This is a topic that has been widely researched by David Jennings and his students [46,66,67,68,69,70,71,72,73] who have carried out extensive studies on marine fungi and their ability to tolerate saline conditions in the marine environment. Do marine fungi require sodium chloride for growth and what are the mechanisms that control osmotic pressure within their mycelium? In an effort to understand these phenomena, a wide range of techniques have been applied to elucidate the mechanisms involved. Jennings [46] highlighted three physiological issues facing a fungus in the marine environment: seawater (1) has a relatively low water potential, (2) contains a relatively high concentration of ions, and (3) has an alkaline pH.

Studies with Paradendryphiella salina showed that sodium stimulates its growth at low salinities but is toxic at high concentrations [36]. This inhibition could be overcome by the addition of magnesium, calcium, strontium and barium (in order of effectiveness), and this was also demonstrated for other marine fungi [36]. The study also demonstrated that the key issue for the growth of the fungus was the permeability of the mycelium to potassium in a high salt medium. This study led to the exploration of a number of factors pertinent to understanding the mechanisms of salt tolerance in marine and other fungi [46]. These included plasma membrane permeability, accumulation of ions in the vacuoles, and the role of polyols in maintaining turgor in the mycelium.

In seawater, how does Paradendryphiella salina control the movement of nutrients and ions into the mycelium? It has already been shown that calcium and other bivalent cations play a role in the movement of potassium and exclusion of sodium from the mycelium. Jones and Jennings [36] proposed that there was a sodium pump for its exclusion from the mycelium. Galpin and Jennings [74] reported the involvement of ATPase in maintaining the K⁺/Na⁺ ratio within the mycelium of P. salina, and ATPase is required for the active transport of cations. When P. salina was grown at high salinity and pH, there was an increased activity of membrane-bound ATPase and this aids in good potassium/sodium balance in the cytoplasm [46]. Thus, ATPase is required for active transport of cations and fulfilled by glycolysis.

These studies continued by exploring the accumulation of ions in fungal vacuoles [68,72,75], plasma-membrane permeability [70,76] and the role of polyols in maintaining turgor in the marine fungal mycelium [73,75]. Ions contributed some 60% of the solute potential in the 48-h old mycelium of P. salina grown in the presence of high concentrations of sodium chloride, while polyols contributed 30% [75]. Jennings and Austin [72] showed the importance of mannitol and arabitol in maintaining the total in vivo carbohydrate content of mycelium of P. salina which is required for making optimum turgor for growth. Mannitol and arabitol synthesis within the hypha increased with increasing salinity because these sugar alcohols play the main role in maintaining osmotic pressure as well as correct differential water potential in the mycelium [66]. This was confirmed by Wethered et al. [75], who provided evidence that the polyol content of the mycelium increased with salinity. Holligan and Jennings [70] proposed two pathways of mannitol synthesis; one is directly from glucose entering the hyphae and another is from hexose phosphate derived from pentose phosphate pathway with ATPase hydrolysis (Figure 2A,B). Arabitol synthesis depends upon a stimulation of the pentose phosphate pathway and is derived from pentose sugar (xylulose and ribulose) via the pentose phosphate pathway (Figure 2C). Jennings [68] concluded that mannitol, arabitol, glycerol, and erythritol are the major polyols which are accumulated by mycelium, and variation in carbon sources, such as glucose and fructose, has an effect in the accumulation of various polyols.

In contrast to the mycelial fungi discussed above, there is evidence that zoosporic genera in the Thraustochytriales require sodium chloride for growth. Siegenthaler et al. [77,78] suggested that phosphate uptake in Thraustochytrium roseum required sodium chloride. Also, they demonstrated that the presence of the amino acid proline in their cells, as well as high levels of inorganic ions which contribute to the solute potential of the cells. Wethered and Jennings [79] noted that proline concentrations in cells increased with the increased salinity of the medium.

Norkrans [80] and Norkrans and Kylin [81] drew attention to marine occurring yeasts that are halotolerant with growth in the range of 0–24% sodium chloride. Gustafsson and Norkrans [82] and Adler and Gustafsson [83] reported that polyols accumulated in the marine occurring yeast Debaryomyces hansenii due to salt-stress, while Adler [84] showed that the accumulation of glycerol in D. hansenii played a role in osmoregulation.

Fungi in man-made salterns, soda lakes, coastal lagoons and the Dead Sea, tolerate very high environmental NaCl concentrations when compared to the marine fungi discussed here [85,86]. Larsen [85] characterised these fungi into four categories depending on tolerance to NaCl concentrations: non-tolerant up to 1%, slight 10%, moderate 20% and extreme 30% NaCl. These fungi too face similar physiological conditions to marine fungi, namely an external environment with relatively low water potential and high concentration of ions [87]. It is speculated that cation transporters prevent intracellular accumulation of Na⁺, which would be toxic but plays a role in maintaining the high K⁺/Na⁺ ratio required for growth in an environment with high salt content. The halophilic Wallemia ichthyophaga accumulates glycerol, while Hortaea werneckii also accumulates erythritol, arabitol and mannitol as solutes [88,89]. The same mechanism applies to the growth of marine fungi in seawater [46].

5. Marine Fungi and Climate Change