Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry.

https://escholarship.org/content/qt21r294gk/qt21r294gk.pdf

Processes and patterns of oceanic nutrient limitation

Ocean acidification is a pervasive stressor that could affect many marine organisms and cause profound ecological shifts. A variety of biological responses to ocean acidification have been measured across a range of taxa, but this information exists as case studies and has not been synthesized into meaningful comparisons amongst response variables and functional groups. We used meta-analytic techniques to explore the biological responses to ocean acidification, and found negative effects on survival, calcification, growth and reproduction. However, there was significant variation in the sensitivity of marine organisms. Calcifying organisms generally exhibited larger negative responses than noncalcifying organisms across numerous response variables, with the exception of crustaceans, which calcify but were not negatively affected. Calcification responses varied significantly amongst organisms using different mineral forms of calcium carbonate. Organisms using one of the more soluble forms of calcium carbonate (high-magnesium calcite) can be more resilient to ocean acidification than less soluble forms (calcite and aragonite). Additionally, there was variation in the sensitivities of different developmental stages, but this variation was dependent on the taxonomic group. Our analyses suggest that the biological effects of ocean acidification are generally large and negative, but the variation in sensitivity amongst organisms has important implications for ecosystem responses.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-0248.2010.01518.x

Meta‐analysis reveals negative yet variable effects of ocean acidification on marine organisms

Ecological risks from metal contaminants are difficult to document because responses differ among species, threats differ among metals, and environmental influences are complex. Unifying concepts are needed to better tie together such complexities. Here we suggest that a biologically based conceptualization, the biodynamic model, provides the necessary unification for a key aspect in risk:  metal bioaccumulation (internal exposure). The model is mechanistically based, but empirically considers geochemical influences, biological differences, and differences among metals. Forecasts from the model agree closely with observations from nature, validating its basic assumptions. The biodynamic metal bioaccumulation model combines targeted, high-quality geochemical analyses from a site of interest with parametrization of key physiological constants for a species from that site. The physiological parameters include metal influx rates from water, influx rates from food, rate constants of loss, and growth rates (whe...

Why Is Metal Bioaccumulation So Variable? Biodynamics as a Unifying Concept

SUMMARY Diatoms and bacteria have cooccurred in common habitats for hundreds of millions of years, thus fostering specific associations and interactions with global biogeochemical consequences. Diatoms are responsible for one-fifth of the photosynthesis on Earth, while bacteria remineralize a large portion of this fixed carbon in the oceans. Through their coexistence, diatoms and bacteria cycle nutrients between oxidized and reduced states, impacting bioavailability and ultimately feeding higher trophic levels. Here we present an overview of how diatoms and bacteria interact and the implications of these interactions. We emphasize that heterotrophic bacteria in the oceans that are consistently associated with diatoms are confined to two phyla. These consistent bacterial associations result from encounter mechanisms that occur within a microscale environment surrounding a diatom cell. We review signaling mechanisms that occur in this microenvironment to pave the way for specific interactions. Finally, we discuss known interactions between diatoms and bacteria and exciting new directions and research opportunities in this field. Throughout the review, we emphasize new technological advances that will help in the discovery of new interactions. Deciphering the languages of diatoms and bacteria and how they interact will inform our understanding of the role these organisms have in shaping the ocean and how these interactions may change in future oceans.

Interactions between Diatoms and Bacteria

Ocean acidiﬁcation is an undisputed fact. The ocean presently takes up one-fourth of the carbon CO2 emitted to the atmosphere from human activities. As this CO2 dissolves in the surface ocean, it reacts with seawater to form carbonic acid, increasing ocean acidity and shifting the partitioning of inorganic carbon species towards increased CO2 and dissolved inorganic carbon, and decreased concentration of carbonate ion.

While our understanding of the possible consequences of ocean acidiﬁcation is still rudimentary, both the scientific community and the society at large are increasingly concerned about the possible risks associated with ocean acidiﬁcation for marine organisms and ecosystems. As this new and pressing ﬁeld of marine research gains momentum, many in our community, including representatives of coordinated research projects, international scientific organisations, funding agencies, and scientists in this ﬁeld felt the need to provide guidelines and standards for ocean acidiﬁcation research.


To initiate this process, the European Project on Ocean Acidiﬁcation (EPOCA) and the International Oceanographic Commission (IOC) jointly invited over 40 leading scientists active in ocean acidiﬁcation research to a meeting at the Leibniz Institute of Marine Science (IFM-GEOMAR) in Kiel, Germany on 19-21 November 2008. At the meeting, which was sponsored by EPOCA, IOC, the Scientific Council on Oceanic Research (SCOR), the U.S. Ocean Carbon and Biogeochemistry Project (OCB) and the Kiel Excellence Cluster “The Future Ocean”, the basic structure and contents of the guide was agreed upon and an outline was drafted. In the following months, the workshop participants and additional invited experts prepared draft manuscripts for each of the sections, which were subsequently reviewed by independent experts and revised according to their recommendations. Starting 15 May 2009, the guide was made publicly available for an open community review.

Guide to best practices for ocean acidification research and data reporting

The acidification caused by the dissolution of anthropogenic carbon dioxide (CO2) in the ocean changes the chemistry and hence the bioavailability of iron (Fe), a limiting nutrient in large oceanic regions. Here, we show that the bioavailability of dissolved Fe may decline because of ocean acidification. Acidification of media containing various Fe compounds decreases the Fe uptake rate of diatoms and coccolithophores to an extent predicted by the changes in Fe chemistry. A slower Fe uptake by a model diatom with decreasing pH is also seen in experiments with Atlantic surface water. The Fe requirement of model phytoplankton remains unchanged with increasing CO2. The ongoing acidification of seawater is likely to increase the Fe stress of phytoplankton populations in some areas of the ocean.

https://science.sciencemag.org/content/sci/327/5966/676.full.pdf

Effect of ocean acidification on iron availability to marine phytoplankton.

. The control of key chemical parameters in phytoplankton cultures, such as pCO2, pH and Ω (the saturation state of calcium carbonate), is made difficult by the interdependence of these parameters and by the changes resulting from the growth of the organisms, such as CO2 fixation, nutrient uptake and, for coccolithophores, calcite precipitation. Even in cultures where pCO2 or pH is maintained constant, other chemical parameters change substantially at high cell densities. Experimentally we observed that various methods of adjustment of pCO2/pH – acid or base addition, use of buffers or pH-stats, or bubbling of CO2-enriched air – can be used, the choice of one or the other depending on the goals of the experiments. At seawater pH, we measured the same growth rates in cultures of the diatom Thalassiosira weissflogii where the pCO2/pH was controlled by these different methods. The pH/pCO2 control method also did not affect the rates of growth or calcification of the coccolithophore Emiliania huxleyi at seawater pH. At lower pH/higher pCO2, in the E. huxleyi strain PLY M219, we observed increases in rates of carbon fixation and calcification per cell, along with a slight increase in growth rate, except in bubbled cultures. In our hands, the bubbling of cultures seemed to induce more variable results than other methods of pCO2/pH control. While highly convenient, the addition of pH buffers to the medium apparently induces changes in trace metal availability and cannot be used under trace metal-limiting conditions.

/pdf/effects-of-the-ph-p-co-2-control-method-on-medium-chemistry-3xugr84ibx.pdf

Effects of the pH/ p CO 2 control method on medium chemistry and phytoplankton growth

Dissolution of anthropogenic CO2 increases the partial pressure of CO2 (pCO2) and decreases the pH of seawater. The rate of Fe uptake by the dominant N2-fixing cyanobacterium Trichodesmium declines as pH decreases in metal-buffered medium. The slower Fe-uptake rate at low pH results from changes in Fe chemistry and not from a physiological response of the organism. Contrary to previous observations in nutrient-replete media, increasing pCO2/decreasing pH causes a decrease in the rates of N2 fixation and growth in Trichodesmium under low-Fe conditions. This result was obtained even though the bioavailability of Fe was maintained at a constant level by increasing the total Fe concentration at low pH. Short-term experiments in which pCO2 and pH were varied independently showed that the decrease in N2 fixation is caused by decreasing pH rather than by increasing pCO2 and corresponds to a lower efficiency of the nitrogenase enzyme. To compensate partially for the loss of N2 fixation efficiency at low pH, Trichodesmium synthesizes additional nitrogenase. This increase comes partly at the cost of down-regulation of Fe-containing photosynthetic proteins. Our results show that although increasing pCO2 often is beneficial to photosynthetic marine organisms, the concurrent decreasing pH can affect primary producers negatively. Such negative effects can occur both through chemical mechanisms, such as the bioavailability of key nutrients like Fe, and through biological mechanisms, as shown by the decrease in N2 fixation in Fe-limited Trichodesmium.

https://www.pnas.org/content/pnas/109/45/E3094.full.pdf

Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions.

The marine clams Mactra veneriformis were collected from three different locations in a contaminated bay in Northern China. Another species of clams Ruditapes philippinarum was collected from the same contaminated bay as well as from a relatively clean site in Hong Kong. The indices of Cd and Zn bioaccumulation (assimilation efficiency, dissolved uptake rate, and efflux rate), tissue concentration, subcellular distribution, metallothionein (MT) content, and clearance rate of the clams were subsequently quantified in these populations in the laboratory. In the two species of clams, the population with a higher Cd tissue concentration assimilated Cd and Zn more efficiently, in correlation with an increase in the Cd associated with the metallothionein-like protein (MTLP) fraction. The subcellular partitioning of Zn was similar among the different populations. The dissolved uptake rates of Cd and Zn were not influenced by the different tissue concentrations of metals in the clams. However, the clam R. philippinarum from the contaminated site reduced their Zn uptake rate constants in response to increasing Zn concentration in the water. Differences in Cd and Zn tissue concentrations had little influence on the metal efflux rate constant and the clams' clearance rate. Our results indicate that the higher Cd and Zn tissue concentrations observed in these two species may be partially caused by the high levels of metal assimilation. Populations living in contaminated environments may be able to modify their physiological and biochemical responses to metal stress, which can subsequently alter trace metal bioaccumulation to aquatic animals. The relative significance of dietary uptake and the potential trophic transfer of metals in the contaminated areas may be substantially different from those in the clean environments.

Understanding the differences in Cd and Zn bioaccumulation and subcellular storage among different populations of marine clams.

Acidification of seawater caused by anthropogenic carbon dioxide (CO 2 ) is anticipated to influence the growth of dinitrogen (N 2 )–fixing phytoplankton, which contribute a large fraction of primary production in the tropical and subtropical ocean. We found that growth and N 2 -fixation of the ubiquitous cyanobacterium Trichodesmium decreased under acidified conditions, notwithstanding a beneficial effect of high CO 2 . Acidification resulted in low cytosolic pH and reduced N 2 -fixation rates despite elevated nitrogenase concentrations. Low cytosolic pH required increased proton pumping across the thylakoid membrane and elevated adenosine triphosphate production. These requirements were not satisfied under field or experimental iron-limiting conditions, which greatly amplified the negative effect of acidification.

https://science.sciencemag.org/content/sci/early/2017/04/26/science.aal2981.full.pdf

Dalin Shi

Papers

Effect of ocean acidification on iron availability to marine phytoplankton.

Effects of the pH/ p CO 2 control method on medium chemistry and phytoplankton growth

Ocean acidification slows nitrogen fixation and growth in the dominant diazotroph Trichodesmium under low-iron conditions.

Understanding the differences in Cd and Zn bioaccumulation and subcellular storage among different populations of marine clams.

The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium