Webster and Hill, 2007:

Nutrient cycling

"Changes in rates of bacterial photosynthesis or inorganic flux through the microbial loop can have major impacts on carbon cycling and on global climate. Bacteria are estimated to be responsible for 20 to 50 per cent of marine primary productivity ^{16, 29} and perform fundamental roles in the degradation of organic matter. In the upper 500 metres of the ocean, microbes consume an estimated 75 percent of the sinking particulate organic carbon flux¹⁶. Marine microbes are also crucial to various bio-geochemical processes such as nitrogen fixation, chemolithoautotrophy, sulfate reduction and fermentation. Environmental perturbations that affect bacterial abundance or community composition are therefore likely to have large-scale effects on ecosystem function.

The traditional view of the marine carbon cycle was that eukaryotic organisms were the only important players in the transfer of carbon between trophic levels. Bacterial processes were largely ignored because bacteria were thought to be inactive and present in low numbers. It is now clear that this historical view of carbon flux from photosynthetic phytoplankton to herbivorous zooplankton to higher organisms is incomplete and the microbial loop needs to be considered in addition to this grazing food chain (Figure 5.3). This paradigm shift has come about over the past 30 years as improvements in microbiological techniques for enumeration, measurement of growth and activity, and assessment of microbial diversity have revolutionised our understanding of marine microbiology²⁴. It is now clear that organic flux into bacteria is a major pathway through which, on average, one-half of oceanic primary production passes^2,1."

Symbiosis

"Symbiosis is considered a permanent association between organisms of different species. Marine microbes are involved in a variety of important symbiotic relationships with marine invertebrates from a range of taxa including sponges, cnidarians, molluscs, echinoderms and nematodes. Proposed symbiotic functions for marine microbes include: nutrition (through direct incorporation of dissolved organic matter in the sea water or translocation of photosynthate¹²¹, quorum sensing⁷⁰, assistance with reproductive processes⁶¹, assistance in chemical defence¹⁰¹, contribution to structural rigidity¹²¹, metabolism of a wide range of waste compounds¹¹⁹, and production of secondary metabolites85. There are also many symbioses where the type of interaction between the host and its symbionts remains unknown. With such a broad range of functions, environmental conditions that affect the distribution or abundance of symbiotic marine microbes could have significant effects on host fitness and survival.

The best studied symbioses in GBR invertebrates are those between corals and their symbiotic zooxanthellae (Hoegh-Guldberg et al. chapter 10), and between sponges and their associated bacteria. In the case of sponges, more than 50 per cent of the wet weight of the organism can be composed of bacteria. These are often remarkably complex symbioses with high microbial diversity, including novel species that have not been found in other ecosystems. There is evidence that some bacteria are ubiquitous in various sponges from different oceans and that some of the phylogenetic clades found in sponges are more similar to each other than to sequences found in other environments^{47, 30, 51}. For example, the bacterial genus Poribacteria has so far been found only in sponges, and these microbes have less than 75 per cent sequence homology to previously known bacteria³¹. Many studies also report that sponges contain distinct microbial communities not found in the surrounding sea water^{108, 47, 95}. Taylor et al.95 distinguished three types of sponge-associated bacteria: specialists found only on one host species, sponge associates found in multiple sponge species but not in sea water, and generalists from multiple hosts and the surrounding sea water".

Recruitment

"For sessile animals such as corals, the choice of a suitable site for settlement is crucial for future survival. Physical and chemical cues are often critical factors in site selection for larval settlement^{76, 50}. Micro-organisms can play an important role in the induction of settlement and metamorphosis in many marine invertebrates, including shellfish such as oysters and abalone¹³, starfish ⁵⁶, polychaete worms¹⁰⁰, hydroids⁷² and corals^{77, 113}. The best known source of chemical cues for corals are the crustose coralline algae, but it is clear that bacterial biofilms can also produce settlement and metamorphic cues^{46,  41, 77, 113}. Marine biofilms have been reported to induce metamorphosis in several classes of cnidarians, including Anthozoa (hard and soft corals)^{75, 46, 113}, Scyphozoa (jellyfish)¹¹ and Hydrozoa⁷¹. Environmental conditions that adversely affect the distribution and abundance of microbes involved in settlement and metamorphosis of reef invertebrates could therefore have large-scale effects on ecosystem structure and the distribution and reproductive fitness of some keystone species."

Disease

"In recent decades, there has been a global increase in reports of disease in marine organisms⁶⁷. Disease epidemics have affected both vertebrate and invertebrate species including fish, seals, dolphins, shellfish (oysters, scallops, abalone and clams), starfish, sea urchins, sponges and corals (reviewed in Harvell et al.⁴⁴). Disease outbreaks have also affected seagrass, kelp and coralline algae populations⁴⁴. On the GBR, the incidence of disease has been most notable in corals^{57, 124, 10} and sponges¹¹². To date, at least eight different coral disease states have been described on the GBR, including pathogens that have had devastating effects on coral communities in the Caribbean (black band disease and white syndrome).

While it appears that the prevalence of marine disease has increased in recent years, this may be an artifact of increased awareness and detection. Determining whether prevalence is changing over time has been problematic due to an absence of baseline data for most marine organisms. Whether these reported disease outbreaks are due to new pathogens, changed environmental conditions or enhanced detection mechanisms is a topic of current debate. In any case, environmental stress such as climate change, which compromises the physiological fitness of marine invertebrates and their symbionts and provides enhanced conditions for disease-causing microbes, will likely increase the prevalence of disease in marine ecosystems.

To date, there are only six coral diseases for which the etiological agent has been described: bleaching of Pocillopora damicornis by the pathogen Vibrio corallilyticus6, black band disease¹⁸, white plague type II⁸², aspergillosis^{89, 39}, white pox⁸⁰, and bleaching of Oculina patagonica by Vibrio shiloi^{64, 65}. In contrast, there are numerous diseases and ‘syndromes’ for which no causative agent has yet been identified (reviewed in Richardson⁸¹, Jones et al.⁵⁷, Bourne and Munn¹⁰). In fact, there is still some controversy about which species are responsible for forming the cyanobacterial mat in black band disease^{18, 32}. The potential role of viruses in coral disease is also being investigated. Heat shocked corals have been shown to produce numerous virus like particles that are evident in animal tissue, zooxanthellae and the surrounding sea water²¹. In addition, these virus like particles appear to induce cell lysis in non stressed corals, suggesting the presence of an infectious agent. However, unequivocal transmission electron microscopy evidence for this has yet to be obtained. On the GBR, virus like particles are abundant and correlate with the spatial dynamics of the bacterioplankton community⁸⁶. It has been suggested that virus like particles on the GBR may significantly influence nutrient cycling rates and food web structure⁸⁶".

 

 

 

 

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Citation and/or URL

Webster, N. and Hill, R. 2007. 'Chapter 5: Vulnerability of marine microbes on the Great Barrier Reef to climate change'. In Climate Change and the Great Barrier Reef: A Vulnerability Assessment, eds. Johnson, J.E. and Marshall, P.A., Great Barrier Reef Marine Park Authority, p. 97-120  

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Spatial Coverage

All of the Great Barrier Reef Marine Park as well as adjacent catchments.

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Temporal Coverage

This volume is a compilation of information collected from  many sources and spanning many time frames.

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Update Frequency

Not applicable as this report is a compilation.

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Other Information

None