Reviews and syntheses: Foraminifera from anaerobic environments – Survival strategies, biogeochemistry, ecology and applications for paleoceanography 

Abstract. The oceans are losing oxygen (O₂) and oxygen minimum zones are expanding, due to climate warming (lower O₂ solubility) and artificial fertilization related to agriculture. This ongoing trend is challenging for most marine taxa that are not well adapted to O₂ depletion. For other taxa this trend might be advantageous, because they can withstand low O₂ concentrations or thrive under anaerobic or even anoxic conditions. Benthic foraminifera are a group of protists that might benefit from ongoing ocean deoxygenation, since several foraminifera species possess adaptations to O₂ depletion that are unique amongst eukaryotes. This paper reviews the current state of knowledge about foraminifera from low O₂ environments. The specific survival strategies of foraminifera to withstand O₂ depletion include an anaerobic metabolism, heterotrophic denitrification, symbiosis with bacteria, kleptoplasty and dormancy. These adaptations, especially the ability to denitrify by some benthic foraminiferal species, have a strong impact on their preferred microhabitat in the sediments, which will be discussed in detail. In addition, due to their high abundances in O₂ depleted environments and their metabolic adaptations, benthic foraminifera are key players in marine nutrient cycling, especially within the marine N and P cycles. Studies about the ecology of benthic foraminifera are scarce but there is evidence that foraminifers have the capacity of phagocytosis, even under anoxia, and some foraminiferal species, which can withstand low O₂ conditions, even seem to prey on meiofauna. Finally, the fact that foraminifera can calcify even under anaerobic conditions makes them important archives for paleoceanographic applications. So this review will briefly summarize O₂ proxies based on foraminiferal morphology, shell geochemistry and composition of foraminiferal assemblages.