A global perspective on dissolved organic matter and the world’s smallest organisms

A global perspective on dissolved organic matter and the world’s smallest organisms

Stiig Markager, Morten Søndergaard, Mathias Middelboe and Colin Stedmon

Background: Approximately 97% of all organic carbon in the world’s oceans exists in the dissolved phase. Living organisms spanning in size from viruses and bacteria up to whales, represent less than 1%, while the remainder is present as detritus (non living organic particulates). The amount of carbon in DOM is similar to that present in the atmosphere as carbon dioxide, however little is known about the processes that control its production, transformation and fate. As a result of its size, even slight changes in the processes regulating the DOM pool can have consequences for the global carbon cycle.  For example it has been estimated that a 1% increase in the annual net remineralisation of oceanic DOM would lead to a greater production of carbon dioxide than that arising from anthropogenic fossil fuels combustion (Hedges 2002).

DOM originates in part from the degradation of terrestrial plant material, which is transported to the open sea by rivers. Additionally aquatic plants, phytoplankton in particular, are also an important source of DOM. In the sea DOM is degraded (and ultimately remineralised) by photochemical and microbial degradation. DOMs concentration and composition varies with geographic position (e.g. distance from land) and depth, however little is known about its properties or characteristics. DOM consists of a complex mixture of organi c compounds where the large majority of the compounds (80%) remain unknown and likely resemble the complex humic compounds found in soils.

                      When one looks at the range of diverse organisms found in the oceans it becomes apparent that organisms smaller than 2 m in diameter (pico- and femtoplankton) dominate, with regards to their number, weight and active biological surface. This does not mean that larger organisms are ecologically irrelevant, but infers that small algae, bacteria, viruses and protozoa are responsible for a large part of the biological turnover in the oceans. Typically, small algae are responsible for 30 ÿ 80 % of the total primary production, and dominate productivity in warm climates and waters with low nutrient concentrations (Søndergaard 1994). Simultaneously much of the material produced via primary production is recycled by bacteria via DOM (Williams 2000), and marine viruses are an important sink for the bacteria (Fuhrman & Noble 1995). As a result there is a tight coupling between the dynamics of DOM and the number and activity of small organisms in the world’s oceans.

Recent research suggests that there are both geographic and temporal differences in the coupling between DOM and the microbial food web (Hansell & Carlson 2002), howeve r systematic relationships across different climates and regions of the oceans have not yet been found. The Galathea 3 expedition presents a unique opportunity to gain a global perspective on the importance of DOM, and the microbial and physicochemical processes that control its generation and fate, which in turn determines its role in global biogeochemical cycles.   The fact that the cruise is carried out on a single ship as a single project, and therefore that all measurements are carried out on the same instruments, using the same approaches and by the same research teams, makes this cruise exceptional unique prospect. All data artefacts common to comparisons of results from different cruises and projects are eliminated, providing the opportunity to produce a unique coherent truly global data set.

Relation to earlier research and perspectives

Characterisation of the DOM pool  DOM is very difficult to chemically characterise due to its complexity. Often “bulk” measurements of carbon (DOC), nitrogen (DON) and phosphorous (DOP) are used. Over the last five years the researchers involved in this project have been developing and adapting optical techniques for characterising DOM, and studying the properties of DOM with regards to its susceptibility to microbial and photochemical degradation.

The characteristics of a fraction of DOM can be investigated and monitored relatively rapidly using fluorescence spectroscopy (Stedmon et al 2003). The approach is not able to provide a detailed chemical description of DOM composition, however is capable of describing the distribution of a number of fluorescent groups present. To date the approach has been used to describe the dynamics of DOM in estuaries, coastal seas and Antarctic lakes (Fulton et al 2004; Stedmon & Markager 2005a, b). These types of fluorescence measurements have been applied for many years and in many different environments (e.g. Coble 1996), however the interpretation of the large amounts of data produced has hindered the use of them to their full potential. This has now been solved by applying a data analysis technique called parallel factor analysis (PARAFAC) (Stedmon et al 2003). This approach allows us to study the variability in fluorescent DOM as a result of production and degradation processes. In the proximity of land and large rivers, DOMs composition is often variable, being influenced by local coastal production and the material carried by rivers from land to the sea. In the open ocean the processes controlling DOM are fewer, and it is likely that its dynamics vary with temperature, light levels and nutrient conditions. A working hypothesis could be that DOMs composition will converge from being very diverse and variable in coastal seas, moving towards a more constant composition in the open ocean. Is there a common pool of oceanic humic material (“oceanic soil”) irrespective of location? The Galathea 3 cruise is an ideal opportunity to test this hypothesis.

DOM – degradation and bacteria   The variability of DOMs concentration and ecosystem role as a function of biological activity, photochemical processes and bacterial degradation is another new and large research field (Carlson 2002). Just 20 years ago, it was thought that the majority of DOM was refractory (i.e. non-degradable). Now we know this is not so, and that its degradation is a complex combination of photochemical and bacterial processes (e.g. Søndergaard & Middelboe 1995). The time scale for the degradation of DOM determines the time it takes for carbon as carbon dioxide to be sequestered by plants and ultimately return to atmosphere after remineralisation. The degradation of DOM is also important for the biogeochemical cycles of nitrogen and phosphorous in the oceans, as the majority of the fixed nitrogen and much of the available phosphorous is bound within DOM. During the cruise we will carry out a series of experiments focused on the long term degradation of DOM and in combination measure compositional changes, oxygen demand, release of nutrients, bacterial activity and sensitivity to temperature changes.

Only bacteria can utilise DOM for growth. In different oceanic  regimes it is thought that bacterial activity and therefore DOM degradation, is controlled by either nutrient concentrations (Carlson et al. 2002), bioavailability of DOM (Kirchman 1990) or other biological factors (Søndergaard et al. 2000). The long-term experiments will therefore be supplemented with short-term experiments aimed at determining what is limiting bacterial activity . The experiments will be designed in such a way to include different temperatures, where inorganic nutrients and organic substrates are added and bacterial productivity is monitored over time.

Virus As mentioned earlier, viruses play an important role in regulating bacterial activity and ultimately DOM turnover in the ocean. Viral lysis of heterotrophic and autotrophic micro-organisms releases labile, nutrient-rich cell material, which is then available to other non-infected bacteria (Furhman 1999, Middelboe et al. 2003). Viral lysates therefore have the potential to be an important food source for bacteria. A fraction of the organic material in the oceans circulates between bacteria and DOM as a result of viral activity. This is termed the “viral loop” and it stimulates microbial respiration and reduces the transport of carbon to higher trophic levels in the marine food web (Fuhrman 1999). As about 75% of the organic material released via cell lysis is utilised within a few hours (Middelboe et al. 2002), this material represents an important source of bacterial carbon and nutrients. Under certain conditions, the release of lysates satisfies the majority of the food requirement of bacteria (Wilhelm & Suttle 1999). The quantitative importance of virus mediated generation of organic material in the oceans in relation to other sources is not yet fully understood. Additionally the factors regulating the importance of this process in different regions of the world’s oceans remain to be studied. This project will therefore investigate the importance of viral processes in the cycling of carbon and nutrients in regions that differ with regards to their productivity, temperature and autotrophic community composition. This will allow us to evaluate the role of viruses in both regional and global carbon cycling, which is possibly the only remaining great unknown with regards to understanding the microbial turnover of carbon, nitrogen and phosphorous in the oceans.

In addition to assessing the abundance and distribution of virus es in the water column in relation to bacterial abundance and activity, auxiliary experiments onboard the ship will be carried out. These experiments will aim to identify the importance of viruses for the production and turnover of DOM. These types of experiments have been tried and tested with success in earlier studies (e.g. Søndergaard et al. 2000, Middelboe & Lyck 2002, Middelboe et al. 2002, 2003).

Picoalgae In 1979 small cyanobacteria of the genus  Synechococcus were suggested to be the most dominant primary producers in the oceans (Waterbury et al. 1979). In the following years it became apparent that small photosynthetic organisms existed in nearly all investigated marine and freshwater environments, however dominated in warm and nutrient poor waters (Søndergaard et al. 1991, Søndergaard 1994). It is also known that picoalgae are vertically distributed in a distinct predictable fashion. For example prochlorophytes are often at the bottom of the photic zone, in particular in warm climates (Chrisholm et al. 1988). Research into the importance of picoalgae's global distribution is by no means completed (Sherr & Sherr 2000), and there are only few  comparative studies where their global distribution is coupled to and compared with the distribution of other organisms in the microbial food web and DOM. This study is attempt to do this and has the potential to represent an important research effort into understanding the significance of these organisms and their community composition across large geographical, climatic and nutrient gradients, spanning temperate, tropical and arctic conditions.

                      There are numerous studies of DOM and microbial activity in the oceans that have been carried out before, however seldom with the global perspective which the Galathea 3 cruise offers. To our knowledge this is the first comprehensive global study of DOM concentrations (DOC, DON, and DOP), composition, turnover and relation to virus distribution in the world’s oceans. We plan to utilise every sampling opportunity for measurements of DOM, picoalgae, bacteria and viruses. Additionally bacterial production with be measured using both the leucine and thymidine incorporation techniques, which will give us an general description of bacterial turnover of DOM. Spectral fluorescence characterisation of DOM will be set up to run more or less continually while the ship is sailing, on surface water taken in by the ship underway.

A flowcytometer will also be onboard for the whole trip which will provide ”near” real time measurements of picoalgae composition (cyanobacteria, eukaryotes, prochlorophytes) and abundance as well as counts of the dominant large phytoplankton present. Water samples are first stained with fluorescent probe compounds which bind to DNA and RNA, and the abundance of bacteria and viruses are counted in the flowcytometer according to their fluorescence and scattering characteristics. The approach can distinguish between bacteria cells with high or low DNA content, which  means it can quantify the number of cells experiencing rapid growth (Søndergaard 2000) and additionally identify up to 4 different groups of viruses, including the separation of algae viruses.

Summary of aims:

To collect a continuous and comprehensive series of samples for describing the distribution of DOM in the oceans.

To characterise DOM using novel optical techniques and test the hypothesis that DOM from different sources final converges towards a common ocean pool.

To couple chemical and optical DOM measurements to the composition and activity of the microbial community (bacteria, viruses and picoalgae).

To carry experiments at specific locations (across climatic and nutrient gradients) that will provide insight into DOM turnover times, identify which parameters limit microbial activity and quantify the importance of viruses.

To apply the knowledge gained to an analysis of the effect of climate change on the future DOM turnover rates.

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