- Phytoplankton and cyanobacteria are the main producers of dissolved organic matter (DOM) in the oceans. Marine DOM is one of Earth’s largest active carbon reservoirs and once produced, it is rapidly utilized by heterotrophic microorganisms (Kirchman 1990; del Giorgio and Cole 2000; Carlson 2002). The DOM-fed microbial biomass is finally transferred via the grazing food chain to higher trophic levels. This microbially-mediated cycling of marine DOM represents approximately one-half of primary production (Cole et al., 1988) and is called the microbial loop (Azam, 1998).
- The continued absorption of atmospheric CO2, at approximately one million tons an hour, into the oceans, can likely affect the microbial loop process, marine productivity and the marine ecosystem services. CO2 reacts with water to form carbonic acid (H2CO3, <1%), which dissociates to bicarbonate (HCO3–, ~90%) and carbonate (CO32–, ~9%), releasing H+. Thus, with increasing atmospheric pCO2, seawater pH decreases, and the equilibrium of the carbonic acid-bicarbonate-carbonate system shifts towards a more bicarbonate-rich state. These changes, collectively referred to as ‘ocean acidification’ (OA), are expected to further intensify in the future (Doney et al. 2009).
- It is hypothesized that climate change will likely be accompanied by loss of species richness and diversity, perturbations in the higher trophic levels, and eventual loss of ecosystem level stability (Smit et al., 2001; Ager et al., 2010). However, our understanding of ecosystem responses to OA is limited as most studies have utilized short-term, rapid perturbations on isolated elements such as single species or small groups of species. To address this limitation, we propose to study the shifts in the marine microbiota in samples collected at a natural CO2 vent system located at the northeastern side of Ischia Island in the Mediterranean Sea (see figure below). Hall-Spencer et al., have extensively demonstrated the usefulness of such ‘natural laboratories’ to assess the long-term impacts of OA (Hall-Spencer et al. 2008; Martin et al. 2008; http://news.bbc.co.uk/2/hi/science/nature/7936137.stm).
To obtain preliminary data, we investigated ecological tipping points of prokaryotes (bacteria, archaeal) and eukaryotes (algae, fungi, protists) along a pH gradient ranging from 8.17 down to 6.57. Samples were collected and filtered on site by Dr. Hall-Spencer and his post-doc which were received at FAMU in September, 2010, November 2010 and March 2011 and April 2013.
We have found, for the first time, that acidification promotes certain bacterial groups such as Firmicutes and Epsilon-proteobacteria (see figures below) likely due to their ability to use alternate electron donors (e.g. H2, formate, elemental sulfur, sulfide, thiosulfate) and acceptors (e.g. sulfite, elemental sulfur, nitrate) that are available at the vent system. Because many of these species are important human and fish pathogens, we speculate whether ocean acidification will increase the likelihood of such opportunistic pathogens to bloom and cause widespread diseases.
Such areas need to be investigated and data can be used to develop predictive models on the effect of changing pH to the microbial functioning and overall marine productivity.