Cryptic biogeochemical cycles: unravelling hidden redox reactions

The turnover of organic matter in the environment is controlled by a series of (often connected) biogeochemical cycles, mainly involving the transformation of C, N, S, O, Fe and Mn under both oxic and anoxic conditions (Druschel and Kappler, 2015). Typically, in order to quantify the extent and rates of turnover, the geochemical end products or reaction intermediates are quantified over time (e.g. sulfide and sulfate or Mn(II) and Mn(IV)). In many cases, the absence of a certain species is used to exclude the relevance of the respective biogeochemical cycle. Recently, however, more evidence has appeared which shows that very reactive, sometimes short-lived intermediates at low concentrations with poorly understood redox properties can play key roles in biogeochemical cycles. For example, redox-active humics are important in carbon cycling, nitrous oxide in the nitrogen cycle and polysulfides in the sulfur cycle (Hansel et al., 2015). The concentrations of these intermediates represent steady-state concentrations arising from the balance between continuous oxidation and reduction. This leads to so-called ‘cryptic element cycles’ where changes in concentrations of a certain redox species cannot be measured but rapid turnover (connected to other element cycles) means that they are key components of the biogeochemical processes that are occurring. In recent years, for example, the importance of cryptic cycling of sulfur has become increasingly clear. The main driver of the sulfur cycle is microbial sulfate reduction which ultimately produces sulfide (H2S). However, this is not a direct process and a large diversity of reactive sulfur species are formed during the six intermediate oxidation states between sulfate and sulfide which are themselves suitable for microbial redox reactions (Zopfi et al., 2004). These reactive intermediates are often below detection limit in the environment, yet are thought to play very important roles in biogeochemical cycling of both sulfur and associated redox species (Holmkvist et al., 2011). Additionally, sulfide can be rapidly recycled making the detection of sulfate reduction difficult. For example, Canfield et al. (2010) found that diverse communities of sulfur cycling microorganisms were present in oxygen minimum zones off the Chilean coast despite very low sulfate and sulfide concentrations. These sulfur cycling communities were sustained through rapid recycling of the H2S which is also linked to nitrogen cycling in these regions. Although cryptic cycling has been described in detail for sulfur, and other elements to a small extent, almost nothing is known about cryptic iron cycling (Hansel et al., 2015).