Diatoms are a type of algae, most of which are unicellular, although they also form colonies that take the shape of filaments or ribbons. A unique feature of diatom cells is that they are encased within a special cell wall made of silica that is called a frustule, which can assume a wide variety of forms but usually consists of two asymmetrical sides with a split between them. Functionally, diatoms serve as primary producers in various marine food chains; and, therefore, it is critically important to know how they may respond to continued increases in the air's CO2 content. Thus, we here review what has been learned about the subject in pertinent experiments that have been conducted over the past few years.
Chen and Gao (2004) grew a strain (2042) of Skeletonema costatum -- a unicellular marine diatom that is widely distributed in coastal waters throughout the world and constitutes a major component of natural assemblages of most marine phytoplankton -- in filtered nutrient-enriched seawater maintained at 20°C under a 12-hour/12-hour light/dark cycle at a light intensity of 200 µmol m-2 s-1, while continuously aerating both cultures with air of either 350 or 1000 ppm CO2 as they simultaneously measured a number of physiological parameters related to the diatom's photosynthetic activity. This work revealed that cell numbers of the diatom "increased steadily throughout the light period and they were 1.6 and 2.1 times higher after the 12-hour light period for the alga grown at 350 and 1000 ppm CO2, respectively." They also say that chlorophyll a concentrations in the bulk of the two CO2 cultures "increased 4.4- and 5.4-fold during the middle 8 hours of the light period for the alga grown at 350 and 1000 ppm CO2, respectively," and that "the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than that at 350 ppm CO2." In addition, they report that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II "increased with increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased." In summation, they say their data showed that "the light-saturated photosynthesis rate based on cell number, the chlorophyll a content, the photosynthetic chemistry of photosysten II and the efficiency of the light reaction all increased to various degrees with elevated CO2."
Three years later, in a study conducted between 15 May and 9 June of 2005 at the Espegrend Marine Biological Station of the University of Bergen, located on a fjord in southern Norway, Riebesell et al. (2007) maintained nine cylindrical mesocosms -- which extended from the water surface to a depth of 9-10 meters -- in equilibrium with air of either ambient CO2 concentration (350 ppm), doubled CO2 (700 ppm) or tripled CO2 (1050 ppm), while they measured several phytoplanktonic physiological parameters. During this period, they report that "net community carbon consumption under increased CO2 exceeded present rates by 27% (2 x CO2) and 39% (3 x CO2)," and they state that continuous oxygen measurements in the mesocosms indicated "enhanced net photosynthesis to be the source of the observed CO2 effect."
Noting further that "the phytoplankton groups dominating in the mesocosm studies -- diatoms and coccolithophores -- are also the main primary producers in high productivity areas and are the principal drivers of biologically induced carbon export to the deep sea," the eleven scientists say their findings "underscore the importance of biologically driven feedbacks in the ocean to global change." And reminding us that "increased CO2 has been shown to enhance fixation of free nitrogen, thereby relaxing nutrient limitation by nitrogen availability and increasing CO2 uptake (Barcelos e Ramos et al., 2007)," Arrigo (2007) states in a News & Views discussion of Riebesell et al.'s paper that "neither these, nor other possible non-steady-state biological feedbacks, are currently accounted for in models of global climate -- a potentially serious omission, given that the biological pump is responsible for much of the vertical CO2 gradient in the ocean." And in this regard they additionally indicate that the diatom and coccolithophore growth-promoting effect of CO2 measured and described by Riebesell et al. has probably been responsible for limiting the rise in atmospheric CO2 experienced since the dawn of the Industrial Revolution to approximately 90% of what it likely would have been in its absence.
The following year, Sobrino et al. (2008) grew cultures of Thalassiosira pseudonana -- which they describe as "a widely distributed diatom" -- while exposing them to either photosynthetically-active radiation (PAR: 400-700 nm) or PAR plus ultraviolet radiation (UVR: 280-400 nm) in 500-mL Teflon bottles maintained at 20°C, using a semi-continuous approach that employed daily dilutions with fresh growth medium (filtered seawater from the Gulf Stream that was enriched with f/2 nutrients) through which air streams of different atmospheric CO2 concentrations (380 or 1000 ppm) were continuously bubbled. In doing so, the three researchers determined that exposure of the seawater medium to air with an extra 620 ppm of CO2 increased the photosynthetic rate of the marine diatoms by approximately 45% in the presence of PAR and about 60% in the presence of both PAR and UVR, while it increased their growth rate by approximately 20% in both of the radiation environments. And in highlighting the significance of their findings, they note that "among the phytoplankton species inhabiting the [ocean's] surface layer, diatoms are responsible for almost 40% of the ocean primary productivity (Nelson et al., 1995)."
Working contemporaneously in the Southern Ocean, Tortell et al. (2008) measured CO2 uptake of in situ phytoplankton assemblages collected at 35 stations in the Ross Sea polynya during Austral spring and summer, together with 14C uptake for a subset of 11 station samples, while they conducted CO2 manipulation experiments with phytoplankton collected at three Ross Sea locations via shipboard incubations using a semi-continuous batch-culture technique. This work indicated, as they describe it, that "for the Phaeocystis-dominated springtime phytoplankton assemblages, there was a statistically significant increase in 14C fixation between 100 and 380 ppm CO2, but no further effects observed at 800 ppm CO2." In the case of the diatom-dominated summertime phytoplankton assemblages, however, the CO2-induced increase in both relative growth rate and primary productivity continued all the way out to the highest CO2 concentration investigated, i.e., 800 ppm, and it promoted "a shift towards larger chain-forming species." As a result, and noting that the larger chain-forming species of diatoms "are prolific bloom formers with a very high capacity for organic carbon export to the sediments (Stickley et al., 2005)," Tortell et al. concluded that "potential CO2-dependent productivity increases and algal species shifts could thus act to increase the efficiency of the biological pump, enhancing Southern Ocean CO2 uptake and contributing to a negative feedback on increased atmospheric CO2."
In conclusion, and has been found to be the case for essentially all types of marine phytoplankton, the real-world data that have been obtained to date suggest that earth's diatoms will manage just fine as the air's CO2 content continues to climb to ever-greater heights. And as diatoms serve as primary producers in numerous marine food chains, the several trophic levels above them should also be similarly benefited by the dreaded phenomenon of "ocean acidification."
References
Arrigo, K.R. 2007. Marine manipulations. Nature 450: 491-492.
Barcelos e Ramos, J., Biswas, H., Schulz, K.G., LaRoche, J. and Riebesell, U. 2007. Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Global Biogeochemical Cycles 21: 10.1029/2006GB002898.
Chen, X. and Gao, K. 2004. Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2. Functional Plant Biology 31: 399-404.
Nelson, D.M., Treguer, P., Brzezinski, M.A., Leynaert, A. and Queguiner, B. 1995. Production and dissolution of biogenic silica in the ocean: Revised global estimates, comparison with regional data and relationship to biogenic sedimentation. Global Biogeochemical Cycles 9: 359-372.
Riebesell, U., Schulz, K.G., Bellerby, R.G.J., Botros, M., Fritsche, P., Meyerhofer, M., Neill, C., Nondal, G., Oschlies, A., Wohlers, J. and Zollner, E. 2007. Enhanced biological carbon consumption in a high CO2 ocean. Nature 450: 545-548.
Sobrino, C., Ward, M.L. and Neale, P.J. 2008. Acclimation to elevated carbon dioxide and ultraviolet radiation in the diatom Thalassiosira pseudonana: Effects on growth, photosynthesis, and spectral sensitivity of photoinhibition. Limnology and Oceanography 53: 494-505.
Stickley, C.E., Pike, J., Leventer, A., Dunbar, R., Domack, E.W., Brachfeld, S., Manley, P. and McClennan, C. 2005. Deglacial ocean and climate seasonality in laminated diatom sediments, Mac Robertson Shelf, Antarctica. Palaeogeography, Palaeoclimatology, Palaeoecology 227: 290-310.
Tortell, P.D., Payne, C.D., Li, Y., Trimborn, S., Rost, B., Smith, W.O., Riesselman, C., Dunbar, R.B., Sedwick, P. and DiTullio, G.R. 2008. CO2 sensitivity of Southern Ocean phytoplankton. Geophysical Research Letters 35: 10.1029/2007GL032583.
Last updated 7 July 2010