Prior to the discovery of hydrothermal vents, chemoautotrophic bacteria were known from anoxic basins such as the Black Sea (Tuttle and Jannasch 1973) but they were generally believed to have little significance in deep ocean environments. Due to high biomass production much research has been centred on the hydrothermal vent communities in recent years, however the expense and in accessibility of such locations have had a severe effect on our knowledge of the microbial assemblage present. Much focus has instead been prioritised on the identification of the larger mega fauna, such as R.pachyptilia, and their nutritional pathways i.e. lipids and fatty acid concentrations. More study on the base bacteria is urgently required to better understand these foreign food webs, in comparison our knowledge of the base photosynthetic organisms of other trophic system is large. As such we may be able to find similarities between the two processes which could aid in our understanding. Analysis of large invertebrates/vertebrates at chemosynthetic sites have led to insights into some of the microbial ecology however much of our knowledge is missing. Three main bacterial growth forms have been noted:
- Free living
- Particulate attached
In most marine systems free living and particulate attached bacteria are viewed as separate, yet interacting groups (Lapoussi et al 2010)however within thisecosystem the separation between the two groups may be more distinct when considering the ecology of the entire system. Free living bacteria have generally been noted to be smaller in size to that of the particulate attached (any species that grows attached to either suspended particulate matter or on substrate) species (Lapoussi’ere et al 2010) as such they energy per individual may be decreased with free living species, an important factor when feeding strategies of higher organisms is concerned. Endosymbionts are bacteria that have developed symbiotic relationships with host species, in which protection and a nutrient supply is offered in exchange for an end metabolic product, in this case carbohydrates. This particular relationship is fairly common within the seep sea, notably cold seeps and whale carcasses (Scott & fisher 1995). The free living species are an important phase in the nutrient cycle however, within this system the majority of the known organisms rely, mostly, on endosymbionts or grazing of particulate species. This could be due to difficulty with filter feeding and a general increase in productivity associated with the attached species (Lapoussi’ere et al 2010). Particulate bacteria can form symbiotic relationships with invertebrates, such as that seen in Alvinocaris spp. Alvinocaris shrimp (Figure 2) species host a “culture” of bacteria on their outer epidermis, which is sporadically fed upon. These bacteria are known as ectosymbionts (attached to the outer surface of the organism). Although ectosymbiotism is present stewart et al (2005) found the majority of organisms used endosymbiotism. I believe the ectosymbiotic nutrition is simply bacterial colonisation of available surfaces and feeding upon such bacteria is a behavioural adaptation.
This method for nutrition will not be as productive as the endosymbionts which will be placed in more optimal conditions within the host organism. Endosymbionts inhabit vacuoles within specialized cells called bacteriocytes (Stewart et al 2005). Bacteriocytes are generally localized in sites of ventilation such as gills as in the bivalve filter feeder Bathymodiolus thermophilus, to allow direct access to the needed reactants for carbohydrate synthesis. Variation is observed, as with species R.pachyptilia. A specialized host organ known as the trophosome, located away from ventilation organ, hold the bacteriocytes. Reasons for this arrangement are unclear however it may be an optimizing response for the endosymbionts. The plume is exposed directly to the vent fluid, the conditions in such an environment may not be optimal for bacterial symbionts of the Riftia worm and as such are removed to area that may be more suitable.
In summary three separate bacterial groups are observed within the ecosystem, however they all occupy separate niches within the habitat, there are some overlapping niches however distinction is noted due to specialization in symbiotic partnerships. The three groups seem to have varying importance with the organisms that occupy this habitat, the endosymbiotic and particulate species of importance to the larger species, most importantly the endosymbionts. The free living species don’t seem to play a large role in the ecology of the vent community, however as with most surface trophic systems free living bacteria may play a vital role in juvenile development and zooplankton species that have yet to be either studied or even discovered.