Bacteria Abundance in Response to Increasingly Acidic Conditions

Abstract

Global warming is a worldwide phenomenon that is the result of anthropogenic input of carbon dioxide into the atmosphere. Although the increase in carbon dioxide levels now is 100 times faster than at the end of the last ice age (IPCC 2001, 2007), the ocean is the most important sink of carbon dioxide and have absorbed approximately half of all anthropogenic CO2 emissions to the atmosphere (Sabine et al. 2004). Many consequences are expected of global warming are occurring presently, one of which is ocean acidification. The average ocean surface water pH has decreased from 8.21 to 8.10 recently (Royal Society 2005). The Intergovernmental Panel on Climate Change predicts CO2 concentrations to reach 800ppmv by the end of the century which is equivalent to a decrease of 0.3-0.4 pH units (Orr et al. 2005). An effect of acidifying seawater is the lowering of calcium carbonate saturation states, a drastic change in environmental condition for shell-forming marine organisms (Doney et al. 2009). The organisms affected directly ranges from plankton to corals, molluscs, and sea urchins (Doney et al. 2009). For example, reduced shell and soft body growth have been reported in mussels (Gazeau et al. 2007) and metabolic depression has been seen in European eels (Anguila anguila) at CO2 levels above 2% (Cruz-Neto and Steffensen 1997). Bacteria are hugely important to all ecosystems, including marine habitats. Marine bacteria are uniquely adapted to saline environments and they need salt for growth (Fenical 1993). Most marine bacteria are unicellular (Fenical 1993) and some such as Prochlorococcus and Synechococcus are abundant oceanic primary producers (Azam and Worden 2004). Although the distribution of marine bacteria is poorly studied, 90% of marine bacterial flora is of the gram-negative genus Vibrio (Fenical 1993). Wang 3 Sediments, animate and inanimate surfaces, and internal spaces of living organisms are important microhabitats of marine bacteria (Fenical 1993). Internal spaces of marine organisms are often more nutrient rich than seawater (Fenical 1993). In particular, sponges and bacteria can form symbioses (Lee et al. 2009). Sponges can acquire their bacterial partners by filter feeding and selectively retaining bacteria (Taylor et al. 2007). The relationship is often mutualistic; bacteria benefit nutritionally while bacteria help sponges eliminate waste more efficiently (Beer and Ilan 1998), stabilize the skeleton (Rutzler 1985), or defend against pathogens and predators (Bewley et al. 1996). Bacteria are observed on the surface of Oriental shrimp (Palaeomon macrodactylus) eggs, American lobster (Homarus americanus) eggs, coral reefs, sea jellies, and other bacteria such as cyanobacterium Microcoleus lyngbyaceus (Fenical 1993). Bacteria are notably associated with chitinous plankton (Vezzulli et al. 2012). They are also a major food source for invertebrates, which reflects the energy available for higher trophic levels (Hall and Meyer 1998). Hall and Meyer (1998) found that invertebrates could derive <10% to 100% of their carbon requirements from bacteria. Marine microorganisms such as bacteria have the potential for rapid growth. Thus, marine microorganisms are critical to global nutrient cycles (Arrigo 2005). Nitrogen, an essential nutrient, is used by organisms to form proteins, chlorophyll, and nucleic acids (Murray et al. 2005). In the ocean, it is present as a dissolved gas or as dissolved inorganic ions: nitrate, nitrite, and ammonium (Murray et al. 2005). Once thought to be restricted to very specific habitats and microbes, now we know that many bacteria are known for their nitrification and denitrification services to the ecosystem (Zehr and Ward 2002). Nitrification is a process of oxidation from ammonia to nitrate with nitrite as an intermediate species (Figure 3). Denitrification is the reverse reaction where nitrate is reduced to nitrite and is converted to nitrogen gas (Figure 3). Bacteria mediate these two processes which makes nutrients available to many organisms, including phytoplankton. Biodiversity of plant and animal communities are dependent on the availability of these nutrients (Bienhold et al. 2012). The objective of this mesocosm experiment is to determine differences in bacteria abundance in normal (600μatm), medium (1000μatm), and high (1300μatm) CO2 concentrations. Mesocosms serve as an intermediate for lab and field studies; mesocosms provide insight about changes to food webs, communities, and ecosystems.