Cyanobacteria-also known as blue-green algae, blue-green bacteria or Cyanophyta (the smallest of which, less than two micrometers in diameter, are typically referred to as picocyanobacteria)-obtain their energy through the process of photosynthesis and are thus important primary producers in many areas of the world's oceans, as well as significant components of the marine nitrogen cycle. This summary briefly reviews the results of studies that indicate how they may be affected by ocean acidification in a CO2-enriched world of the future, several of which findings challenge alarming negative projections of the IPCC.
Contending that cyanobacteria "should be one of the focus points regarding biological responses to the rise in atmospheric CO2 concentration," Lu et al. (2006) studied physiological changes in phycocyanin (PC)-rich and phycoerythrin (PE)-rich Synechococcus strains of picocyanobacteria under atmospheric CO2 concentrations of 350, 600 and 800 ppm in batch cultures maintained in one-liter glass flasks under a 12-hour:12-hour light:dark regime for periods of 12 days, during which time intervals they measured a number of physiological parameters related to the growth and well-being of the picocyanobacteria. And in doing so, they found the growth of the PE strain was unaffected by atmospheric CO2 enrichment, but that the PC strain grown at 800 ppm CO2 experienced a 36.7% increase in growth compared to when it was grown at 350 ppm CO2.
On the other hand, the PC strain showed no significant change in carbohydrate content over the CO2 range investigated; but the PE strain exhibited a CO2-induced increase of 37.4% at 800 ppm CO2. Nevertheless, the PC strain exhibited a 36.4% increase in its RNA/DNA ratio between 350 and 800 ppm CO2, which ratio, in the words of Lu et al., "provides a good estimate of metabolic activities and has been used extensively as a biochemical indicator of growth rate in a variety of marine organisms." In addition, in both Synechococcus strains, cellular pigment contents were generally greater in the CO2-enriched treatments than in the ambient-air controls. At day 12 in the PE strain, for example, they averaged in excess of 70% greater at 800 ppm CO2 than at 350 ppm CO2.
From these results it is clear that both strains of the Synechococcus picocyanobacteria benefited greatly from the extra CO2 that was supplied to them, albeit in a variety of different ways; and in comparing the different responses of the two strains, Lu et al. conclude "the PC strain would probably benefit more than the PE strain from future increases in atmospheric CO2 concentration." However, they note "differences in photosynthetic characteristics may allow the coexistence of the two picocyanobacterial strains through a subtle form of niche differentiation," citing the work of Ernst et al. (2003) and Stomp et al. (2004). Consequently, there is reason to believe there may be a significant increase in primary production and nutrient cycling throughout the world's oceans as the air's CO2 content continues on its upward trajectory in the years and decades to come, driven by these several positive impacts of ocean acidification on these very tiny organisms.
One year later, Levitan et al. (2007) wrote "among the principal players contributing to global aquatic primary production, the nitrogen (N)-fixing organisms (diazotrophs) are important providers of new N to the oligotrophic areas of the oceans," and they cited several studies that demonstrated "cyanobacterial (phototrophic) diazotrophs in particular fuel primary production and phytoplankton blooms which sustain oceanic food-webs and major economies and impact global carbon (C) and N cycling." These facts thus compelled them to examine how the ongoing rise in the air's CO2 content might impact these relationships by exploring the response of the cyanobacterial diazotroph Trichodesmium to changes in the atmosphere's CO2 concentration, choosing this particular diazotroph because it dominates the world's tropical and sub-tropical oceans in this respect, contributing over 50% of total marine N fixation.
More specifically, the eight Israeli and Czech researchers grew Trichodesmium IMS101 stock cultures in YBCII medium (Chen et al., 1996) at 25°C and a 12-hour:12-hour light/dark cycle (with the light portion of the cycle in the range of 80-100 µmol photons m-2 s-1) in equilibrium with air of three different CO2 concentrations (250, 400 and 900 ppm, representing low, ambient and high concentrations, respectively), which was accomplished by continuously bubbling air of the three CO2 concentrations through the appropriate culture vessels throughout various experimental runs, each of which lasted a little over three weeks, during which time intervals they periodically monitored a number of diazotrophic physiological processes and properties.
So what did they learn? Levitan et al. found Trichodesmium in the high CO2 treatment "displayed enhanced N fixation, longer trichomes, higher growth rates and biomass yields." In fact, they state in the high CO2 treatment there was "a three- to four-fold increase in N fixation and a doubling of growth rates and biomass," and that the cultures in the low CO2 treatment reached a stationary growth phase after only five days, "while both ambient and high CO2 cultures exhibited exponential growth until day 15 before declining."
In discussing possible explanations for what they observed, the researchers suggest "enhanced N fixation and growth in the high CO2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms required under low and ambient CO2." Consequently, they conclude "in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO2 to increase N fixation and growth of Trichodesmium," and "other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans." To emphasize these points, they state in the final sentence of their paper "Trichodesmium's dramatic response to elevated CO2 may consolidate its dominance in subtropical and tropical regions and its role in C and N cycling, fueling subsequent primary production, phytoplankton blooms, and sustaining oceanic food-webs."
Shortly thereafter, employing semi-continuous culturing methods that used filtered, microwave-sterilized surface Sargasso seawater that was enriched with phosphate and trace nutrients, Fu et al. (2008) examined "the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera [watsonii] at glacial (190 ppm), current (380 ppm), and projected year 2100 (750 ppm) CO2 levels." In doing so, they learned that when the seawater was replete with iron, daily primary production at 750 ppm CO2 was 21% greater than it was at 380 ppm, while at 190 ppm CO2 it was 38% lower than it was at 380 ppm. When the seawater was iron-limited, however, daily primary production at 750 ppm CO2 was 150% greater than it was at 380 ppm, while at 190 ppm CO2 it was 22% lower than it was at 380 ppm. With respect to N2 fixation, on the other hand, rates varied little among the three CO2 treatments when the seawater was iron-limited; but when the seawater was replete with iron, N2 fixation at 750 ppm CO2 was 60% greater than it was at 380 ppm, while at 190 ppm CO2 it was 33% lower than it was at 380 ppm.
In discussing their results, Fu et al. write "several studies examining the marine diazotrophic cyanobacterium Trichodesmium have shown significant increases in N2 fixation and photosynthesis in response to elevated CO2 concentration (Hutchins et al., 2007; Levitan et al., 2007; Ramos et al., 2007)," and they say their data "extend these findings to encompass the marine unicellular N2-fixing cyanobacterium Crocosphaera," which group, they add, "is now recognized as being perhaps equally as important as Trichodesmium to the ocean nitrogen cycle (Montoya et al., 2004)." Consequently, they conclude-and rightly so-"anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation," which two-pronged positive phenomenon would be a tremendous boon to the marine biosphere.
Kranz et al. (2009) introduced their study by writing "marine phytoplankton contribute up to 50% of global primary production (Falkowski et al., 1998) and influence Earth's climate by altering various biogeochemical cycles (Schlesinger, 2005)." They also note, with respect to the latter subject, that among diazotrophic cyanobacteria (dinitrogen-fixers), Trichodesmium species contribute "about half of all marine N2 fixation (Mahaffey et al., 2005)," supporting "a large fraction of biological productivity in tropical and subtropical areas and exerting, over long timescales, a significant influence on global carbon cycles by providing a major source of reactive N to the water column (Falkowski and Raven, 1997)."
To see how the ongoing and projected increase in the air's CO2 concentration might impact one of the global ocean's most important diazotrophic cyanobacteria (Trichodesmium erythraeum IMS101), Kranz et al. grew the ubiquitous marine N2-fixer in semi-continuous batch cultures through which they bubbled air with CO2 concentrations of either 370 or 1000 ppm. And after the cultures were acclimated to their respective CO2 concentrations for at least 14 days (covering more than 5 generations), they measured their rates of particulate organic carbon (POC) and particulate organic nitrogen (PON) fixation, discovering that there was "a strong increase in photosynthesis and N2 fixation under elevated CO2 levels," such that POC and PON production rates rose "by almost 40%."
In discussing the generality of their results, the German scientists note, while working with the same Trichodesmium species, "Barcelos e Ramos et al. (2007) and Levitan et al. (2007) observed stimulation in N2 fixation by approximately 40% and even up to 400%, while Hutchins et al. (2007) obtained stimulation by up to 35%." And in discussing the significance of these findings, they state "the observed increase in photosynthesis and N2 fixation could have potential [global] biogeochemical implications, as it may stimulate productivity in N-limited oligotrophic regions and thus provide a negative feedback on rising atmospheric CO2 levels."
Two years later, Kranz et al. (2011) produced a review paper wherein they write "marine phytoplankton are responsible for almost half of all photosynthetic carbon fixation on Earth and play a vital role in altering the CO2 exchange between ocean and atmosphere," citing Maier-Reimer et al. (1996) and Gruber (2004). However, they note lack of nitrates often limits phytoplanktonic growth, and that diazotrophic cyanobacteria that fix nitrogen, such as Trichodesmium species, thus play "a crucial role in many marine ecosystems by providing a new source of biologically available nitrogen." And they thus went on to describe how atmospheric CO2 enrichment helps to enhance both halves of this important two-pronged phenomenon.
First of all, the three researchers-all from the Alfred Wegener Institute for Polar and Marine Research located in Bremerhaven, Germany-report "four recent studies tested the effect of different CO2 concentrations on the growth, biomass production and elemental composition of Trichodesmium (Barcelos e Ramos et al., 2007; Hutchins et al., 2007; Kranz et al., 2009; Levitan et al., 2007)," and they say these studies "concordantly demonstrated higher growth and/or production rates under elevated pCO2, with a magnitude exceeding those CO2 effects previously seen in other marine phytoplankton."Focusing next on particulate organic nitrogen (PON) production, they note Trichodesmium species are particularly effective in this regard, writing "the stimulation in N2 fixation and/or PON production between present-day pCO2 values (370-400 ppm) and those predicted for the year 2100 (750-1000 ppm) ranged between 35 and 240%." And last of all, they state "data on CO2 dependency of N2 fixation rates from recent publications suggest that N2 fixation by Trichodesmium species might increase by more than 20 Tg N per year to about 100 Tg N per year until the end of this century," citing Hutchins et al. (2009).
In light of these several findings, it appears Earth's oceans are primed to do their part in terms of preserving and protecting the biosphere, as they (1) ramp up their productivity to sustain a greater population of aquatic organisms that may be tapped to supply additional food for the planet's burgeoning human population, and as they (2) remove from the atmosphere and sequester in their sediments ever more carbon, as anthropogenic CO2 emissions continue to rise.
Garcia et al. (2011) additionally emphasized that N2 fixation by marine diazotrophic cyanobacteria (such as various species of Trichodesmium) contributes substantial new nitrogen to marine environments, including the North Atlantic, Pacific, and Indian Oceans (Carpenter et al., 1993; Capone et al., 1997, 2005; Karl et al., 2002)." And they further note phosphorus and iron have been identified as key factors that control N2 fixation in those environments. However, they also report several other studies suggest the current low partial pressure of CO2 in the atmosphere "may be another possible limiting factor for N2 fixation and CO2 fixation by Trichodesmium," citing the work of Barcelos e Ramos et al. (2007), Hutchins et al. (2007), Levitan et al. (2007, 2010) and Kranz et al. (2009). Thus, in a laboratory study designed to explore this latter possibility, Garcia et al. examined the effects of near-present-day (~380 ppm) and elevated (~750 ppm) atmospheric CO2 concentrations on CO2 and N2 fixation by T. erythraeum isolates from the Pacific and Atlantic Oceans under a range of irradiance conditions.
According to the seven scientists, "the positive effect of elevated CO2 on gross N2 fixation was large (~50% increase) under mid and/or low irradiances compared with that at high light (~20% increase)," noting that data from Kranz et al. (2010) and Levitan et al. (2010) corroborated their findings. In fact, they report in the Kranz et al. study, "under low light, gross N2-fixation rates were 200% higher in a high-CO2 treatment (900 ppm) compared with a low-CO2 treatment (150 ppm), whereas under high light, gross N2-fixation rates were only 112% higher under elevated CO2." In the case of CO2 fixation, on the other hand, they found CO2-fixation rates increased significantly "in response to high CO2 under mid- and high irradiances only."
As the atmosphere's CO2 concentration continues to rise, therefore, this phenomenon should boost the growth rates of marine diazotrophic cyanobacteria and enable them to make more nitrogen available to themselves and co-occurring species, which should ultimately act to significantly increase both the quantity and quality of the worldwide phytoplanktonic food base that ultimately supports all marine animal life.
One year later, Holland et al. (2012) introduced their intriguing study of the potentially-toxic cyanobacterium Cylindrospermopsis raciborskii, which was originally described as a tropical-subtropical species but is increasingly found in temperate regions. Noting "climate change is hypothesized to be a factor in this expansion," the five researchers rightly state "identifying future risk from this, and other nuisance cyanobacteria, is paramount," and so they proceeded to conduct their own study of the subject to help identify that risk. More specifically, working with a strain of the cyanobacterium that was originally isolated from a lake near Brisbane (Australia), Holland et al. say they "used continuous (turbidostats) and batch cultures grown under two different light regimes, and adjusted the alkalinity of the media (with an associated change in pH, HCO3- and CO2) to assess the effect of these parameters on the specific growth rate, inorganic carbon acquisition and photosynthetic parameters of C. raciborskii."
Although there were insufficient data to confirm results obtained from the low-light experiments, the Australian researchers discovered "there was a positive linear relationship in the 'high' light turbidostats between the growth rate and pH," whereby the potentially-toxic C. raciborskii grew more profusely when atmospheric CO2 concentrations were low and water pH was high, leading Holland et al. to conclude high-CO2/low-pH conditions may change the composition of marine communities "to favor species that are better adapted to these new growth conditions, such as Chrysophytes," which are known to produce "more than half of the food consumed by aquatic animals," additionally citing Maberly et al. (2009) in this regard.
About this same time, Tiera et al. (2012) wrote that they "tested the direct effect of an elevated CO2 concentration (1,000 ppm) on the biomass and metabolic rates (leucine incorporation, CO2 fixation and respiration) of two isolates belonging to two relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217)," the former of which they refer to as simply Roseobacter and the latter of which they designate Cytophaga. And in doing so, they found "contrary to some expectations, lowering pH did not negatively affect bacterial growth." In fact, they indicated that it actually increased growth efficiency in the case of Cytophaga, noting "in both cases, the bacterial activity under high CO2 would increase the buffering capacity of seawater." Their final words on the subject, therefore, were that the responses of both of the marine bacterial families "would tend to increase the pH of seawater, acting as a negative feedback between elevated atmospheric CO2 concentrations and ocean acidification." And, of course, their work also revealed the identities of two more sets of marine organisms that are in no way threatened by the seawater pH changes induced by the historical and still-ongoing rise in the atmosphere's CO2 concentration.
Lomas et al. (2012) introduced their work by noting "marine cyanobacteria, both unicellular Prochlorococcus and Synechococcus and colonial Trichodesmium spp., play important roles in the ocean carbon cycle and the biological carbon pump, particularly in the subtropical and tropical gyres (e.g. Partensky et al., 1999; Capone et al., 2005)," while adding that Trichodesmium "is thought to account for about half of the total N2-fixation in the oceans," citing Barcelos e Ramos et al. (2007), Hutchins et al. (2007), Levitan et al. (2007) and Kranz et al. (2009, 2010).
Against this backdrop of information, and working on board the RV Atlantic Explorer between July 2009 and April 2010 at the Bermuda Atlantic Time-series Study (BATS) site in the subtropical North Atlantic Ocean about 86 km southeast of Bermuda, Lomas et al. examined the C-fixation responses of natural assemblages of cyanobacteria dominated by Synechococcus and Prochlorococcus and the N2- and C-fixation responses of isolated Trichodesmium colonies to changes in pH/pCO2 conditions between the time of the last glacial minimum (8.4/150 ppm) and projected year 2100 values (7.8/800 ppm). And this effort revealed, in their words, "whole community assemblages dominated by Prochlorococcus and Synechococcus, whether nutrient-replete or P-limited, did not show a clear response of C-fixation rates to changes in pH/pCO2." However, they found "Fe- and P-replete colonies of Trichodesmium increased N2-fixation rates at pH 7.8 by 54% over ambient pH/pCO2 conditions, while N2-fixation at pH 8.4 was 21% lower than at ambient pH/pCO2." Likewise, they found C-fixation rates of Trichodesmium "were on average 13% greater at low pH than at ambient pH and 37% greater than at high pH." And they made a point of noting "these results for natural populations of all three cyanobacteria concur with previous research and suggest that one important response to changes in ocean pH and pCO2 might be an increase in N2 and C fixation by Trichodesmium under nutrient-replete conditions."
Quoting the team of researchers that conducted the study, their results for Trichodesmium, along with the similar results of several other marine scientists, suggest "ocean acidification would likely result in a positive feedback on the growth and physiology of natural populations, resulting in a positive change in their role in ocean carbon and nitrogen cycles," which is, of course, great news for the biosphere!
More recently, Kerfahi et al. (2014) investigated the effect of a natural gradient in seawater pCO2 on sediment bacterial communities off Vulcano in the Mediterranean, hoping to get some idea about the sensitivity of these microorganisms to ocean acidification. To accomplish their objective, they "sampled the upper 2 cm of volcanic sand in three zones, ambient (median pCO2 419 µatm, minimum Ωarag 3.77), moderately CO2-enriched (median pCO2 592 µatm, minimum Ωarag 2.96), and highly CO2-enriched (median pCO2 1611 µatm, minimum Ωarag 0.35)." Results indicated that "the relative abundances of most of the dominant genera were unaffected by the pCO2 gradient." In fact, the six scientists actually found that "bacterial diversity increased toward higher pCO2." These findings, according to Kerfahi et al., "support the view that globally increased ocean pCO2 will be associated with changes in sediment bacterial community composition but that most of these organisms are resilient."
Contemporaneously, Spungin et al. (2014) set out to examine the combined effects of phosphorus (P) limitation and atmospheric CO2 enrichment on Trichodesmium erythraeum IMS 101 cultures in a study where they measured nitrogen acquisition, glutamine synthetase activity, carbon (C) uptake rates, intra-cellular Adenosine Triphosphate (ATP) concentration and the pool sizes of related key proteins, where the two CO2 concentrations tested were 400 and 900 µatm and the two P concentrations were 0.5 and 50 µM.
With respect to their findings the three Israeli researchers report -- among a large number of other related things -- that the cell-per-day growth rate of T. erythraeum at 0.5 µM P was enhanced by 41% when going from 400 to 900 µatm CO2 at 0.5 µM P and by 57% at 50 µM P. Spungin et al. state that their results suggest several possible cellular mechanisms by which Trichodesmium could adapt to predicted changes in pCO2 and P availability in future oceans. They conclude, for example, that "by modifying its metabolic and physiological characteristics, Trichodesmium in the future oceans could maintain inputs of new nitrogen to the upper mixed layer of oceanic waters even in P-limited areas." And they further suggest that "this behavior may extend Trichodesmium's dominance in the acidified ocean ... and possibly increase its contribution of new N and C to these regions."
Lastly, in an effort to improve knowledge of the CO2 responses of lesser-studied diazotrophs, Eichner et al. (2014) acclimated the single-celled Cyanothece sp. and two heterocystous species, Nodularia spumigena and the symbiotic Calothrix rhizosoleniae, to two different pCO2 levels -- 380 and 980 µatm -- after which they measured their growth rates, cellular composition, and carbon and N2 fixation rates, which they then compared to what had been found in prior similar studies with still other related species. In doing so the three researchers report that "the three species of functionally different N2 fixers investigated in this study responded differently to elevated pCO2, showing enhanced, decreased as well as unaltered growth and production rates." However, it is important to note that Barcelos e Ramos et al. (2007) concluded that Trichodesmium, with its large positive response to atmospheric CO2 enrichment, is "the single most important nitrogen fixer in today's ocean." And in addition to Trichodesmium, Eichner et al. indicate that a number of single-celled and symbiotic open-ocean species have also been found "to respond positively to high pCO2," as in their study, thus confirming prior "predictions of an increase in N2 fixation with ocean acidification on a global scale."
In conclusion, it would appear that the cyanobacteria of Earth's oceans are doing their part to preserve and protect the entire biosphere, as they increase their productivity to sustain a greater population of aquatic organisms that may be tapped to supply additional food for the planet's burgeoning human population, and as they remove from the atmosphere and sequester in seabed sediments ever more carbon, as anthropogenic CO2 emissions continue to rise.
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