Typical marine water columns are divided into multiple layers, each with its own biological community adapted to the surrounding chemistry and oxygen levels. Global warming is causing major changes in these habitats by amplifying the density differences between layers, decreasing the penetration depth of oxygen, and affecting the solubility of gases in water. Consequently, scientists have noticed that the regions depleted of oxygen are expanding, and therefore the ocean’s global balance of carbon, oxygen, nitrogen and sulphur is shifting. As new nutrients pour into the coastal ocean from the land – a phenomenon known as eutrophication – the so-called dead zones expand throughout nearshore regions and estuaries, impacting fauna occupying these waters and the sea floor. We should recall that this is the case because oxygen levels generally decrease with depth, and depending on the affected region, the intensity and duration of oxygen depletion varies from seasonal hypoxia to permanent anoxia. Eutrophication leads to the proliferation of algae, some of which produce harmful toxins. Moreover, excessive algal populations promote oxygen depletion. ‘In extreme cases, this chain of events leads to anoxic waters that may produce hydrogen sulfide which is toxic to most animals,’ Professor Taylor warns. Lack of oxygen leads to an entire chain of consequences. Aside from being harmful to animal life, it impacts the way that essential minerals are cycled. An example provided by Professor Taylor is that ‘loss of biologically available nitrogen is accelerated in oxygen minimum zones (OMZs) found along the eastern margins of the Pacific, Atlantic and Indian Oceans and other oxygen-depleted bodies of water. Without a compensating mechanism to replenish bioavailable nitrogen, expanding zones of oxygen depletion imply less nutrition to support plankton production in future oceans.’

This is one of Professor Taylor’s reasons for studying the Cariaco Basin. The basin acts as a natural laboratory, allowing researchers to gain insight into the microbiological and biogeochemical processes in the transition zones between oxygenated and deoxygenated waters. Conveniently, it is a single system where the whole range of oxygen conditions are available for study. ‘What we learn here informs us about how oxygen-depleted water columns elsewhere currently function and how they might function in the future as deoxygenation progresses.’ he explains. ‘Furthermore, the CARIACO Ocean Time-series program has provided monthly data on physical, chemical, biological and meteorological conditions since late 1995. This outstanding database provides a unique and very important context to facilitate understanding our molecular results on community composition, functional genes present and gene expression.’

Despite the wealth of knowledge researchers have already gathered, fundamental questions still persist about life forms feeding on inorganic and organic chemicals and living without oxygen, especially how they cycle nutrients present in the water. Although DNA and RNA sequencing technologies have enabled scientists to acquire an understanding of the genetic diversity and classification of microbes in nature, these technologies do not adequately capture the functional dynamics of microbial communities present in any given environment. Thus, there is more to learn about potential and realized capabilities of marine microbes. Their functional diversity and redundancy and their responses to chemical and physical changes in their ecosystems are still poorly understood. A particularly interesting part of Professor Taylor’s research aims to find out how microbes adapt to oxygen depletion by studying their behaviour in the context of geochemical gradients in their environment. These gradients represent distributions of chemical species in the water and are influenced by depth, currents and other local characteristics of the medium. In order to feed, microbes organize along these gradients and, through time, modify their metabolism and gene expression to better adapt to current conditions.

Future research directions

Since most marine microbes to date cannot be cultivated in the lab, scientists need ways to explore their functions without the benefit of laboratory cultures. DNA and RNA inventories of microbial communities tell us about the genes present and whether they are being used at the time of collection. These inventories are powerful tools for gaining a better understanding of metabolic pathways and the response of microbial communities to environmental conditions. Previous experience and the database amassed during Professor Taylor’s research, has enabled his team to make important steps towards developing techniques capable of linking key players to particular biogeochemical functions and activities of microbes. ‘We do so by allowing active members to incorporate stable isotopic tracers of a process and produce isotopically heavy biomass and we fluorescently-label subsamples with a selection of genetic probes. We then interrogate individual probe-positive cells in each taxonomic subsample by confocal Raman microspectrometry to determine extent of isotope incorporation. These single-cell analyses are performed in the NARMIL.’ Professor Taylor tells us. Among many other types of research, the lab enables scientists to determine which organisms dominate carbon, nitrogen and sulfur cycling in anoxic systems and to explore elemental cycling within anaerobic symbiotic associations.