Global distribution of deep water heterotrophic protists: Some expected results, some interesting questions

The Malaspina 2010 circumnavigation was an important study into the deep ocean microbiomes. It was the first to look at the abundance and biomass of heterotrophic protists (HP) on a global scale.

In the bathypelagic zone, microbes interact with a wide range of organic molecules, leading to the breakdown of this organic matter into its simplest inorganic forms. Along with the stable homogenous mesopelagic zone, this makes the deep oceans one of the earths largest biomes, which has an important role in remineralisation and carbon sequestration on a global scale (Nagata et al., 2010).

Photosynthetic organisms do not form the basis of the deep oceans food web because little to no light is able to penetrate down to the depths of the ocean. This lack of light means that the basis of the deep oceans microbial food web is sustained by organic matter that descends to the depths and by the production of prokaryotes. This is evidenced by the in-situ chemosynthesis that uses reduced inorganic compounds, such as ammonia or carbon monoxide, which Jiao and Zheng (2011) argue has a big impact on global carbon sequestration.

This makes prokaryotes the entry point for carbon in the deep oceans food web. In surface waters, HP are known to be an important grazer of prokaryotes, but little is known about the deep oceans HP. Disagreements about the importance of deep ocean grazing by HP concern the variation in abundance and biomass of the deep oceans HP.

This study was the first to globally assess the abundance and biomass of HP in mesopelagic and bathypelagic waters. A combination of epifluorescence microscopy and flow cytometry was used to analyse deep water samples from the Malaspina 2010 circumnavigation. The study correlated a global trend in decreasing HP abundance with depth. At the same depth ranges across the globe, however, the decrease in biomass and abundance of HP was not equal. On average the HP biomass would equate to roughly 20% of the prokaryote biomass, suggesting the possibility of HP grazing on the prokaryotes.

However, there was substantial variation regionally in the HP to prokaryote ratio, for example, the Atlantic community had no significant difference between the bathypelagic and mesopelagic layers, and in contrast, the Great Australian Blight had the highest levels of variation. In areas where there where high levels of HP compared to prokaryotes, counting osmotrophs and parasites as HP grazers, when in fact they are independent to prokaryote abundance, could be the causal factor to this discrepancy. In the areas with a higher prokaryote to HP ratio, there was a large contribution of fungi to total abundance. This could mean that fungi are being used as an additional carbon source for HP and that the presence of osmotrophs and parasites could relax the grazing pressure on prokaryotes.

Ref

Jiao, N. and Zheng, Q., 2011. The microbial carbon pump: from genes to ecosystems. Applied and environmental microbiology, 77(21), pp.7439-7444.

Nagata, T., Tamburini, C., Arístegui, J., Baltar, F., Bochdansky, A.B., Fonda-Umani, S., Fukuda, H., Gogou, A., Hansell, D.A., Hansman, R.L. and Herndl, G.J., 2010. Emerging concepts on microbial processes in the bathypelagic ocean–ecology, biogeochemistry, and genomics. Deep Sea Research Part II: Topical Studies in Oceanography, 57(16), pp.1519-1536.

Article reviewed

Pernice, M.C., Forn, I., Gomes, A., Lara, E., Alonso-Sáez, L., Arrieta, J.M., del Carmen Garcia, F., Hernando-Morales, V., MacKenzie, R., Mestre, M. and Sintes, E., 2015. Global abundance of planktonic heterotrophic protists in the deep ocean. The ISME journal, 9(3), pp.782-792.