The impact of iron availability on the ecology of Prochlorococcus populations in the Sargasso Sea

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Prochlorococcus spp. are tiny (1-2 um diameter) autotrophic cyanobacteria and are the most numerically dominant photosynthetic species in the world. They are ubiquitous in the subtropical and tropical ocean gyres that comprise vast areas of the open ocean. Due to their abundance and distribution, they are key organisms in many marine systems, and are responsible for a large proportion of gross primary productivity and nutrient cycling, which affect the global climate and are the basis of many marine ecosystems. Prochlorococcus was unknown to science until recently, when its discovery by Penny Chisholm (MIT) in 1998 revolutionised our understanding of marine photosynthesis. As a result of this relatively recent discovery, we have only a rudimentary understanding of the biogeochemical functions and ecological roles of natural Prochlorococcus populations in the open ocean. Prochlorococcus cells dominate when the water column is highly stratified, a situation that occurs seasonally in the summer in the Bermuda region of the Sargasso Sea (subtropical North Atlantic). In such highly stratified waters, there is a characteristic layer of high chlorophyll at the base of the sunlit zone (euphotic layer) and the top of the nutracline. This layer, termed the deep chlorophyll maximum, is dominated by Prochlorococcus spp. in tropical and subtropical waters. Molecular and physiological studies have identified two Prochlorococcus ecotypes, known as 'high-light' and 'low-light' ecotypes. The low-light ecotypes dominate the lower euphotic zone, whereas the high-light ecotypes dominate the upper euphotic zone, but are not exclusively restricted to this region. This proposal aims to determine the reason for this seasonal accumulation and the vertical distribution of the two ecotypes, and so help to define the ecological role of this numerically dominant species. Iron is a fundamental requirement for photosynthetic cells and iron distributions and concentrations, which are often sparse and low in oceanic systems, have been shown to significantly affect phytoplankton distribution and community composition. Recently, a sub-surface minimum in iron concentration has been found to develop at the Bermuda region of the Sargasso Sea in the summer, and this minimum is co-incident with both the DCM and the accumulation of the low-light Prochlorococcus ecotype. This situation has been shown to develop even when iron concentrations at the surface remain high. The objective of this proposal is to determine whether a low iron concentration is the main factor in determining the abundance of the low-light ecotype of Prochlorococcus cells that are numerically dominant in the Bermuda region of the Sargasso Sea in the summer. This will be achieved by participation in a number of cruises aboard the R/V Atlantic Explorer in the Bermuda region of the Sargasso Sea, which are timed to coincide with the early, mid and late stages of development of both the sub-surface iron minimum and the accumulation of Prochlorococcus cells. During these cruises, water-column profile measurements will be made to correlate Prochlorococcus ecotype distribution and physiology with available iron concentration. On-deck incubation experiments will be performed under controlled light and temperature conditions, and iron will be artificially added to water samples taken from the DCM and the surface water (sampled at the same location as iron profiles). The resulting response of the community will then be monitored to detect a response of the phytoplankton to added iron, and to identify which if any species respond to the addition. The different responses of phytoplankton communities at the surface and DCM will determine the limiting factors of the system. Conducting similar experiments during the summer and over two seasons will confirm the temporal development of these features.