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The biological carbon pump is crucial for the export of particulate organic matter in the ocean. Recent studies on marine microbes have shown the profound influence of bacteria and archaea as regulators of particulate organic matter export.

BackgroundOceans are crucial regulators of the global carbon cycle. Understanding the oceanic biological carbon pump (BCP) and its contribution to carbon export has been the subject of extensive research. These studies have provided quantitative evidence regarding the centrality of phytoplankton throughout the water column. In the Southern Ocean, the biological carbon pump is driven primarily by phytoplankton productivity and is an effective organic matter sink. There is evidence showing that sinking particulate organic matter (POM) sustains microbial communities with different ecological strategies (i.e., r-/K-strategists). These results suggest that the role of microbial communities on the effectiveness of the biological carbon pump should not be underestimated. However, we lack mechanistic insights regarding the importance of these microorganisms, their diversity and influence on the efficiency of the BCP.Results Here, we provide the first insights regarding prokaryotic metabolic capacity linked to suspended and sinking particles to improve our understanding of microbial contributions towards POM export in the Southern Ocean. A Marine Snow Catcher (MSC) was deployed at several stations southwest of Tasmania in the Subantarctic zone to obtain suspended and sinking particulate material for determining carbon and nitrogen flux. Metagenomic analysis and metagenomic-assembled genomes showed that both the suspended and sinking particle-pools were dominated by bacteria with metabolic capacity for degrading POM (e.g. Gammaproteobacteria MAGs). Archaeal genomes (Poseidoniia and Nitrososphaeria) appear to drive nitrogen metabolism via nitrite and ammonia oxidation in these communities. Results suggest that for bacteria r-strategists were more ubiquitous in the suspended pool while K-strategists were more prevalent in the sinking particle-pool, with the opposite being true for archaea. In addition, metabolic reconstructions suggest that prokaryotes harbour substantial genetic capacity for degrading complex POM and chemoautotrophic synthesis of recalcitrant dissolved organic carbon (RDOC) from CO2. ConclusionThe metagenome assembled genomes from sinking and suspended size fractions and carbon flux determinations revealed striking trends regarding prokaryotic contributions in the water column. The results show that the predicted lifestyles for bacteria and archaea may differ substantially in sinking and suspended fractions, suggesting niche specificity. Together, our data suggest that prokaryotes in suspended and sinking particles may enhance POM export via the production of RDOC in the Southern Ocean.

Although most knowledge on the biology of Sousa plumbea has primarily come from South African waters, a number of research gaps remain on the natural history and status of the species in the region. Research on two populations in South African waters for which some historical data exist may aid in highlighting long-term changes in the biology and natural history of this little known coastal delphinid. Recent studies on the age, growth and reproduction of animals incidentally caught in shark nets in Richards Bay, KwaZulu-Natal, yielded a lower maximum age estimate of 24 (previously 46) growth-layer-groups (GLGs), sexual maturity of 7.5 and 8 GLGs in males and females (previously 12-13 and 10 GLGs, respectively), an ovulation rate of 0.2 and a 5-year calving interval (previously 0.3 and 3-year calving interval) than previously reported. These differences may be due to a difference in the interpretation of GLGs between observers or a predominance of young males being caught in the shark nets. Stomach content analysis revealed a change in the relative proportions of the main prey items over the past 25 years, but no difference in species richness or diversity was found between the sexes. No change in trophic level was recorded between 1972 and 2009. Field studies in Algoa Bay, Eastern Cape, conducted 16 years apart indicated a decline in the mean group size (from 7 to 3 animals), a decline in the maximum group size (from 24 to 13 animals), an increase in solitary individuals (15.4-36%), and a change in behaviour from predominantly foraging (64-18%) to mainly travelling (24-49%). The observed changes are suggestive of a change in food availability, resulting in a range shift or a potential decline in numbers. These studies indicate the importance of long-term studies to monitor population changes and their possible causes. A number of threats, such as shark nets, pollution (noise and chemical), and coastal development and disturbance, to the humpback dolphin populations in South Africa have been identified. Urgent action is required to ensure continued existence of the species in South African waters.