Marine sponges are hosts to complex microbial communities that may comprise up to 60% of the sponge biomass. Little is known about the roles of these symbiotic bacteria and their contributions to the sponge and thereby indirectly to the coral reef ecosystem in which the sponges live. The hypothesis driving the project is that diverse assemblages of nitrogen-fixing bacteria play an important role in providing sponges with nitrogen, taking nitrogen gas and converting it to organic forms of nitrogen that can be used by sponges. The role of bacterial symbionts of two sponges from Key Largo, Florida, Ircinia strobilina and Mycale laxissima in nitrogen fixation will be determined. The project will include biochemical approaches to demonstrate nitrogen fixation in whole sponges and bacterial isolates, and molecular approaches to study the pattern of where and when the nifH gene is turned on and makes the NifH protein, a key protein in nitrogen fixation. Training will be provided in marine microbiology to graduate students and undergraduate minority students will be encouraged to enter careers in scientific research by participating in a summer course in Marine Microbiology. The project will advance our understanding of microbial diversity and function in complex sponge symbioses, promoting the study of sponge symbioses as models of the complex symbiotic relationships found between microbes and many higher organisms. The project will provide new information on sources of nitrogen for fragile coral reef environments.
Coral reef ecosystems have long been a paradox for ecologists: How do the low nutrient concentrations in the water surrounding reefs support one of the most productive ecosystems in the world with remarkably high bio-diversity? This seems counter-intuitive because we would expect such high productivity to need intensive nutrient inputs to support it. It is becoming clear that efficient nutrient recycling between the water column and the diverse benthic community is a key factor in supporting these productive ecosystems. Our study focused on abundant, key habitat-forming organisms in the benthic community: marine sponges. We study how marine sponges affect the cycling of nutrients in the local reef environment. Sponges are sessile marine invertebrates that rely on filter-feeding to obtain their food from the surrounding water. It is estimated that a sponge can pump up to 24,000 liter of water per kilogram of sponge tissue per day, a volume equal to tens of thousands times its body size. The particles filtered from seawater by sponges range in size from 0.2-2 μm, mainly composed of marine bacteria, archaea and single-cell algae. Interestingly, some of these microbes are not consumed by sponges as food, in contrast, they are retained as microbial symbionts and live inside the sponge body. Previous studies has found that sponges can receive many benefits through symbiosis with selected microbes, including obtaining additional nutrients and secondary metabolites. Sponges and their associated microbes provide an excellent model to study complex microbe-animal symbiosis that can provide insights into other symbioses, including the bacterial community living inside or associated with humans. A balanced nutrient supply is essential in the growth of organisms. Some sponges harbor cyanobacteria, a group of photosynthetic bacteria that can convert carbon dioxide to carbohydrates through photosynthesis. In these sponges, the supply of carbon may be greater than the nitrogen needed to maintain a balanced diet. We speculate that nitrogen fixing bacteria, a group of bacteria that can convert nitrogen gas in the air to amino acids, may play an important role in these sponges to produce additional biologically available nitrogen compounds to fulfill the demand and maintain the carbon and nitrogen balance. Our study showed that these nitrogen-fixing bacteria are present inside sponges, and we confirmed that they expressed a key nitrogen fixation gene, the nifH gene. Different groups of bacteria present in two sponges from the Florida Keys, Ircinia strobilina (black ball sponge) and Mycale laxissima (strawberry vase sponge) were shown to contain nifH genes and to actively express these genes. The nifH gene encodes the enzyme that mediates the nitrogen fixation process, suggesting that these bacteria are very likely to be active in fixing nitrogen. Additionally, we found that some members of the nitrogen fixing bacterial community were more active during the day and others are more active at night, implying a joint effort by diverse bacteria to provide fixed nitrogen to the sponges. Ours is the first study to detect changes in patterns of nitrogen fixation in sponge symbionts during day-night cycles. We also studied a group of sponge-associated microorganisms called Archaea. This archaea group can obtain energy by oxidizing ammonia excreted from the sponge host, and thereby removes this toxic waste from the sponge host. In a novel and unexpected finding, we discovered that microbes within sponges can accumulate another important element, phosphorus. This phosphorus is present in the form of polyphosphate granules. This exciting finding opens up a new area of research. Initial suggestions are that the bacteria within sponges may be playing an important role in storing and cycling phosphorus in the reef environment. Further studies are underway to understand how this microbial process affects the sponge and benthic phosphorus cycle. Our study showed that the diverse microorganisms live inside sponges contribute to various aspects of nutrient cycling in the sponges and the surrounding benthic community. With a declining population of coral and increasing sponge abundance in many coral reefs, our study can help in predicting the future of coral reefs. Studies on nutrient cycling in marine sponges contribute to better understanding of the factors shaping this unique ecosystem.
