct types of symbiosis among poriferan hosts have been identified. The first type of symbiosis (HMA sponges) is one that involves sponges that harbour populations of bacteria two to four orders of magnitude higher than seawater concentrations within their mesohyl. Bacterial populations in HMA sponges are very dense with remarkable levels of species richness. The second type of symbiosis (LMA sponges) involves sponges that harbour sparse populations of bacteria. These hosts have bacterial densities that are up to fivefold lower than HMA sponges with correlated reductions in species richness.

Despite increasing knowledge of the constituents of the sponge microbial communities, we have a poor understanding of reciprocal aspects of the association (i.e. effects of bacteria on sponge phenotype). This paper set to compare the community structure of symbionts harboured by HMA and LMA sponges and to determine whether microbial associates influenced phenotypic characteristics of the host.

Poppell et al. used a variety of microscopic techniques  to compare aspects of host biology (e.g. choanocyte chamber morphology and density) in the context of symbiont status. Molecular approaches uncovered components of ecological structure of bacterial communities in HMA versus LMA sponges (e.g. species richness, evenness).

Several intriguing patterns in poriferan-bacterial interactions were revealed through the combined molecular and microscopic approach to studying HMA and LMA sponges. Choanocyte chamber density is greater in LMA sponges than in HMA sponges. There are also distinct patterns of organisation for bacterial communities in HMA and LMA sponges. These results suggest that the large bacterial communities found in HMA sponges may allow the host to decrease their heterotrophy versus that of LMA sponges with minimal bacterial communities.

The microbial associates found in HMA and LMA sponges showed strikingly different patterns in the composition and structure of their respective symbiont communities. Species richness was highest among HMA sponges. HMA sponges harboured significantly more diverse bacterial communities than LMA sponges, and the HMA communities were not dominated by one bacterial type as was detected in LMA host. Cluster analysis identified distinct types of communities hosted by HMA and LMA sponges. The status of the host’s microbial abundance (i.e. LMA or HMA) as well as environmental factors appeared to drive patterns in bacterial species. DGGE indicates that LMA sponges are dominated by a small number of bacterial species, and this technique provides clear evidence that bacterial communities are more diverse and even in HMA sponges.

Symbiont density was visually assessed from scanning electron micrographs. Symbiont status of host sponges appears truly dichotomous; microbes were either densely packed in the host (i.e. HMA) or bacteria were difficult to locate (i.e. LMA). They did not observe any intermediate bacterial densities.

As abundant, filter-feeding metazoans, sponges occupy an important position in marine food webs.

Sponges link two dynamic ecosystems (pelagic and benthic) through nutrient cycling, nutrient transformations, energy flow, and remodelling of the physical environment. Sponges can reduce bacterioplankton loads by three orders of magnitude. However, the sponge feeding behaviour is a product of complex interactions between the host and its symbiotic residents). If we are to understand the central role sponges play in marine food webs, we must include careful analysis of the manner in which symbionts might affect the trophic behaviour of their hosts.

The observation that sponge-bacterial symbioses appear to adopt one of two strategies (HMA versus LMA) is one of the more intriguing patterns among animal symbioses. Although interest in sponge-microbe symbiotic relationships is high, we have only begun to understand key aspects of the ecological interactions that occur among host and symbiont and how those interactions might shape the evolutionary pressures experienced by the partners. More research is needed to better understand these relationships and to answer questions such as the causes of this divergence in microbial community structure. It is also worth noting the methodological limitations inherent in PCR-DGGE, which include poor band resolution, co-migration of amplicons and potential contamination from non-sponge bacteria. When conducting analysis of the data these factors need to be taken into consideration and if these are significantly affecting the reliability of the conclusions better methods may need to be developed.

Poppell, E., Weisz, J., Spicer, L., Massaro, A., Hill, A., & Hill, M. (2013). Sponge heterotrophic capacity and bacterial community structure in high‐and low‐microbial abundance sponges. Marine Ecology. 

http://onlinelibrary.wiley.com/doi/10.1111/maec.12098/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false