Global warming may cause the microscopic organisms habiting the polar regions of the Earth's oceans, such as the Arctic, to be slowly conquered by their counterparts from the tropical regions, affecting multiple ecosystems, according to two major studies which assessed more than 35,000 samples collected from all the world's oceans between 2009 and 2013.
The two studies, published in the journal Cell on Thursday, found that the diversity of microscopic organisms habiting the Earth's oceans are the least at the higher latitudes -- nearer to the poles -- and increase in a gradient towards the equator. The two studies analyzed the diversity of these organisms in samples collected from ocean water currents to predict how these communities might adapt to changing environmental conditions.
The researchers, including Chris Bowler from Institut de Biologie de l'Ecole Normale Superieure (IBENS) in France, said one of the studies focused on the diversity of groups of microscopic marine organisms called plankton, and the other assessed the activity patterns of genes among microbial communities across the Earth's oceans.
"Our study focused on plankton because it's a major contributor to marine ecosystems in terms of biomass, abundance, and diversity," said Lucie Zinger of IBENS, co-senior author of the plankton study.
"All types of life have representatives in the plankton -- bacteria, archaea, protists, animals and plants, as well as viruses. But the large majority of this diversity is invisible to the naked eye," he said.
The findings of the study revealed that plankton groups across the planet follow a gradient of diversity along the latitudes -- with the lowest level of their diversity closest to the poles, and the diversity increasing towards the equator.
"Ocean temperature is mainly responsible for this pattern. Ocean warming due to climate change is likely to lead to a 'tropicalization,' or increase, of plankton diversity in temperate and polar waters," Zinger said.
The other study assessed ocean microbial communities to see what genes they had, and which ones were actively involved in their cellular processes, across gradients of both depth and latitude.
"Looking at transcriptomes is important for determining not just which microbes are present, but what those microbes are doing with regard to activities like photosynthesis and nutrient uptake," said Shinichi Sunagawa, senior author of the microbe genome analysis study from ETH Zurich in Switzerland.
"One of our goals was to learn whether microbial communities adjust to environmental and temperature variations with changes in their composition relative to each other or with changes in the gene expression patterns within these communities," he said.
The species of bacteria, the genes present, and the activities of genes varied distinctly in the waters with boundaries separating them both across surface water to deep water, and across poles towards the equator, the study said.
"We did not expect to find biogeographic patterns for the underlying mechanisms of metatranscriptomic composition variation. Specifically, we found differences in polar communities to be dominated by changes in organismal composition, while in nonpolar waters, the differences were dominated by changes in the expression of genes," Sunagawa said.
He added that the researchers were surprised to find evidence for a nitrogen-fixing bacterium -- commonly present in the soil or plant roots on land -- in the deep Arctic waters.
"And what this means, together with the results of the diversity study, our findings suggest that microbial communities to warmer waters have a larger gene pool, which may allow them to be quite flexible and responsive temperature changes by the cold counterpart and colder waters seem to be more hardwired to them," Sunagawa explained in a telephonic press briefing where PTI was also present.
While the studies did not probe into the diversity changes that may result in communities of larger organisms like fishes, they said the changes in microbial composition in the Arctic might lead to cascading effects related to nutrition on the larger eukaryotic organisms in the food chain.
"In the Arctic one thing that is very prominent in high latitudes like the Arctic is the importance of lipids and fats in the diet," Bowler told PTI.
He explained that the higher organisms in the food web of Arctic ecosystems need to nourish themselves with organisms rich in fat to build fat reserves that can help them withstand the extreme cold conditions of the region.
"So if the food sources that these organisms are eating or less rich in fats, then we could perhaps imagine that these large organizations will struggle to survive," Bowler said.
Sunagawa said due to climate change cold water microbial communities may experience displacement of some species, possibly by those with better adaptive capacity. Sunagawa said in the press briefing that it is critical is to continue to monitor the state of the oceans by certain interdisciplinary efforts, and to try and capture temporal trends in microbial community changes.
"Every drop of marine water is full of microbes, which play a central role in many processes relevant to life on Earth," he said.