How increased ocean acidification may remove the positive impact the mud shrimp has on the nitrogen cycle

Within the past 200 years due to increased levels of CO₂ in the atmosphere, the pH of the ocean’s surface water has decreased by 0.1 units to an average of pH 8.10, worryingly by the end of the Century its predicated to decrease by an additional 0.3 - 0.5 units increasing ocean acidification.

In light of this there have been few studies that have investigated the impacts of ocean acidification on microbial processes, It is assumed that the nitrogen cycle will be particular susceptible to this decrease in pH due to the reduction of ammonia oxidation rates. Sediments are important sites for microbial driven transformation of nitrogen. A study by Laverock (2013) looks at the effects ocean acidification may have on ammonia oxidation rates in sediment, the authors also looks at how sediment in borrows produced from bioturbation may be impacted, This is important because bioturbation from organism may play a big role in nitrification processes for example the large, semi-permanent burrows of the mud shrimp U.deltaurais support bacterial communities that are distinct with a higher diversity than, the surrounding surface and subsurface sediment. Importantly the presence of U.deltaurais has been shown to increase threefold the rates of denitrification within sediment therefore having an impact on the ecosystem functioning.

Methods

·         Manipulative experiments were used for a period of 14 weeks to look at ocean acidification on benthic ammonia oxidation rates in the overlying water surface, surface sediment and sediment in burrows there were 4 target treatments  pH 7.90, 7.70,7.35 and 6.8 and one control of pH 8.10

·         Sediment was collected using a box core and used in the experiment there was a 9 week settling period to allow shrimps to establish their burrows and to also allow microbial communities to regain their spatial structure after sediment disturbance this was important because it allowed conditions to occur to what is more representative of the natural environment.

·         To measure microbial gene abundance q-PCR assays was performed to both quantify bacteria and archaeal 16S rRNA and amoA genes

·         The pH of the mud shrimp’s haemolymph was measured to allow acidosis to be measured.

                                                                                                                                            

Findings

As expected ammonia oxidation rates in the overlying water decreased with reduced pH, however within surface sediments, there was no significant effect of pH on ammonia oxidation rates interestingly within burrow wall sediments, ammonia oxidation rates decreased by 80% from pH 8.10 to 7.90 followed by a further 83% between pH 7.90 and 7.70. However at pH 7.70 to 6.80 the ammonia oxidation became lower than in the surface sediment, this is important to mention because at pH 8.10 burrow wall rates were, on average, five times higher than surface rates showing that ocean acidification has a negative impact of the rates in borrows. Due to increased acidic conditions the two shrimps in the lowest pH treatment 6.80 and one form pH 7.35 were unable to survive the conditions highlighting the physiological stress the change in conditions caused an interesting finding that in the borrow treatment pH 6.8 although the ammonia oxidation rates were well below that of the control there was no significant different between the burrow sediment and the surface sediment and the rate had actually increased indicating that relicts structures still continue to act as important sites for biogeochemical cycling through passive flushing of the borrow. Since the burrow sediment increased in ammonia oxidation rates in the absence of mud shrimp its suggested that the change in behaviour of the mud shrimp induced by pH changes may alter the phsico-chemical conditions within the burrow reducing the rate of ammonia oxidation at low pH.

Even Though Bacterial and archaeal 16S rRNA gene abundances increased significantly with a reduction in pH the results suggested that on the contrary archaeal cell-specific ammonia oxidation rates actually decreased at low pH. Indicating that there is higher inactivity in the AOA possible explanation of this is that pH directly affects enzyme activity, Chemolithoautotrophic nitrification is also an energetically expensive process  meaning that bacteria and archaea may find the conditions to stressful to actively uptake ammonium in low pH. There were limitations of the PCR primers used, The primers used only targeted betaproteobacterial ammonia oxidizers. Although Gammaproteobacterial AOB have been detected in low abundance within the water column they may conversely be widespread in marine sediments. It is also likely that some member of the community for both AOB and AOA, are missed by the use of single amoA primer sets.

This paper provides a good starting point, however to obtain a more detailed picture of how ocean acidification may affect ammonia oxidation rates a limiting factor in the nitrogen cycle, a number of factors still require further study including identifying if the decline in ammonia oxidation rates in reduced pH is due to decline in cell activity due to physiologically stress of alternately an shift in the active members of the microbial community. Species interactions should also be considered, although it appears ocean acidification may have a negative effect on mud shrimp other bioturbating species may benefit from increased acidified conditions especially due to decreased competition from other species.

Laverock, B., Kitidis, V., Tait, K., Gilbert, J., Osborn, A. and Widdicombe, S. (2013). Bioturbation determines the response of benthic ammonia-oxidizing microorganisms to ocean acidification. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1627), pp.20120441-20120441.