Warming endangers sea sponges

Sea sponges, which host a complex community of microbes in a mutually-beneficial relationship, are at risk from higher sea surface temperatures because the symbiotic relationship between the sponge and its microbes breaks down at 33 degrees Celsius.

This is identical to the temperature threshold at which corals experience bleaching, which is also a breakdown in the symbiotic relationship between the host and its microbes.

Work by AIMS scientist Dr Nicole Webster has shown that the elevated sea temperatures expected as climate change progresses would threaten the survival of sponges.  These remarkable living creatures are an essential part of many marine ecosystems, including the Great Barrier Reef.  In some sponges, up to 40 per cent of their body weight is made up of microscopic symbiotic bacteria, contributing a range of benefits including chemical defence systems and the processing of nutrition and waste.

The delicate equilibrium of this relationship appears to be at risk.  “Global climate change will have a significant impact on the marine ecosystem by altering microbial diversity, microbial function and community dynamics,” Dr Webster said.

In a series of experiments in which Dr Webster and her colleagues exposed a common GBR sponge (Rhopaloeides odorabile) to water temperatures ranging from 27 degrees to 33 degrees, the clear line of demarcation between a continued healthy symbiotic microbial community in the sponge and a switch to harmful microbes that cause disease consistently emerged at 33 degrees.

By 2100, the 33 degree sea surface temperature threshold would be crossed regularly on the waters of the GBR, based on current climate change projections.

As microbes have symbiotic relationships with nearly all marine invertebrates, profound changes to the marine world are likely to flow from these higher sea temperatures.  Dr Webster’s work points to the need for greater emphasis on the unseen world of microbes in understanding threats from climate change.

“Although microbes constitute by far the largest diversity and biomass of all marine organisms, the likely widespread effects of climate change on microbial communities have been largely overlooked,” Dr Webster said.

Dr Webster presented her findings on 21 August 2008 at the 12th International Society for Microbial Ecology paper at the Cairns Convention Centre.